Class 11 Biology MCQs | 85 MCQs | Cell: The Unit Of Life

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401. The main protein subunit forming microfilaments is:

ⓐ. Actin

ⓑ. Tubulin

ⓒ. Keratin

ⓓ. Collagen

Correct Answer: Actin

Explanation: Microfilaments are primarily composed of actin, a globular protein that polymerizes into long, thin filaments. Actin monomers assemble into F-actin, creating a flexible cytoskeletal network beneath the plasma membrane and throughout the cytoplasm. This network supports cell shape, provides force for movement, and participates in processes like cytokinesis and muscle contraction. Because actin filaments are dynamic, the cell can rapidly remodel them for changes in cell behavior. The actin-based nature is the defining micro-point that distinguishes microfilaments from tubulin-based microtubules and intermediate filaments. Hence, actin is the correct subunit.

402. The typical diameter of an actin microfilament is about:

ⓐ. 25 nm

ⓑ. 10 nm

ⓒ. 7 nm

ⓓ. 2 nm

Correct Answer: 7 nm

Explanation: Microfilaments are the thinnest major cytoskeletal elements and have a typical diameter close to 7 nm. This size reflects their construction from two intertwined strands of polymerized actin. The small diameter supports flexibility and rapid remodeling, which is crucial for cell shape changes, membrane dynamics, and motility. The value is also a standard identification feature used to distinguish microfilaments from microtubules (~25 nm) and intermediate filaments (~10 nm). Remembering these characteristic sizes helps in diagram-based and concept questions. Therefore, about 7 nm is correct.

403. A process most directly driven by actin microfilaments is:

ⓐ. Formation of the mitotic spindle

ⓑ. Chromosome condensation in nucleus

ⓒ. Electron transport chain in cristae

ⓓ. Cytokinesis by cleavage furrow

Correct Answer: Cytokinesis by cleavage furrow

Explanation: During cytokinesis in many animal cells, actin microfilaments form a contractile ring at the cell equator. Interaction of actin with myosin generates a tightening force that creates the cleavage furrow and pinches the cell into two daughter cells. This actin-based contraction is a classic, high-yield example of microfilament function. It differs from spindle formation, which relies mainly on microtubules, and from nuclear condensation processes, which are not microfilament-driven. The cleavage furrow mechanism demonstrates how microfilaments convert chemical energy into mechanical force. Hence, cytokinesis by cleavage furrow is most directly driven by actin microfilaments.

404. The motor protein most directly associated with actin filaments is:

ⓐ. Kinesin

ⓑ. Dynein

ⓒ. Myosin

ⓓ. Tubulinase

Correct Answer: Myosin

Explanation: Myosin is the principal motor protein that interacts with actin microfilaments to generate movement and force. In muscle, myosin slides actin filaments to produce contraction, while in non-muscle cells, myosin helps drive cytokinesis, cell shape changes, and contractile events. Myosin uses ATP to change conformation and “walk” along actin, translating chemical energy into mechanical work. This actin–myosin pairing is a foundational concept for understanding microfilament-based motility. In contrast, kinesin and dynein are mainly microtubule-based motors. Therefore, myosin is the correct motor protein.

405. A structure rich in actin microfilaments just beneath the plasma membrane is the:

ⓐ. Glycocalyx coat

ⓑ. Microvillus core

ⓒ. Nuclear lamina layer

ⓓ. Mitochondrial matrix mesh

Correct Answer: Microvillus core

Explanation: Microvilli contain a core bundle of actin microfilaments that provides structural support and maintains their finger-like projections. Actin bundles give microvilli stiffness and allow them to increase surface area for absorption without collapsing. This is a common exam example linking microfilaments to membrane specializations. The actin network beneath the membrane also contributes to shape and dynamic changes, but microvillar cores are a clear, named structural association. Because microvilli function in absorption, their actin support is crucial for maintaining large surface area under mechanical stress. Hence, the microvillus core is rich in actin microfilaments.

406. A common outcome when actin polymerization is blocked in a motile cell is:

ⓐ. Loss of pseudopodia formation

ⓑ. Faster chromosome segregation

ⓒ. Increased cristae surface area

ⓓ. Enhanced thylakoid stacking

Correct Answer: Loss of pseudopodia formation

Explanation: Cell crawling and pseudopodia formation depend heavily on actin polymerization at the leading edge of the cell. Actin filaments push the plasma membrane forward to create protrusions that help the cell move and adhere. If actin polymerization is blocked, the cell loses the ability to generate these membrane protrusions and its motility is strongly reduced. This directly demonstrates the role of microfilaments in cell movement and shape change. The effect is not primarily on spindle-based chromosome segregation or on organelle membrane stacking processes. Therefore, loss of pseudopodia formation is the expected outcome.

407. Compared with microtubules, actin microfilaments are generally:

ⓐ. Thicker and hollow cylinders

ⓑ. Always nonpolar structures

ⓒ. Made of tubulin dimers

ⓓ. Thinner and more flexible

Correct Answer: Thinner and more flexible

Explanation: Actin microfilaments are thinner than microtubules and are generally more flexible, supporting rapid remodeling of cell shape. Their small diameter and filamentous nature allow cells to form fine networks and bundles near the membrane and within the cytoplasm. This flexibility is useful for processes such as motility, endocytosis-related shape changes, and contractile functions. Microtubules, by contrast, are thicker, hollow, and more rigid, supporting long-range transport and spindle formation. Actin filaments are also polar, which supports motor-protein movement, but the key contrast asked here is thickness and flexibility. Hence, thinner and more flexible is correct.

408. A bacterial cell typically lacks which of the following cytoskeletal systems in the same form as eukaryotes?

ⓐ. Actin microfilament system

ⓑ. Tubulin microtubule system

ⓒ. Intermediate filament system

ⓓ. All three in identical form

Correct Answer: Intermediate filament system

Explanation: Intermediate filaments are a prominent cytoskeletal component of many eukaryotic cells, providing mechanical strength and structural stability. Prokaryotic cells do not generally possess intermediate filaments in the same organized, standard form found in eukaryotes. This makes intermediate filaments a classic distinguishing feature in cytoskeleton comparisons. While prokaryotes may have proteins that perform some similar structural roles, the typical intermediate filament network is considered a eukaryotic feature. This micro-point helps separate cytoskeletal organization across cell types in exam questions. Therefore, the intermediate filament system is the best answer.

409. The contractile ring during cytokinesis is mainly made of:

ⓐ. Actin and myosin

ⓑ. Tubulin and kinesin

ⓒ. Keratin and collagen

ⓓ. DNA and histones

Correct Answer: Actin and myosin

Explanation: The contractile ring that forms at the equator of a dividing cell is built from actin filaments and myosin motor proteins. Myosin interacts with actin to generate sliding forces that tighten the ring, pulling the membrane inward to form the cleavage furrow. This mechanism is a key example of microfilament function in cell division, distinct from microtubule-based spindle activity. The actin–myosin system converts ATP energy into mechanical constriction, physically separating the cytoplasm. Because this process is widely tested, the actin-plus-myosin composition is a high-yield fact. Hence, actin and myosin is correct.

410. A structural feature that helps actin filaments support cell shape in many cells is that they form:

ⓐ. A hollow tube lattice in the cytosol

ⓑ. A pore-lined nuclear envelope mesh

ⓒ. A stacked disc system in plastids

ⓓ. A dense cortical network under membrane

Correct Answer: A dense cortical network under membrane

Explanation: Actin filaments commonly form a cortical network just beneath the plasma membrane, often called the actin cortex. This network supports the membrane, maintains cell shape, and allows controlled deformation during processes like endocytosis, cell migration, and cytokinesis. The cortex can rapidly remodel because actin polymerization and depolymerization are dynamic, enabling cells to respond quickly to mechanical and signaling changes. This membrane-associated actin organization is a standard micro-point explaining why microfilaments are closely linked to cell surface events. It also helps explain surface projections and contractile behaviors in many cells. Therefore, a dense cortical network under the membrane is the key supporting feature.

411. Intermediate filaments are primarily known for providing:

ⓐ. ATP synthesis in membranes

ⓑ. Structural tensile strength

ⓒ. DNA replication control

ⓓ. Pigment storage function

Correct Answer: Structural tensile strength

Explanation: Intermediate filaments form strong, rope-like fibers that resist stretching and help cells withstand mechanical stress. Their key role is to provide tensile strength, maintaining cell integrity when cells are pulled, compressed, or subjected to shear forces. They are especially important in tissues that face frequent mechanical strain, where stable support is required rather than rapid remodeling. Unlike microtubules and microfilaments, intermediate filaments are less involved in fast transport or contractile movement and more in long-term structural reinforcement. This property explains why defects in certain intermediate filament proteins can make tissues fragile. Hence, structural tensile strength is the primary function.

