Cell Cycle and Cell Division MCQs with Answers – Part 3 (Class 11 Biology)

Explanation: Diakinesis completes prophase I. By its later part, the chromosomes are fully condensed, chiasmata have undergone terminalisation and the meiotic spindle is being assembled. The nucleolus disappears, and the nuclear envelope loses its visible integrity, allowing the bivalents to interact with the spindle during the approaching metaphase I. Zygotene would show developing synapsis, and pachytene would contain fully paired tetrads with recombination nodules. Diplotene would show partial homologue separation at visible chiasmata but less advanced terminalisation and nuclear reorganisation. The complete feature set places the cell at the transition from prophase-I chromosome preparation to equatorial bivalent alignment. Stage recognition is strongest when homologue relationship, associated structures and nuclear-envelope status are interpreted together.

Explanation: Pachytene contains fully organised tetrads, recombination nodules and crossing over between non-sister chromatids. Diplotene follows after recombination has been completed; the synaptonemal complex dissolves, homologues begin separating and chiasmata become evident. During diakinesis, chromosomes become fully condensed and chiasmata move towards terminal positions. Metaphase I then places the bivalents at the equatorial plate. The mapping follows a continuous progression from genetic exchange to visible crossover connections, final prophase-I preparation and spindle-dependent alignment. Each stage is distinguished by a different chromosome relationship rather than merely by increasing chromosome condensation. The correct correspondence preserves both the sequence of stages and the identity of the structures involved. Column labels are reference numbers only; their visual position does not imply a biological pairing.

Explanation: Terminalised chiasmata and fully condensed bivalents show that the cell has progressed through diplotene into diakinesis. Spindle assembly also indicates preparation for metaphase I. The abnormal observations are the continued visibility of the nucleolus and nuclear envelope, structures that normally disappear by late diakinesis. The treatment affects this specific nuclear reorganisation rather than earlier synapsis or crossing over. Anaphase-I segregation has not yet begun, as the cell remains in the preparatory interval before metaphase alignment. Persistence of the nuclear envelope could interfere with normal interaction between bivalents and the spindle even though chromosome condensation and chiasma terminalisation have occurred. The case demonstrates that several late-diakinesis events can be experimentally separated. The absence of a later product is interpreted through the first failed prerequisite, not as evidence that every earlier event also failed.

Explanation: During metaphase I, paired homologous chromosomes align as bivalents at the equatorial plate. Spindle microtubules from opposite poles interact with the kinetochores of the two homologues, preparing them to move into different daughter regions. The sister chromatids of each chromosome remain joined at their centromere, so statement IV is valid. Statement III describes anaphase II, not metaphase I or anaphase I. In the first meiotic division, homologous chromosomes separate while sister-centromere association is retained. The valid statements define the reductional geometry of metaphase I: the unit aligned at the equator is a homologous pair, and the forthcoming segregation separates homologues rather than sisters. After meiosis I, DNA content per cell is \(2C\), but the summed DNA across both products remains \(4C\) when no DNA is lost.

Explanation: Cell P aligns a bivalent rather than an individual replicated chromosome. Its two homologous members are related to opposite spindle poles, while their centromeres remain unsplit. This is the defining metaphase-I arrangement and prepares homologues for separation in anaphase I. Cell Q represents the general geometry of mitotic metaphase, where sister kinetochores of an individual replicated chromosome face opposite poles. Cell R fits metaphase II in a haploid cell, which also aligns chromosomes individually. Cell S has already entered anaphase after centromere division. The table distinguishes three superficially similar equatorial or spindle arrangements by identifying the aligned unit, the chromosome relationship oriented towards opposite poles and the state of the centromeres. The first biologically meaningful difference among the rows locates the process or stage represented by the data. Row interpretation requires the same counting convention and observation unit to be applied consistently across the table.

Explanation: Anaphase I separates the two homologous chromosomes of each bivalent. One replicated homologue moves towards one pole, while the corresponding homologue moves towards the opposite pole. The centromere of each chromosome remains intact, so its two sister chromatids continue travelling together as one replicated chromosome. Statement IV describes the change occurring in mitotic anaphase and anaphase II, where centromeres split and former sister chromatids become independent daughter chromosomes. The first meiotic division is reductional precisely since homologous chromosome sets, rather than sister chromatids, are separated. Each future daughter nucleus receives one member of every homologous pair while retaining chromosomes composed of two sister chromatids. Bivalent orientation at metaphase I prepares homologue separation; sister-chromatid separation is reserved for the second meiotic division.

Explanation: The separating units are replicated homologous chromosomes. P moves as one chromosome with its two sister chromatids still joined, while Q moves in the opposite direction with its sister pair also intact. This is the defining chromosome behaviour of anaphase I. During mitotic anaphase and anaphase II, the centromeres divide and sister chromatids separate from each other. Metaphase II would show individual replicated chromosomes aligned at an equator rather than moving poleward. The described movement also explains the reductional nature of meiosis I. Each pole receives one homologue from every pair, reducing the chromosome-set number, but the chromosomes arriving there still contain the sister chromatids produced during premeiotic DNA replication.