412. The typical diameter of intermediate filaments is closest to:

ⓐ. 25 nm

ⓑ. 7 nm

ⓒ. 10 nm

ⓓ. 2 nm

Correct Answer: 10 nm

Explanation: Intermediate filaments have a characteristic diameter around 10 nm, placing them between microfilaments (~7 nm) and microtubules (~25 nm). This intermediate size is part of why they are named “intermediate” filaments. Their diameter reflects a rope-like assembly that supports mechanical stability rather than a hollow tube or thin flexible strand. This measurement is commonly tested as a quick identification feature in cytoskeleton questions. Remembering the three major filament diameters helps students avoid confusion in diagrams and conceptual comparisons. Therefore, the closest diameter is about 10 nm.

413. A common intermediate filament protein in epithelial cells is:

ⓐ. Myosin

ⓑ. Tubulin

ⓒ. Actin

ⓓ. Keratin

Correct Answer: Keratin

Explanation: Keratin is a major intermediate filament protein in epithelial cells, forming networks that strengthen cell structure and help resist mechanical stress. In skin and other epithelia, keratin filaments are crucial for maintaining tissue integrity under friction and pressure. Their stable filament network supports the idea that intermediate filaments primarily provide strength rather than rapid movement. This is why keratin-related defects can lead to fragile epithelial tissues. Keratin is therefore a classic example used in exams to link intermediate filaments with epithelial strength. Hence, keratin is correct.

414. The intermediate filament network that lines the inner surface of the nuclear envelope is called:

ⓐ. Nuclear lamina

ⓑ. Axoneme

ⓒ. Stroma lamella

ⓓ. Contractile ring

Correct Answer: Nuclear lamina

Explanation: The nuclear lamina is a meshwork of intermediate filament proteins that supports the inner surface of the nuclear envelope. It helps maintain nuclear shape, organizes nuclear pores, and provides structural support to the nucleus. This reinforces the mechanical stability role of intermediate filaments at the level of the nucleus, not just the cytoplasm. The lamina is made mainly of lamins, which are intermediate filament proteins specific to the nuclear region. Its presence is a standard micro-point connecting intermediate filaments to nuclear architecture. Therefore, nuclear lamina is the correct term.

415. A key property of intermediate filaments compared with microtubules is that intermediate filaments:

ⓐ. Are hollow tubes with protofilaments

ⓑ. Are highly dynamic with GTP caps

ⓒ. Are mainly for tensile support

ⓓ. Act as tracks for kinesin motors

Correct Answer: Are mainly for tensile support

Explanation: Intermediate filaments are designed to provide durable mechanical strength rather than serve as rapidly growing or shrinking polymers. They form stable, rope-like assemblies that resist stretching, making them well suited for reinforcing cells and tissues. Microtubules, in contrast, are hollow and show dynamic instability, supporting transport and spindle functions. Intermediate filaments do not typically serve as tracks for kinesin or dynein, and they do not use GTP-driven polymerization caps like tubulin-based microtubules. Their primary contribution is maintaining cell integrity under mechanical stress. Hence, they are mainly for tensile support.

416. The polarity of intermediate filaments is best described as:

ⓐ. Strongly polar with plus/minus ends

ⓑ. Nonpolar in overall structure

ⓒ. Polar only during mitosis

ⓓ. Polar only in axonemes

Correct Answer: Nonpolar in overall structure

Explanation: Intermediate filaments are generally considered nonpolar because their subunits assemble in a way that does not create distinct plus and minus ends like actin filaments and microtubules. This nonpolarity is consistent with their major function in structural support rather than directional transport. Because they lack clear polarity, they are not typically used as tracks for directional motor proteins in the way microtubules and actin are. The absence of polarity also supports their stable, rope-like mechanical role. This is a common conceptual point tested in cytoskeleton comparisons. Therefore, intermediate filaments are best described as nonpolar in overall structure.

417. A cell type expected to have especially abundant intermediate filaments for mechanical stability is:

ⓐ. Bacterial cell

ⓑ. Red blood cell

ⓒ. Sieve tube element

ⓓ. Epidermal skin cell

Correct Answer: Epidermal skin cell

Explanation: Epidermal skin cells experience constant mechanical stress from friction, pressure, and stretching, so they require strong internal reinforcement. Keratin intermediate filaments provide tensile strength to these epithelial cells, helping tissues resist tearing and maintaining integrity. This is why keratin-based networks are particularly well developed in skin and similar epithelia. The link between intermediate filaments and mechanically stressed tissues is a standard exam logic point. In contrast, mature red blood cells lack many organelles and are not reinforced by typical intermediate filament networks in the same way. Hence, epidermal skin cells are expected to have abundant intermediate filaments.

418. A correct match of intermediate filament protein to location is:

ⓐ. Lamins—nuclear envelope support

ⓑ. Tubulin—nuclear envelope support

ⓒ. Actin—nuclear envelope support

ⓓ. Myosin—nuclear envelope support

Correct Answer: Lamins—nuclear envelope support

Explanation: Lamins are intermediate filament proteins that form the nuclear lamina beneath the inner nuclear membrane. This network supports nuclear shape and provides mechanical stability to the nuclear envelope. The lamin-based lamina also helps organize nuclear pores and contributes to structural organization of the nucleus. This match is a high-yield fact connecting a named intermediate filament protein to a specific cellular location and function. It distinguishes lamins from tubulin or actin, which form different cytoskeletal systems. Therefore, lamins—nuclear envelope support is the correct match.

419. Intermediate filaments are most directly linked to which type of junction that strengthens tissues?

ⓐ. Tight junctions

ⓑ. Gap junctions

ⓒ. Desmosomes

ⓓ. Plasmodesmata

Correct Answer: Desmosomes

Explanation: Desmosomes are cell–cell junctions that provide strong mechanical coupling between adjacent cells, especially in tissues under stress. Intermediate filaments, particularly keratin in epithelial cells, anchor into desmosomes to distribute tensile forces across a tissue layer. This linkage allows stress on one cell to be shared with neighboring cells, reducing the chance of tearing. The intermediate filament–desmosome relationship is a standard micro-point in cell structure questions. It directly connects the tensile role of intermediate filaments with tissue-level strength. Hence, desmosomes are most directly linked with intermediate filaments.

420. A main difference between intermediate filaments and actin microfilaments is that intermediate filaments:

ⓐ. Drive amoeboid movement at leading edge

ⓑ. Provide stable resistance to stretching

ⓒ. Form the core of microvilli

ⓓ. Build the contractile ring alone

Correct Answer: Provide stable resistance to stretching

Explanation: Intermediate filaments are specialized for long-lasting mechanical support and resistance to stretching, acting as internal “cables” that reinforce cells. Actin microfilaments are more involved in dynamic processes such as cell movement, membrane protrusions, and contraction with myosin. Intermediate filaments are therefore less about rapid remodeling and more about maintaining integrity under mechanical strain. This distinction explains their abundance in tissues like skin and their connections to desmosomes and the nuclear lamina. The functional contrast is a common conceptual trap in exams, where students may confuse movement roles with support roles. Therefore, providing stable resistance to stretching is the main difference.

421. The centriole is best identified by which microtubule arrangement?

ⓐ. Nine triplet microtubules

ⓑ. Nine doublets with central pair

ⓒ. Thirteen protofilaments in ring

ⓓ. Nine singlets in simple circle

Correct Answer: Nine triplet microtubules

Explanation: A centriole is a cylindrical structure built from nine sets of microtubule triplets arranged in a ring. This “9 triplet” pattern is the key ultrastructural marker that distinguishes centrioles from axonemal structures of cilia/flagella. The triplet design provides rigidity and stability, which is important because centrioles serve as organizing platforms rather than beating structures. Centrioles help organize the centrosome and contribute to microtubule nucleation during spindle formation. Their architecture supports the formation of spindle poles and orderly microtubule arrays. Hence, the defining arrangement is nine triplet microtubules.

422. In animal cells, the main microtubule-organizing center for spindle formation is the:

ⓐ. Nucleolus region

ⓑ. Centrosome

ⓒ. Golgi stack region

ⓓ. Plasma membrane domain

Correct Answer: Centrosome

Explanation: The centrosome is the primary microtubule-organizing center in most animal cells and is central to mitotic spindle formation. It nucleates and anchors microtubules so they radiate in a controlled pattern, creating two focused poles during mitosis. This organization is essential for building a bipolar spindle that can capture and separate chromosomes accurately. The centrosome typically contains a pair of centrioles plus surrounding pericentriolar material that supports nucleation. By defining spindle poles, it helps coordinate spindle geometry and timing of chromosome movement. Therefore, the centrosome is the key MTOC for spindle formation.

423. The pericentriolar material of the centrosome is most directly important because it:

ⓐ. Stores ATP for spindle energy

ⓑ. Packages spindle proteins into vesicles

ⓒ. Nucleates microtubules for spindle

ⓓ. Produces rRNA for ribosome assembly

Correct Answer: Nucleates microtubules for spindle

Explanation: The pericentriolar material is the protein-rich matrix around centrioles that serves as the main site of microtubule nucleation. It contains nucleation factors that initiate microtubule growth and help anchor microtubule minus ends at the centrosome. During mitosis, this nucleation capacity increases, enabling rapid assembly of spindle microtubules. A strong nucleation center is crucial for forming a stable bipolar spindle with robust microtubule arrays. Without efficient nucleation, spindle formation becomes slow or abnormal, risking errors in chromosome segregation. Hence, the pericentriolar material is vital because it nucleates microtubules for the spindle.

424. The spindle microtubules that attach directly to chromosome kinetochores are:

ⓐ. Astral microtubules

ⓑ. Polar microtubules

ⓒ. Interzonal microtubules

ⓓ. Kinetochore microtubules

Correct Answer: Kinetochore microtubules

Explanation: Kinetochore microtubules are the spindle fibers that make direct connections to kinetochores on chromosomes. This attachment allows the spindle to exert controlled forces needed to align chromosomes at the equatorial plane and later separate sister chromatids. The connection is highly regulated to ensure proper bi-orientation, meaning each sister chromatid attaches to opposite poles. Correct kinetochore attachment is central to accurate chromosome segregation and prevents unequal distribution of genetic material. Because movement depends on microtubule dynamics at these attachment sites, their identity is a common exam focus. Therefore, the microtubules that attach to kinetochores are kinetochore microtubules.

425. Microtubules radiating toward the cell cortex from spindle poles are called:

ⓐ. Polar spindle fibers

ⓑ. Chromosomal spindle fibers

ⓒ. Interpolar spindle fibers

ⓓ. Astral microtubules

Correct Answer: Astral microtubules

Explanation: Astral microtubules extend outward from the spindle poles toward the cell cortex, forming star-like arrays around centrosomes. Their main role is to help position and orient the mitotic spindle within the cell, supporting correct plane of division. By interacting with cortical factors, astral microtubules contribute to pulling forces that keep spindle poles properly located. This is especially important in animal cells where spindle positioning influences how cytoplasm is partitioned. Astral arrays therefore support spatial control of division rather than direct chromosome attachment. Hence, the microtubules radiating to the cortex are astral microtubules.

426. Centriole duplication in animal cells typically occurs during:

ⓐ. Interphase before mitosis

ⓑ. Late anaphase stage only

ⓒ. Telophase after cytokinesis

ⓓ. Prophase after spindle begins

Correct Answer: Interphase before mitosis

Explanation: Centrioles duplicate during interphase so that the cell enters mitosis with two centrosomes capable of forming two spindle poles. This duplication ensures a bipolar spindle can assemble, allowing chromosomes to be pulled toward opposite poles during division. The timing is important because centrosome separation and spindle formation require two organizing centers before metaphase alignment. If duplication fails or is abnormal, spindle poles may be defective, increasing the risk of segregation errors. Thus, centriole duplication is an interphase event that prepares the cell for mitosis. Therefore, it typically occurs during interphase before mitosis.

427. In many higher plant cells, spindle formation is notable because:

ⓐ. Spindle is absent in plant division

ⓑ. Centrioles are typically absent

ⓒ. Chromosomes do not condense

ⓓ. Microtubules cannot polymerize

Correct Answer: Centrioles are typically absent

Explanation: Many higher plant cells form mitotic spindles without typical centrioles, showing that centrioles are not universally required for spindle assembly. Instead, plant cells use other microtubule-organizing regions to nucleate and organize spindle microtubules. This difference is a key conceptual comparison between animal and plant cell division mechanisms. Even without centrioles, plants still assemble bipolar spindles to segregate chromosomes accurately. The principle is that microtubule organization can be achieved by alternative organizing centers. Hence, spindle formation in many higher plants is notable because centrioles are typically absent.

428. A centriole can act as a basal body mainly to:

ⓐ. Store pigments in plastids

ⓑ. Build nuclear pores in envelope

ⓒ. Initiate cilia or flagella growth

ⓓ. Generate ATP in inner membrane

Correct Answer: Initiate cilia or flagella growth

Explanation: A centriole can function as a basal body when it serves as the organizing base for cilia or flagella formation. In this role, it anchors at the cell surface and organizes microtubule growth that extends outward to form the axoneme. This connection explains why basal bodies and centrioles share closely related structural features and why cells with motile projections often maintain such organizing structures. The basal body provides the template and stability needed for orderly microtubule arrangement during projection assembly. This is a standard link between centrosome biology and ciliary structure. Therefore, a centriole acts as a basal body to initiate cilia or flagella growth.

429. A direct consequence of centrosome malfunction during mitosis is most likely:

ⓐ. Abnormal spindle pole formation

ⓑ. Blocked glycolysis in cytosol

ⓒ. Loss of cell wall cellulose layers

ⓓ. Reduced chlorophyll pigment content

Correct Answer: Abnormal spindle pole formation

Explanation: The centrosome organizes microtubules and helps establish spindle poles, so malfunction can produce poorly focused poles or abnormal numbers of poles. Such defects disrupt bipolar spindle geometry and impair correct kinetochore attachment, increasing the risk of chromosome mis-segregation. Spindle pole abnormalities can lead to lagging chromosomes, unequal distribution of genetic material, or failed division outcomes. Because the spindle depends on a robust organizing center, centrosome defects commonly show up as spindle assembly problems rather than metabolic or pigment issues. This is why centrosome integrity is tightly linked with reliable mitosis. Hence, centrosome malfunction most likely causes abnormal spindle pole formation.

430. The microtubules that overlap at the spindle midzone and help push poles apart are:

ⓐ. Astral microtubules

ⓑ. Kinetochore microtubules

ⓒ. Interpolar microtubules

ⓓ. Cortical actin microfilaments

Correct Answer: Interpolar microtubules

Explanation: Interpolar microtubules extend from opposite spindle poles and overlap in the spindle midzone, forming a supportive framework for spindle elongation. Their overlap allows coordinated interactions that help maintain spindle shape and contribute to forces that separate poles during later stages of mitosis. This midzone arrangement supports the spindle’s structural integrity while chromosomes are being segregated. Because they do not attach directly to kinetochores, their role is more about spindle architecture and pole dynamics. Their presence is essential for stable bipolar organization and proper pole separation mechanics. Therefore, the overlapping midzone microtubules are interpolar microtubules.

431. The main microtubule-nucleating complex at the centrosome is:

ⓐ. Actin–myosin contractile unit

ⓑ. γ-tubulin ring complex

ⓒ. α/β-tubulin heterodimer unit

ⓓ. Keratin intermediate filament bundle

Correct Answer: γ-tubulin ring complex

Explanation: Microtubules begin (nucleate) efficiently at centrosomes because the pericentriolar material contains γ-tubulin ring complexes that act as a template for microtubule initiation. This complex provides a stable starting scaffold so tubulin subunits can be added rapidly to build a new microtubule. By anchoring the minus end near the centrosome, it helps organize a radial array that can be remodeled into a mitotic spindle during division. This nucleation role explains why centrosomes are major organizers of the microtubule network in animal cells. Hence, γ-tubulin ring complex is the correct answer.

432. During mitosis, the formation of two spindle poles in animal cells most directly requires:

ⓐ. Only actin ring constriction

ⓑ. Only nuclear pore breakdown

ⓒ. Only lysosome fusion events

ⓓ. Centrosome duplication in interphase

Correct Answer: Centrosome duplication in interphase

Explanation: A bipolar spindle needs two organizing centers so microtubules can be arranged from opposite sides of the cell. Centrosomes duplicate during interphase, producing two centrosomes that later move apart to establish two spindle poles. From each pole, microtubules grow and interact to capture chromosomes and set up a balanced, bipolar geometry. If duplication does not occur, a cell commonly cannot form two proper poles, increasing the risk of abnormal spindle organization and segregation errors. Therefore, centrosome duplication in interphase is the key requirement.