Explanation: A diploid number of \(18\) represents \(9\) homologous pairs. During anaphase I, one replicated chromosome from each pair moves towards each pole. Each pole receives: \[ \frac{18}{2}=9\ \text{chromosomes} \] The centromeres remain intact, so every chromosome still consists of two sister chromatids: \[ 9\times2=18\ \text{chromatids} \] The original meiocyte contained \(4C\) DNA after premeiotic replication. Equal homologue segregation places half of that amount, or \(2C\), at each pole. The chromosome set at either pole is haploid, but its chromosomes remain replicated. The condition per pole is \(n,2C\), not \(n,1C\), which is reached only after sister-chromatid separation in meiosis II. Meiosis I is reductional since homologous chromosome sets move to opposite poles while sister centromeres remain intact. Each pole receives one member of every homologous pair, so ploidy falls to \(n\) even though each chromosome still contains two sister chromatids. Bivalent orientation at metaphase I prepares homologue separation; sister-chromatid separation is reserved for the second meiotic division.

Explanation: Normal bivalent alignment shows that homologous pairing, prophase-I organisation and metaphase-I positioning have already occurred. The defect appears only after anaphase I begins, when individual homologues should move towards opposite poles. Failure of selected homologues to migrate points to disruption of the spindle-related movement required for their segregation. A replication defect would have affected chromosome structure before prophase I, while failed synapsis would have prevented normal bivalent formation and alignment. Telophase-II nuclear-envelope formation occurs much later and cannot explain the immediate movement failure. The experiment distinguishes successful equatorial preparation from successful poleward segregation and identifies spindle-dependent homologue movement as a separate requirement of the reductional division. The conclusion remains limited to the supplied treatment and observations; broader causal claims would require additional evidence. The treatment is located by finding the earliest event that differs from the control and then tracing its expected downstream consequence.

Explanation: The assertion is true because anaphase I separates homologous chromosomes rather than sister chromatids. Each chromosome moving towards a pole therefore remains replicated and consists of two sister chromatids. The reason is also true and provides the direct explanation: centromeres do not divide during anaphase I, so the sister chromatids remain physically associated and move together as one chromosome. This behaviour distinguishes anaphase I from mitotic anaphase and anaphase II, in which centromeres divide and sister chromatids separate as daughter chromosomes. Separation of homologous chromosomes reduces the chromosome-set number at each pole from diploid to haploid, while the retention of joined sister chromatids preserves \(2C\) DNA in each resulting cell. The absence of centromere division is therefore the immediate structural basis for each chromosome reaching the pole with both sister chromatids still attached.

Explanation: Cell P shows the defining event of anaphase I: homologous chromosomes move towards opposite poles while sister centromeres remain intact. Separation of homologous sets reduces the chromosome-set number in each product, so this is the reductional event. Cell Q shows centromere division followed by separation of sister chromatids into daughter chromosomes. That behaviour is equational with respect to ploidy and occurs in mitotic anaphase or anaphase II, depending on the cell's history. Cell R is still at metaphase I, before homologue segregation, while Cell S is undergoing telophase reorganisation after chromosome movement. Comparing P with Q isolates the decisive distinction between the two separation programmes: meiosis I resolves homologous pairs without splitting centromeres, whereas an equational anaphase resolves sister chromatids after centromere division. The retained or divided centromere is the decisive structural clue.

Explanation: Normal anaphase I separates homologous chromosomes while preserving the centromeric connection between sister chromatids. Premature centromere division would release the sisters during a stage in which they should move together as one replicated chromosome. The usual division pattern would be disrupted: homologue segregation and sister separation could occur at the same time instead of being assigned to meiosis I and meiosis II respectively. This would not restore diploidy at either pole or create new bivalents. It also would not represent orderly completion of meiosis II, since the second division involves its own spindle organisation and chromosome alignment. The immediate defect is loss of the sister association that normally preserves replicated chromosomes after the reductional division.

Explanation: Homologous chromosome segregation is completed as the replicated homologues reach opposite poles during late anaphase I. Telophase I then begins, and the chromosomes may disperse from their highly condensed condition without necessarily becoming as extended as ordinary interphase chromatin. In the stated meiocyte, nuclear membranes and nucleoli reappear around the chromosome groups. Cytokinesis can subsequently divide the cytoplasm and form two haploid cells, collectively described as a dyad. The broad order follows the transition from chromosome segregation to partial nuclear reorganisation and cellular separation. Some events may overlap, and nuclear reappearance is not equally prominent in every organism, but cytokinesis cannot normally produce the dyad before the homologous chromosome sets have been established. Centromere state distinguishes anaphase I from anaphase II: intact sister centromeres accompany homologue movement in the first division.