433. In a typical animal cell, a centrosome usually contains:

ⓐ. Two centrioles at right angles

ⓑ. Two nuclei inside one envelope

ⓒ. Two lysosomes in a shared sac

ⓓ. Two chloroplasts near the nucleus

Correct Answer: Two centrioles at right angles

Explanation: The centrosome is a microtubule-organizing center that commonly includes a pair of centrioles arranged roughly perpendicular to each other. This paired arrangement, along with surrounding pericentriolar material, supports microtubule nucleation and organization. During cell division, the two centrosomes act as the two spindle poles, helping create the bipolar spindle required for chromosome separation. The centriole pair is therefore a structural identifier of the centrosome in many animal cells. This organization supports both interphase microtubule arrays and mitotic spindle formation. Hence, two centrioles at right angles is correct.

434. The structure that acts as the foundation for cilia formation and is closely related to a centriole is the:

ⓐ. Nucleolus

ⓑ. Golgi cisterna

ⓒ. Basal body

ⓓ. Mitochondrial crista

Correct Answer: Basal body

Explanation: A basal body is essentially a centriole-like structure positioned near the cell surface that organizes the growth of a cilium or flagellum. It serves as the template from which the microtubule-based axoneme extends outward. Because it is an organizing platform, it is strongly linked to microtubule arrangement and stability during cilium assembly. This is why cells that form cilia often use a centriole-derived basal body to initiate the projection. The concept connects centrosome/centriole biology to ciliary structure and function. Therefore, basal body is the correct answer.

435. During centriole duplication, the new centriole typically forms:

ⓐ. Inside the nuclear envelope space

ⓑ. Perpendicular to the existing centriole

ⓒ. Within the Golgi lumen stacks

ⓓ. On the outer plasma membrane coat

Correct Answer: Perpendicular to the existing centriole

Explanation: Centriole duplication is tightly controlled so that each existing centriole can guide the formation of a new one in a defined orientation. Typically, a new centriole begins to assemble near and at right angles to the pre-existing centriole, helping maintain orderly centrosome architecture. This orientation supports later centrosome separation into two poles and contributes to proper spindle geometry. Regulating duplication and orientation prevents abnormal centrosome numbers that could disrupt mitosis. Thus, perpendicular formation is a key micro-point linking centriole duplication to spindle pole organization. Hence, the new centriole forms perpendicular to the existing centriole.

436. A correct statement about spindle microtubule polarity in animal cells is that:

ⓐ. Minus ends are anchored at centrosomes

ⓑ. Plus ends are permanently fixed at centrosomes

ⓒ. Both ends are sealed with peptidoglycan

ⓓ. Polarity does not exist in microtubules

Correct Answer: Minus ends are anchored at centrosomes

Explanation: Centrosomes organize microtubules by nucleating them and anchoring their minus ends in the pericentriolar material. This anchoring creates a directional arrangement in which the plus ends extend outward toward the cytoplasm or toward chromosomes during mitosis. Such polarity is essential for controlled growth and shrinkage at the plus ends and for directional interactions involved in spindle dynamics. The anchored-minus-end setup helps build focused spindle poles that can coordinate chromosome movement. This structural rule is a core micro-point for understanding spindle organization. Therefore, minus ends are anchored at centrosomes.

437. A key feature of spindle formation in many higher plant cells is that:

ⓐ. Chromosomes remain uncondensed throughout

ⓑ. Microtubules cannot assemble in plant cells

ⓒ. Cytokinesis occurs only by cleavage furrow

ⓓ. Centrioles are generally not present

Correct Answer: Centrioles are generally not present

Explanation: Many higher plant cells form functional mitotic spindles without typical centrioles, showing that centrioles are not essential in all eukaryotic spindle systems. Instead, plants use other organizing regions to nucleate and arrange microtubules into a bipolar spindle. This difference is frequently tested to compare animal and plant cell division organization. Even in the absence of centrioles, plants still achieve accurate chromosome separation through well-organized microtubule arrays. The key point is the alternative organization strategy, not the absence of the spindle itself. Hence, centrioles are generally not present in many higher plant cells.

438. If centrosomes fail to separate properly before metaphase, the most likely spindle abnormality is:

ⓐ. A completely absent microtubule network

ⓑ. A fully normal bipolar spindle

ⓒ. A monopolar spindle arrangement

ⓓ. A cell wall forming inside the nucleus

Correct Answer: A monopolar spindle arrangement

Explanation: Two distinct spindle poles are needed to form a bipolar spindle that can attach chromosomes from opposite sides. If centrosomes do not separate, microtubules can be organized from a single pole-like region, producing a monopolar spindle. This prevents proper bi-orientation of chromosomes and disrupts alignment and segregation. Such abnormal geometry increases the chance of failed division or unequal distribution of genetic material. The defect is directly tied to centrosome positioning, not to unrelated organelle functions. Therefore, a monopolar spindle arrangement is the most likely outcome.

439. The pericentriolar material is most directly important during mitosis because it:

ⓐ. Provides sites for microtubule nucleation

ⓑ. Stores starch grains for cell energy

ⓒ. Forms chromatin fibers in the nucleus

ⓓ. Creates thylakoid stacks in plastids

Correct Answer: Provides sites for microtubule nucleation

Explanation: The pericentriolar material surrounds centrioles and acts as the functional zone where microtubules are nucleated and anchored. During mitosis, this nucleation capacity supports rapid assembly of spindle microtubules needed to build a robust bipolar spindle. By organizing many microtubules in a focused region, it helps establish clear spindle poles and stable spindle architecture. This organization is essential for efficient chromosome capture and accurate segregation. Without strong nucleation sites, spindle formation becomes inefficient and error-prone. Hence, providing sites for microtubule nucleation is the key role.

440. In animal cells, the spindle type showing star-like asters around poles is termed:

ⓐ. Anastral spindle

ⓑ. Amphiastral spindle

ⓒ. Multinucleate spindle

ⓓ. Diffuse nucleoid spindle

Correct Answer: Amphiastral spindle

Explanation: In many animal cells, centrosomes organize radiating astral microtubules, producing star-like asters around each spindle pole. When asters are present at both poles, the spindle is described as amphiastral. This reflects the strong role of centrosomes in organizing microtubule arrays and positioning the spindle within the cell. The presence of asters is a classic structural feature distinguishing many animal spindles from typical higher plant spindles, which generally lack prominent asters. Therefore, the aster-bearing animal spindle is termed amphiastral spindle.

441. The “9+2” arrangement in eukaryotic cilia/flagella refers to:

ⓐ. Nine singlet microtubules with two rings

ⓑ. Nine triplet microtubules with two caps

ⓒ. Nine random microtubules plus two centrioles

ⓓ. Nine outer doublets plus two central singlets

Correct Answer: Nine outer doublets plus two central singlets

Explanation: The 9+2 arrangement describes the axoneme, the internal microtubule skeleton of most motile cilia and flagella. It has nine peripheral microtubule doublets arranged in a circle and two single microtubules in the center. This geometry supports coordinated bending because accessory proteins can link and regulate interactions between the outer doublets. The central pair helps organize and modulate the beat pattern via associated structures. This is a classic ultrastructure marker used to identify motile cilia/flagella in cells. Therefore, “9+2” means nine outer doublets plus two central singlets.

442. In a typical axonemal doublet, the “A” tubule is:

ⓐ. A complete microtubule

ⓑ. An incomplete microtubule segment

ⓒ. A protein rod without tubulin subunits

ⓓ. A membrane tube without microtubules

Correct Answer: A complete microtubule

Explanation: Each peripheral doublet in the axoneme has two parts: an A tubule and a B tubule. The A tubule is the complete microtubule with a full set of protofilaments, providing a stable scaffold for attachment of motor proteins. The B tubule is incomplete and shares part of its wall with the A tubule, which helps form the doublet architecture. This A–B arrangement is essential for the sliding mechanism that generates bending in motile cilia and flagella. Recognizing “A complete, B incomplete” is a high-yield micro-point in competitive questions. Hence, the A tubule is a complete microtubule.

443. The immediate force for bending in motile 9+2 axonemes is generated mainly by:

ⓐ. Actin filament treadmilling at the membrane

ⓑ. Dynein-driven microtubule sliding

ⓒ. Collagen fiber recoil in the cytosol

ⓓ. DNA helicase activity along chromosomes

Correct Answer: Dynein-driven microtubule sliding

Explanation: Motility in 9+2 axonemes depends on dynein arms attached to one microtubule doublet that “walk” along the neighboring doublet. This dynein activity uses ATP to produce sliding between adjacent outer doublets. Because the doublets are cross-linked by other proteins, the sliding is converted into bending rather than simple separation. The repeated, regulated sliding along the axoneme length creates the wave-like beat pattern. This is the core mechanical principle behind ciliary and flagellar movement in eukaryotes. Therefore, dynein-driven microtubule sliding generates the bending force.

444. The cross-links that help convert microtubule sliding into bending in a 9+2 axoneme are mainly:

ⓐ. Plasmodesmata channels between cells

ⓑ. Histone bridges on chromatin fibers

ⓒ. Nexin links between outer doublets

ⓓ. Ribosomal proteins on the ER surface

Correct Answer: Nexin links between outer doublets

Explanation: In motile axonemes, dynein activity tends to slide adjacent microtubule doublets relative to each other. Nexin links mechanically connect neighboring outer doublets, limiting excessive sliding and thereby forcing the axoneme to bend. This “sliding-to-bending” conversion is essential for producing a coordinated beat rather than disorganized displacement. Nexin therefore contributes to structural integrity and to the geometry required for rhythmic movement. Understanding nexin’s role helps explain why dynein alone is not sufficient to create bending. Hence, nexin links between outer doublets enable bending.

445. A structure that most closely resembles a centriole and anchors a cilium at the cell surface is the:

ⓐ. Basal body

ⓑ. Nucleolus organizer region

ⓒ. Lysosome-associated vesicle

ⓓ. Golgi trans-face network

Correct Answer: Basal body

Explanation: The basal body is a centriole-like microtubule organizing structure positioned at the base of a cilium or flagellum. It anchors the axoneme to the cell cortex and provides the template for orderly microtubule arrangement during cilium formation. Because it is structurally related to centrioles, it supports the idea that cilia/flagella assembly is linked to centrosome components. The basal body helps organize the transition from the triplet microtubules of the base to the doublets of the axoneme. This anchoring is necessary for stable beating and proper orientation. Therefore, the cilium-anchoring centriole-like structure is the basal body.

446. A key structural difference between a typical motile cilium and a primary (non-motile) cilium is that primary cilia commonly show:

ⓐ. Nine outer triplets plus two central singlets

ⓑ. Nine outer doublets plus two central singlets

ⓒ. Nine outer doublets plus three central singlets

ⓓ. Nine outer doublets with no central pair

Correct Answer: Nine outer doublets with no central pair

Explanation: Primary cilia are commonly described as having a 9+0 arrangement, meaning they retain the nine peripheral doublets but lack the central pair. This structural difference is strongly associated with reduced or absent motility compared to typical 9+2 motile cilia. The absence of the central pair changes the internal regulatory architecture that supports rhythmic beating in motile axonemes. Primary cilia often function more in sensing and signaling rather than in propulsion. This contrast is a common exam trap where students assume all cilia are 9+2. Hence, primary cilia commonly show nine outer doublets with no central pair.

447. The main energy source used by axonemal dynein to drive movement is:

ⓐ. NADPH molecules from chloroplast stroma

ⓑ. ATP hydrolysis at dynein heads

ⓒ. GTP hydrolysis at tubulin caps

ⓓ. Proton gradient across thylakoid sacs

Correct Answer: ATP hydrolysis at dynein heads

Explanation: Dynein is a motor protein that converts chemical energy into mechanical work by hydrolyzing ATP. In the axoneme, dynein arms use ATP-driven conformational changes to generate force along adjacent microtubule doublets. This ATP-dependent walking action produces sliding, which is then converted into bending by structural linkers. Without ATP hydrolysis, the dynein motors cannot generate the repetitive force cycles needed for beating. This explains why ciliary motion is energy-dependent and stops when ATP supply is blocked. Therefore, ATP hydrolysis at dynein heads powers axonemal movement.

448. In the 9+2 axoneme, the “2” central microtubules are best described as:

ⓐ. Two microtubule triplets inside the basal body

ⓑ. Two incomplete B-tubules of outer doublets

ⓒ. Two single microtubules in the center

ⓓ. Two actin bundles aligned to the membrane

Correct Answer: Two single microtubules in the center

Explanation: The central pair of a 9+2 axoneme consists of two single microtubules located at the center of the axoneme. They are not doublets or triplets, and they are distinct from the peripheral doublets that form the “9” ring. The central pair contributes to the internal organization and regulation of the beat pattern through associated structures. Its presence is a hallmark of most motile cilia and flagella and helps distinguish them from 9+0 primary cilia. This structural identification is frequently tested with diagram-based questions. Thus, the “2” refers to two single microtubules in the center.

449. The “9” in the 9+2 arrangement specifically refers to:

ⓐ. Nine peripheral microtubule doublets

ⓑ. Nine peripheral microtubule singlets

ⓒ. Nine peripheral microtubule triplets

ⓓ. Nine peripheral actin filament bundles

Correct Answer: Nine peripheral microtubule doublets

Explanation: The “9” denotes the nine outer microtubule doublets that form a circular ring around the axoneme. Each doublet is composed of an A tubule and a B tubule, creating the repeating peripheral units essential for dynein-based sliding. These outer doublets are the main structural and functional tracks along which dynein motors act to generate movement. Their symmetric placement supports coordinated bending waves and stable axonemal architecture. This is a key ultrastructural fact used to define motile cilia/flagella organization. Therefore, the “9” refers to nine peripheral microtubule doublets.

450. The basic cytoskeletal element forming the axoneme of cilia and flagella is:

ⓐ. Intermediate filaments made of keratin

ⓑ. Collagen fibers in extracellular matrix

ⓒ. Microfilaments made of actin only

ⓓ. Microtubules made of tubulin

Correct Answer: Microtubules made of tubulin

Explanation: The axoneme is fundamentally a microtubule-based structure built from tubulin polymers arranged into doublets and central singlets. This microtubule framework provides the rigid yet flexible scaffold needed for controlled bending. Motor proteins interact specifically with microtubules in the axoneme to generate force, making microtubules central to both structure and function. Because the axoneme is internal and organized, it differs from extracellular fibers like collagen and from actin-based systems that drive other types of cell movement. Identifying microtubules as the axonemal core is a standard exam micro-point. Hence, cilia and flagella axonemes are formed by microtubules made of tubulin.

451. The basal body that anchors a cilium is structurally most similar to a:

ⓐ. Centriole

ⓑ. Ribosome

ⓒ. Lysosome

ⓓ. Nucleolus

Correct Answer: Centriole

Explanation: A basal body is a centriole-like structure positioned at the base of a cilium or flagellum, acting as the anchoring and organizing platform for axoneme formation. Its similarity to a centriole is seen in its microtubule-based cylindrical architecture and role in organizing microtubule assembly. This relationship explains why basal bodies are often described as modified centrioles that move to the cell surface to initiate cilium formation. The basal body provides a stable base so the axoneme can extend outward and beat effectively. Therefore, the basal body is most similar to a centriole.

452. The microtubule arrangement typically associated with a basal body is:

ⓐ. Thirteen singlet protofilaments ring

ⓑ. Nine doublets plus central pair

ⓒ. Nine singlets plus central pair

ⓓ. Nine triplet microtubules

Correct Answer: Nine triplet microtubules

Explanation: Basal bodies share the same characteristic microtubule pattern as centrioles, which is nine sets of microtubule triplets arranged in a ring. This triplet architecture provides rigidity and a reliable template for initiating the microtubule organization of the growing cilium. As the axoneme extends, the arrangement transitions into the doublet-based pattern typical of cilia. The triplet pattern is a high-yield ultrastructure detail used to distinguish basal bodies from axonemes, which commonly show 9+2 or 9+0. Hence, nine triplet microtubules is the correct arrangement for a basal body.

453. The basal body is located:

ⓐ. Free in the cytosolic matrix far from membrane

ⓑ. Embedded within the nuclear envelope

ⓒ. At the base of cilium near plasma membrane

ⓓ. Inside the mitochondrial matrix compartment

Correct Answer: At the base of cilium near plasma membrane

Explanation: A basal body is positioned at the base of a cilium or flagellum, closely associated with the plasma membrane where the projection emerges. This location allows it to anchor the axoneme and organize the growth of microtubules into the ciliary shaft. Being near the cell surface also helps align the cilium’s orientation and provides mechanical stability during beating or sensory activity. The basal body thus serves as the junction between the cytoplasmic microtubule organizing system and the membrane-bound ciliary extension. Therefore, it is located at the base of the cilium near the plasma membrane.