Explanation: Telophase I follows separation of homologous chromosomes. Nuclear membranes and nucleoli may re-form around the chromosome groups, although the extent of nuclear reconstitution varies. The chromosomes may become less condensed, but they often do not reach the highly extended condition typical of a complete interphase. Cytokinesis can partition the cell into two haploid products, forming a dyad. Statement IV belongs to anaphase II rather than telophase I. Sister chromatids remain joined after the first division, and each telophase-I chromosome is still replicated. The valid statements describe a transitional state in which chromosome-set reduction has occurred, cellular products may form and the chromosomes remain prepared for the second meiotic division. Telophase I can re-establish nuclei around replicated haploid chromosome sets; sister-chromatid separation remains reserved for meiosis II.

Explanation: The chromosome groups have already separated to opposite poles, eliminating metaphase I as the stage. Each chromosome remains composed of two sister chromatids, showing that the centromeres did not divide during the preceding anaphase. Nuclear-envelope formation and beginning cytoplasmic separation identify telophase. Since replicated chromosomes are being enclosed after homologue segregation, the stage is telophase I rather than telophase II. In telophase II, the sister chromatids would already have separated and each chromosome would be represented by a single chromatid. The forming cytoplasmic partition may produce two haploid cells, but each cell will retain chromosomes at the \(n,2C\) condition until meiosis II separates the sisters. Each pole receives one member of every homologous pair, so ploidy falls to \(n\) even though each chromosome still contains two sister chromatids. The metaphase-I arrangement directs homologues towards opposite poles, while the joined sister chromatids are retained for separation in meiosis II.

Explanation: Each cell has one member of every homologous pair, so the reduction from \(2n\) to \(n\) has already occurred. The chromosomes still possess two sister chromatids, indicating that meiosis II has not yet separated them. The presence of nuclear membranes and partly condensed chromosome sets is consistent with telophase I. Cytokinesis after this stage produces two haploid cells known collectively as a dyad. Mitotic daughters would retain the diploid chromosome complement, while telophase-II products would usually form four cells whose chromosomes contain one chromatid each. The combined evidence of product number, ploidy, replication state and nuclear organisation identifies the products of the first meiotic division rather than merely any pair of newly formed cells.

Explanation: Interval Q represents the replicated diploid meiocyte immediately before meiosis I, with the condition \(2n,4C\). During the first meiotic division, homologous chromosomes are allocated to different products while sister chromatids remain joined. Each product enters the between-division interval with the condition \(n,2C\), represented by R. The change Q → R records both the reduction in chromosome-set number and the partitioning of DNA from \(4C\) to \(2C\) per cell. The table also states that no DNA synthesis occurs in R, which identifies the interval as interkinesis rather than a second S phase. P → Q represents premeiotic replication, while R → S represents sister-chromatid separation in meiosis II. The ordered pair Q → R captures the specific reductional outcome of meiosis I and the unchanged replicated state carried into interkinesis. No second replication intervenes between those two conditions. The result records a halving of homologous chromosome sets while each retained chromosome still carries two sister chromatids.

Explanation: Incorporation of a labelled DNA precursor would indicate synthesis of new DNA. The absence of nuclear labelling during the interval between the two meiotic divisions shows that no additional genome replication occurred. The later completion of meiosis II demonstrates that the second division can proceed using the sister chromatids formed during the single premeiotic S phase. Homologues have already separated in meiosis I and do not remain paired during interkinesis. Ploidy stays haploid rather than returning to \(2n\), and sister chromatids remain associated until anaphase II. The experimental evidence directly supports the defining feature of interkinesis: it is an interval between divisions, but unlike a complete interphase, it contains no S phase.

Explanation: Meiosis I partitions the \(4C\) DNA of the replicated diploid meiocyte into two cells, each containing \(2C\). The horizontal segment during interval P shows that DNA content per cell remains unchanged. This is the expected pattern during interkinesis, where no replication occurs. Meiosis II then separates sister chromatids and reduces DNA content per final cell from \(2C\) to \(1C\). A second S phase would produce a rising segment rather than a plateau. Pachytene occurs before meiosis I while the cell still contains \(4C\) DNA, and \(G_2\) is also a predivision diploid interval. The graph demonstrates that the two meiotic divisions are separated by a pause without an intervening genome-duplication event. Axes, interval labels and curve shape must be interpreted together before assigning a stage. A change in slope marks a change in rate or process, whereas a horizontal segment represents persistence of the current state.

Explanation: The assertion is false. Interkinesis is the interval between meiosis I and meiosis II, but it does not contain another S phase. The reason is true and directly states the defining replication rule. Each chromosome entering meiosis II already consists of two sister chromatids produced during the single premeiotic S phase. Meiosis I separates homologous chromosomes while retaining those sisters, leaving each interkinesis cell in the \(n,2C\) condition. A second replication would increase DNA content and disturb the normal pattern of one genome duplication followed by two divisions. Interkinesis may involve limited preparation for the next division, but that preparation must not be confused with renewed DNA synthesis. Interkinesis preserves the replicated chromosomes produced before meiosis I and carries them into the second division without another genome-duplication step.