454. The most direct function of the basal body in ciliogenesis is to:

ⓐ. Hydrolyze macromolecules using enzymes

ⓑ. Synthesize secretory proteins for export

ⓒ. Produce ATP for dynein motors directly

ⓓ. Nucleate and organize axonemal microtubules

Correct Answer: Nucleate and organize axonemal microtubules

Explanation: The basal body acts as the organizing template for assembling the axoneme by nucleating and positioning microtubules in the correct geometry. This is essential because the cilium requires a precise microtubule arrangement to form a stable structure capable of movement or sensing. By serving as an organized base, it ensures the correct alignment of microtubule elements as they extend into the ciliary shaft. This organizing role is the key reason basal bodies are centriole-like and microtubule-based. Without proper basal body function, cilia formation and orientation can be defective. Hence, its direct function is to nucleate and organize axonemal microtubules.

455. A correct structural distinction is that a basal body contains:

ⓐ. Peripheral doublets and a central pair

ⓑ. Peripheral triplets without a central pair

ⓒ. Only central pair microtubules

ⓓ. Actin bundles arranged in a ring

Correct Answer: Peripheral triplets without a central pair

Explanation: Basal bodies characteristically show nine peripheral microtubule triplets arranged in a cylinder, and they do not include a central pair as part of their basic triplet core. This distinguishes the basal body from the axoneme of motile cilia, which typically has nine doublets and a central pair. The triplet architecture provides a stable foundation from which the axoneme extends, and the geometry shifts as the cilium forms. This distinction is frequently tested to separate base structures from the shaft structure. Therefore, peripheral triplets without a central pair is the correct description.

456. A basal body is most directly involved in the formation of:

ⓐ. Spindle kinetochores at chromosome

ⓑ. Thylakoid stacks in chloroplast

ⓒ. Cilium/flagellum projection from surface

ⓓ. Nucleolus granules inside nucleus

Correct Answer: Cilium/flagellum projection from surface

Explanation: Basal bodies are dedicated organizing centers that initiate and anchor the growth of cilia and flagella at the cell surface. They provide the structural base from which the axoneme extends, ensuring correct microtubule arrangement and stable attachment near the plasma membrane. This role links basal bodies to cellular motility or sensory functions depending on the cilium type. Because they are centriole-like, basal bodies represent a specialized use of the same microtubule organizing architecture. Their involvement is therefore directly tied to forming the surface projection itself. Hence, basal bodies are involved in forming cilium/flagellum projections.

457. In many cells, a basal body is formed when a centriole:

ⓐ. Loses all microtubules permanently

ⓑ. Converts into a lysosomal vesicle

ⓒ. Enters the nucleolus during interphase

ⓓ. Migrates to the cell surface region

Correct Answer: Migrates to the cell surface region

Explanation: Basal bodies often arise when a centriole moves toward the plasma membrane and becomes positioned to organize a cilium. This migration allows the centriole-like structure to anchor at the surface and serve as the template for axoneme assembly. The relocation is critical because cilia project from the cell surface and require a stable base at the membrane interface. This concept explains why centrioles and basal bodies are closely related and why cells can repurpose a centriole for ciliogenesis. The transformation is thus primarily positional and functional rather than a loss of microtubules. Therefore, a basal body forms when a centriole migrates to the cell surface region.

458. The transition from basal body to axoneme is most correctly described as:

ⓐ. Triplets in base become doublets in shaft

ⓑ. Doublets in base become triplets in shaft

ⓒ. Central pair becomes nine triplets

ⓓ. Actin cortex becomes axoneme core

Correct Answer: Triplets in base become doublets in shaft

Explanation: The basal body has nine microtubule triplets, while the ciliary axoneme typically contains nine microtubule doublets (with or without a central pair depending on cilium type). As the structure extends from the basal body into the ciliary shaft, the microtubule organization shifts so that triplets at the base are not maintained as triplets in the axoneme. This transition supports appropriate axoneme mechanics and motor interactions needed for function. Understanding this base-to-shaft change is a key ultrastructural micro-point in cilia/flagella questions. Hence, triplets in the base become doublets in the shaft.

459. A cell structure most directly responsible for anchoring the cilium to the cell cortex is the:

ⓐ. Basal body

ⓑ. Chromosome kinetochore

ⓒ. Nuclear lamina

ⓓ. Golgi trans network

Correct Answer: Basal body

Explanation: The basal body sits at the base of the cilium and anchors the entire ciliary structure to the cell near the plasma membrane. This anchoring is essential to withstand mechanical forces generated during beating or external fluid movement. By providing a stable attachment and organized microtubule template, the basal body ensures the cilium remains properly oriented and functional. It also connects ciliary microtubules with cytoplasmic organization, helping maintain structural continuity. This anchoring role is the central reason basal bodies are required for stable cilia. Therefore, the basal body is responsible for anchoring the cilium.

460. A correct concept link is that basal bodies and centrioles are both:

ⓐ. Actin-based contractile rings

ⓑ. Microtubule-organizing cylinders

ⓓ. Membrane sacs with hydrolases

Correct Answer: Microtubule-organizing cylinders

Explanation: Both basal bodies and centrioles are cylindrical, microtubule-based structures that organize microtubule assembly in a controlled pattern. Centrioles typically function within centrosomes to help organize spindle poles and cytoplasmic microtubules, while basal bodies anchor at the cell surface to initiate cilium or flagellum formation. Despite different roles, they share a common organizing architecture and are often interconvertible by relocation and functional switching. Their shared identity as microtubule-organizing cylinders explains their similar structural patterns and their role in building organized microtubule arrays. Hence, both are microtubule-organizing cylinders.

461. In the 9+2 axoneme, radial spokes are most directly involved in:

ⓐ. Making ATP for axonemal motors

ⓑ. Regulating dynein activity patterns

ⓓ. Anchoring basal body into nucleus

Correct Answer: Regulating dynein activity patterns

Explanation: Radial spokes extend from the outer doublets toward the central pair and act as a regulatory system that helps coordinate bending. By transmitting positional and mechanical information between the central apparatus and the outer doublets, they help control when and where dynein arms generate sliding. This regulation is essential because ciliary beating requires an ordered, rhythmic pattern rather than random microtubule movement. Proper spoke-mediated control supports synchronized waveform production along the axoneme. As a result, radial spokes contribute to effective propulsion or fluid movement at the cell surface. Hence, their most direct role is regulating dynein activity patterns.

462. The central pair in a typical motile 9+2 axoneme is mainly important because it:

ⓐ. Stores reserve starch for ciliary action

ⓑ. Forms the basal body triplet template

ⓓ. Helps coordinate the beat pattern

Correct Answer: Helps coordinate the beat pattern

Explanation: The central pair forms part of the central apparatus that influences how bending is patterned and coordinated in motile cilia and flagella. Its presence supports regulated interaction with radial spokes and associated proteins, which helps define the plane and rhythm of the beat. This coordination ensures that dynein-driven sliding is translated into a repeatable waveform rather than irregular motion. The central pair therefore contributes to consistent, directional beating required for effective movement of the cell or surrounding fluid. Because 9+0 structures lack this central pair, they commonly show reduced motility and more sensory roles. Hence, the central pair helps coordinate the beat pattern.

463. A correct ultrastructural feature of the basal body at the base of a cilium is:

ⓐ. Nine microtubule triplets present

ⓑ. Nine microtubule doublets present

ⓓ. Two central singlet microtubules present

Correct Answer: Nine microtubule triplets present

Explanation: The basal body is a centriole-like structure that anchors the cilium and provides the template for axoneme organization. Its characteristic design is a ring of nine microtubule triplets, which gives strong structural support at the ciliary base. This triplet arrangement is a key marker that distinguishes the base from the axoneme shaft, where the common repeating unit is a doublet. The basal body’s architecture helps initiate ordered microtubule arrangement as the cilium extends outward. This organized base is essential for stable attachment and correct alignment of the cilium. Therefore, nine microtubule triplets are present in the basal body.

464. A typical eukaryotic flagellum is best described as:

ⓐ. A rigid, peptidoglycan-supported filament

ⓑ. A DNA-containing filament for movement

ⓓ. A protein tube lacking microtubules

Correct Answer: A membrane-bound 9+2 structure

Explanation: Eukaryotic flagella are extensions of the cell surface and are covered by a plasma membrane continuity with the cell. Internally, they contain an axoneme that commonly shows the 9+2 microtubule arrangement responsible for motility. The microtubule framework provides the scaffold on which dynein-based forces act to generate bending waves. This design links structure to function: a membrane-covered, microtubule-based core supports controlled, rhythmic movement. The arrangement also aligns with the basal body foundation that organizes the axoneme at the base. Hence, a typical eukaryotic flagellum is a membrane-bound 9+2 structure.

465. In an axonemal outer doublet, dynein arms are classically associated with the:

ⓐ. A tubule of the doublet

ⓑ. B tubule of the doublet

ⓓ. C tubule of basal body

Correct Answer: A tubule of the doublet

Explanation: The A tubule is the complete microtubule of each outer doublet and serves as the main scaffold for attachment of dynein arms. Dynein motors anchored on the A tubule interact with the neighboring doublet to generate ATP-driven sliding. This sliding is then shaped into bending by structural constraints within the axoneme. Because the A tubule is structurally complete and stable, it provides a reliable platform for motor attachment and repetitive force generation. This A-tubule association is a standard micro-point in axoneme ultrastructure questions. Therefore, dynein arms are classically associated with the A tubule.

466. In the 9+2 axoneme, the number of peripheral microtubule doublets is:

ⓐ. Eight arranged in a partial ring

ⓑ. Ten arranged in a complete ring

ⓓ. Nine arranged in a ring

Correct Answer: Nine arranged in a ring

Explanation: The defining “9” of the 9+2 arrangement refers to nine peripheral microtubule doublets positioned in a circular ring. These outer doublets create the main structural track system for dynein-driven interactions that produce sliding and bending. The ring geometry provides symmetry and stability, enabling coordinated bending waves along the axoneme length. This repeated arrangement is a key identifier of motile cilia and flagella under ultrastructural examination. It also helps differentiate axonemes from basal bodies, which use triplets rather than doublets. Hence, nine peripheral doublets are arranged in a ring.

467. The basal body contributes to ciliary function most directly by:

ⓐ. Replicating chromosomal DNA in nucleus

ⓑ. Producing ribosomal subunits for cytosol

ⓓ. Degrading proteins inside a vesicle

Correct Answer: Anchoring and templating axoneme

Explanation: The basal body sits at the base of the cilium and anchors the entire projection to the cell surface region. It also acts as the organizing template that initiates correct microtubule geometry as the axoneme extends outward. This dual role ensures the cilium is both structurally stable and properly oriented for effective movement or sensing. By providing a structured foundation, the basal body supports consistent assembly and long-term maintenance of the ciliary core. This link explains why basal bodies are centriole-like microtubule organizers. Therefore, the basal body most directly anchors and templates the axoneme.

468. Primary cilia (commonly 9+0) are most often associated with:

ⓐ. High-frequency beating for locomotion

ⓑ. Sensory and signaling roles

ⓓ. Starch storage in non-green tissue

Correct Answer: Sensory and signaling roles

Explanation: Primary cilia are commonly non-motile and function as cellular “antennae” that detect extracellular signals and mechanical cues. Their structure often lacks the central pair typical of motile 9+2 axonemes, which aligns with reduced or absent rhythmic beating. Because of their positioning on the cell surface, they can concentrate receptors and signaling components to coordinate cellular responses. This makes them important in sensing the environment and regulating pathways that influence growth, differentiation, and tissue organization. The key concept is functional specialization rather than propulsion. Hence, primary cilia are most often associated with sensory and signaling roles.

469. The most direct reason a motile cilium produces a wave-like beat is:

ⓐ. Random diffusion of tubulin dimers

ⓑ. Constant fusion of ciliary vesicles

ⓓ. Coordinated dynein action along length

Correct Answer: Coordinated dynein action along length

Explanation: Motile beating requires controlled activation and inactivation of dynein motors along different sides and regions of the axoneme. When dyneins act in a coordinated sequence, they generate localized sliding between specific outer doublets at specific times. Structural constraints convert this sliding into bending, and the sequential pattern of bending along the shaft produces a propagating wave. This coordination yields rhythmic, directional movement rather than unorganized motion. Proper regulation is therefore essential for effective fluid movement or cell propulsion. Hence, the wave-like beat arises from coordinated dynein action along the axoneme length.

470. If nexin links between outer doublets are severely disrupted, the most direct mechanical outcome is:

ⓐ. Sliding fails to become bending

ⓑ. ATP synthesis in axoneme increases

ⓓ. Chromosomes attach to kinetochores

Correct Answer: Sliding fails to become bending

Explanation: Dynein motors generate sliding forces between adjacent outer doublets, but effective beating requires that this sliding be constrained and redirected into bending. Nexin links provide key mechanical connections between neighboring doublets, limiting excessive displacement and helping preserve axonemal integrity. When these links are disrupted, doublets can slide too freely, reducing the controlled curvature needed for a rhythmic beat. The result is loss of efficient bending mechanics even if dynein motors remain capable of producing force. This illustrates how structural linkers are essential for converting motor-driven sliding into coordinated motion. Therefore, sliding fails to become bending when nexin links are severely disrupted.

471. A key enzyme commonly present in peroxisomes helps detoxify hydrogen peroxide by converting it to:

ⓐ. Glucose and oxygen

ⓑ. Starch and water

ⓓ. Amino acids and water

Correct Answer: Water and oxygen

Explanation: Peroxisomes contain oxidative enzymes that can generate hydrogen peroxide during metabolic reactions, so they also carry enzymes to neutralize it. Catalase is the classic peroxisomal enzyme that breaks down hydrogen peroxide into harmless products. This reaction protects cellular components from oxidative damage and maintains redox balance during metabolism. The formation of water and oxygen is the characteristic outcome of catalase action on hydrogen peroxide. Because hydrogen peroxide is reactive, rapid conversion is essential for normal cell function. Hence, “water and oxygen” correctly describes the detoxification products.

472. Glyoxysomes in germinating seeds are mainly associated with:

ⓐ. Conversion of fats to sugars

ⓑ. Synthesis of ribosomal subunits

ⓓ. Replication of nuclear DNA

Correct Answer: Conversion of fats to sugars

Explanation: Glyoxysomes are specialized microbodies common in germinating seeds where stored lipids are used as an energy source. They contain enzymes that support pathways converting fatty acids into intermediates that can be used to form sugars. This conversion is important because the young seedling needs soluble carbohydrates before it becomes fully photosynthetic. The glyoxysome-based reactions help mobilize lipid reserves efficiently during early growth stages. This functional link is a standard exam point connecting organelle specialization with seed physiology. Therefore, glyoxysomes are mainly associated with conversion of fats to sugars.

473. The membrane that encloses the large central vacuole in many plant cells is called:

ⓐ. Plasmalemma

ⓑ. Middle lamella

ⓓ. Tonoplast

Correct Answer: Tonoplast

Explanation: The vacuole is bounded by a distinct membrane that separates vacuolar contents from the cytoplasm. This membrane is called the tonoplast and it regulates movement of ions, water, and solutes into and out of the vacuole. By controlling transport, the tonoplast helps maintain osmotic balance, pH conditions, and storage functions of the vacuole. The vacuole also contributes to turgor pressure, and the tonoplast is central to maintaining that pressure by managing water flux. Recognizing the tonoplast as the vacuolar membrane is a frequent micro-point in organelle questions. Hence, the correct term is tonoplast.

474. The main function of a contractile vacuole in many freshwater protozoans is:

ⓐ. Protein synthesis for secretion

ⓑ. Osmoregulation by expelling water

ⓓ. Formation of spindle microtubules

Correct Answer: Osmoregulation by expelling water

Explanation: Freshwater protozoans live in a hypotonic environment, so water tends to enter their cells continuously by osmosis. A contractile vacuole collects the excess water from the cytoplasm and periodically expels it to the outside. This prevents swelling and bursting, maintaining internal osmotic stability. The process is therefore a direct adaptation for surviving in freshwater conditions where water influx is constant. The contractile vacuole’s repeated filling and contraction cycles are tightly linked to maintaining cell volume. Hence, its main function is osmoregulation by expelling water.

475. The middle lamella between two plant cells is mainly composed of:

ⓐ. Lignin polymers

ⓑ. Cellulose microfibrils

ⓓ. Chitin molecules

Correct Answer: Calcium pectate

Explanation: The middle lamella is the cementing layer that helps hold adjacent plant cells together. It is rich in pectic substances, and calcium pectate is commonly emphasized as a major component providing adhesion. This composition supports cell-to-cell bonding and helps maintain tissue integrity in plants. Because the middle lamella lies between primary walls of neighboring cells, its adhesive nature is central to forming stable plant tissues. This is a classic structural micro-point that distinguishes the middle lamella from cellulose-rich primary walls. Therefore, calcium pectate is the correct composition.

476. Plasmodesmata are best described as:

ⓐ. Cytoplasmic connections between plant cells

ⓑ. Ribosomal clusters on rough ER

ⓓ. Vesicles formed by Golgi stacks

Correct Answer: Cytoplasmic connections between plant cells

Explanation: Plasmodesmata are channels that traverse plant cell walls and connect the cytoplasm of adjacent cells. They allow transport and communication by enabling movement of small molecules and, in some cases, larger signaling components between cells. This connectivity supports coordination across tissues, which is especially important because plant cells are separated by rigid walls. Plasmodesmata therefore function as an intercellular continuity system rather than a membrane-bound transport vesicle system. The concept is frequently tested to distinguish plant intercellular communication from animal junctions. Hence, plasmodesmata are cytoplasmic connections between plant cells.

477. A typical bacterial flagellum produces movement mainly by:

ⓐ. Actin–myosin contraction

ⓑ. Dynein-driven bending waves

ⓓ. Rotation of the filament

Correct Answer: Rotation of the filament

Explanation: Bacterial flagella differ fundamentally from eukaryotic cilia/flagella in how they generate motion. In many bacteria, the flagellum acts like a rotary propeller, where a basal motor rotates the filament to push the cell through liquid. This rotation-based mechanism allows rapid changes in direction and speed by altering rotational patterns. The movement does not rely on a 9+2 axoneme or dynein-driven sliding, which are features of eukaryotic motile structures. The rotation concept is a common competitive-style comparison point between prokaryotic and eukaryotic motility systems. Therefore, bacterial movement is mainly by rotation of the filament.

478. The most direct role of a sex pilus in bacteria is:

ⓐ. Making ATP in the cytoplasm

ⓑ. Transfer of DNA during conjugation

ⓓ. Building peptidoglycan cross-links

Correct Answer: Transfer of DNA during conjugation

Explanation: A sex pilus is a specialized appendage that helps establish contact between a donor and a recipient bacterial cell. This contact enables conjugation, during which genetic material is transferred from one cell to another. The sex pilus therefore supports horizontal gene transfer, which can rapidly spread traits within bacterial populations. The key conceptual point is that it is involved in cell-to-cell DNA transfer rather than general attachment or basic metabolism. This role is frequently tested because it links structure, process, and genetic consequence. Hence, the sex pilus is directly involved in transfer of DNA during conjugation.

479. A plasmid in a bacterial cell is best described as:

ⓐ. Extra-chromosomal circular DNA

ⓑ. The main nucleoid DNA only

ⓓ. A protein-only genetic element

Correct Answer: Extra-chromosomal circular DNA

Explanation: Plasmids are typically small, circular DNA molecules that exist in addition to the main bacterial chromosome. They can replicate independently and often carry genes that provide selective advantages under certain conditions. This makes plasmids important in bacterial adaptation and variation, even though they are not essential for basic survival in all contexts. The key identifying feature is that they are extra-chromosomal DNA rather than part of the main nucleoid chromosome. Their independent nature helps explain why plasmid traits can be gained or lost without changing the core genome. Therefore, a plasmid is extra-chromosomal circular DNA.

480. Gas vacuoles in many aquatic prokaryotes are most directly associated with:

ⓐ. Protein secretion to the outside

ⓑ. Formation of mitotic spindle poles

ⓓ. Hydrolysis of macromolecules

Correct Answer: Buoyancy control in water

Explanation: Gas vacuoles are intracellular, gas-filled structures that help certain aquatic prokaryotes regulate their position in the water column. By adjusting buoyancy, cells can move toward optimal light levels or nutrient zones without using complex motility systems. This is especially relevant for photosynthetic prokaryotes that benefit from staying at suitable depths for light capture. The vacuoles therefore contribute to ecological fitness by enabling vertical positioning in aquatic habitats. The concept is commonly tested as an example of a prokaryotic inclusion aiding survival and resource optimization. Hence, gas vacuoles are linked to buoyancy control in water.

481. In freeze-fracture electron microscopy, the “intramembranous particles” seen on the fracture faces mainly represent:

ⓐ. Peripheral proteins loosely attached to membrane surface

ⓑ. Integral membrane proteins embedded in the lipid bilayer

ⓓ. Ribosomes bound to the cytosolic side of rough ER

Correct Answer: Integral membrane proteins embedded in the lipid bilayer

Explanation: Freeze-fracture splits the membrane through its hydrophobic interior, exposing the internal faces of the lipid bilayer. Integral proteins that span or deeply penetrate the bilayer remain within these fracture faces and appear as raised particles or pits. Because peripheral proteins are mostly on the surface, they do not create the same consistent intramembranous particle pattern. The distribution of these particles supports the idea that proteins are not arranged as a continuous coat but are dispersed within the bilayer. This observation strongly supports the fluid mosaic model of membrane structure. Therefore, the particles mainly represent integral membrane proteins.

482. Which organelle is NOT considered part of the endomembrane system?

ⓐ. Golgi apparatus with flattened cisternae stacks

ⓑ. Rough endoplasmic reticulum with ribosomes attached

ⓓ. Mitochondria with inner membrane cristae folds

Correct Answer: Mitochondria with inner membrane cristae folds

Explanation: The endomembrane system refers to functionally connected membrane-bound components that exchange materials through vesicular transport, such as ER, Golgi, lysosomes, and related vesicles. These compartments coordinate processing, sorting, and trafficking of proteins and lipids via membrane flow. Mitochondria do not participate in this vesicular exchange pathway and maintain a distinct membrane system with their own genetic and bioenergetic machinery. Their membranes are not part of the ER–Golgi–lysosome trafficking route. This separation is an exam-favorite conceptual trap in organelle grouping questions. Hence, mitochondria are not part of the endomembrane system.

483. A newly synthesized protein destined for secretion is targeted to the rough ER primarily by:

ⓐ. N-terminal signal peptide sequence

ⓑ. C-terminal SKL tag for peroxisomes

ⓓ. Nuclear localization signal rich in basic residues

Correct Answer: N-terminal signal peptide sequence

Explanation: Secretory proteins typically carry an N-terminal signal peptide that is recognized early during translation. This signal directs the ribosome–mRNA complex to the rough ER membrane, where translation continues with the growing polypeptide entering the ER lumen. This co-translational targeting ensures the protein enters the secretory pathway at the correct starting compartment. After entry, folding and initial processing steps occur in the ER before the protein is routed onward. The presence of the signal peptide is therefore the key determinant for ER targeting of secretory proteins. Hence, the N-terminal signal peptide sequence is correct.

484. The initial step of N-linked glycosylation for many secretory proteins begins on the:

ⓐ. Cytosolic face of the plasma membrane bilayer

ⓑ. Mitochondrial intermembrane space compartment

ⓓ. Golgi trans cisternae region

Correct Answer: Rough ER lumen side

Explanation: N-linked glycosylation begins in the rough ER where an oligosaccharide is transferred onto specific asparagine residues of a nascent polypeptide as it enters the ER lumen. This early modification helps proper folding and quality control by enabling interactions with ER chaperones. Starting glycosylation in the ER ensures that proteins entering the secretory pathway are processed before they reach later sorting stations. Subsequent trimming and further modification can occur as the protein moves forward, but the initial attachment step is ER-based. This is a standard high-yield point for tracing the secretory pathway. Therefore, the rough ER lumen side is correct.

485. If a cytosolic enzyme is engineered to include an ER signal peptide, it will most likely be synthesized on:

ⓐ. Free cytosolic ribosomes only

ⓑ. Mitochondrial 70S ribosomes in matrix

ⓓ. Rough ER–bound ribosomes

Correct Answer: Rough ER–bound ribosomes

Explanation: Ribosomes are not permanently “free” or “bound”; they become bound to rough ER when the translating polypeptide displays an ER-targeting signal peptide. The signal is recognized during translation, and the ribosome is directed to the ER membrane so synthesis continues with the polypeptide entering the ER pathway. This rerouting changes the site of translation without changing the ribosome type itself in the cytosol. As a result, a protein engineered with an ER signal peptide will be made on ER-attached ribosomes and enter the secretory route. This scenario tests the concept that targeting information in the protein controls ribosome location. Hence, rough ER–bound ribosomes is correct.

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