101. Match each compound with its most appropriate chemical category. Column II entries may be reused.
| Column I | Column II |
|---|
| P. Adenine | \(1\). Nitrogen-containing heterocyclic base |
| Q. Uracil | \(2\). \(\alpha\)-amino acid |
| R. Glycine | \(3\). Trihydroxy alcohol |
| S. Glycerol | |
ⓐ. P-\(1\), Q-\(2\), R-\(1\), S-\(3\)
ⓑ. P-\(2\), Q-\(1\), R-\(3\), S-\(2\)
ⓒ. P-\(1\), Q-\(3\), R-\(2\), S-\(1\)
ⓓ. P-\(1\), Q-\(1\), R-\(2\), S-\(3\)
Correct Answer: P-\(1\), Q-\(1\), R-\(2\), S-\(3\)
Explanation: Adenine and uracil are both nitrogen-containing heterocyclic bases, so the first Column II entry must be reused for P and Q. Glycine has the general amino-acid framework and belongs to the \(\alpha\)-amino-acid category. Glycerol is trihydroxypropane, a three-carbon alcohol bearing three hydroxyl groups. The correct mapping is P-\(1\), Q-\(1\), R-\(2\) and S-\(3\). The shared presence of nitrogen cannot place glycine with adenine and uracil, since chemical classification depends on the complete structural plan. Likewise, glycerol is recognised from its hydroxyl groups rather than from a nitrogen-containing ring or amino-acid backbone. The comparison becomes clear once purines and pyrimidines classify nitrogenous bases by ring structure, whereas DNA–RNA distinctions also involve the pentose sugar and the occurrence of thymine or uracil. The reused mapping is necessary because more than one named compound can belong to the same broad structural family. Each row is resolved by its defining groups rather than by one shared element.
102. Four compounds are characterised below.
| Compound | Structural evidence |
|---|
| P | Nitrogen occurs within a heterocyclic ring; no sugar is attached |
| Q | An amino, carboxyl, hydrogen and \(R\)-group are attached to one \(\alpha\)-carbon |
| R | A three-carbon chain carries three hydroxyl groups |
| S | A carboxyl group is attached to a hydrocarbon chain |
Which compound belongs to the nitrogenous-base family?
ⓐ. P
ⓑ. Q
ⓒ. R
ⓓ. S
Correct Answer: P
Explanation: Compound P contains nitrogen within a heterocyclic ring and has no attached sugar, which identifies it as a free nitrogenous base. Compound Q has an amino group, carboxyl group, hydrogen atom and variable \(R\)-group attached to the same \(\alpha\)-carbon, so it is an \(\alpha\)-amino acid. Compound R is glycerol since it is a three-carbon alcohol with three hydroxyl groups. Compound S follows the general fatty-acid plan, with a carboxyl group attached to a hydrocarbon chain. Classification must therefore use the complete structural evidence rather than the mere presence of carbon or nitrogen. Since P lacks an attached pentose, it is not a nucleoside; without both sugar and phosphate, it is also not a nucleotide. The nitrogen-containing ring system makes P the compound belonging to the nitrogenous-base family. The table therefore separates four neighbouring small-biomolecule families by their defining functional groups and attachments, making P the only heterocyclic nitrogenous compound without a sugar.
103. A molecule from the adenine–guanine–cytosine–uracil–thymine set consists only of its nitrogen-containing heterocyclic structure. It has neither an attached sugar nor phosphate. The molecule should be classified as:
ⓐ. a phosphate-containing nucleotide
ⓑ. a sugar-containing nucleoside
ⓒ. an unbound nitrogenous base
ⓓ. a protein-forming amino acid
Correct Answer: an unbound nitrogenous base
Explanation: The molecule belongs to the stated five-member set of nitrogenous heterocyclic bases and contains no additional sugar or phosphate component. A free member of this set is classified as a nitrogenous base. Attachment of a sugar would convert it into a nucleoside, while the further addition of an esterified phosphate group to the sugar would produce a nucleotide. The evidence excludes those later component levels. An amino acid has a different framework centred on an \(\alpha\)-carbon carrying amino and carboxyl groups. The component description thus places the molecule at the first level of the base–nucleoside–nucleotide progression.
104. Arrange the following stages in the order that converts a free nitrogenous base into a nucleotide.
P. Begin with a nitrogenous base.
Q. Attach a sugar to the base.
R. Form a nucleoside.
S. Add an esterified phosphate group to the sugar.
T. Form a nucleotide.
ⓐ. P → R → Q → T → S
ⓑ. Q → P → R → S → T
ⓒ. P → Q → S → R → T
ⓓ. P → Q → R → S → T
Correct Answer: P → Q → R → S → T
Explanation: A free nitrogenous base is the starting component. When a sugar attaches to that base, the resulting base–sugar combination is a nucleoside. The nucleoside identity must be established before the next component is added. Esterification of a phosphate group to the sugar then produces a nucleotide. The valid sequence is base, sugar attachment, nucleoside, phosphate addition and nucleotide. The order reflects component dependency rather than arbitrary naming: a nucleotide contains all three components, while a nucleoside contains only the base and sugar. Placing phosphate addition before nucleoside formation would skip the defined intermediate represented by the base–sugar unit. For these observations, a nitrogenous base joined to a pentose forms a nucleoside, and addition of phosphate forms a nucleotide.
105. Assertion: A base–sugar unit carrying an esterified phosphate group is classified as a nucleotide rather than a nucleoside.
Reason: Adenine, guanine, cytosine, uracil and thymine are nitrogen-containing heterocyclic bases.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true and Reason is false; therefore Reason cannot explain Assertion
ⓓ. Assertion is false and Reason is true; therefore Reason cannot explain Assertion
Correct Answer: Both Assertion and Reason are true, but Reason does not correctly explain Assertion
Explanation: The Assertion is true since a nucleotide contains a nitrogenous base, a sugar and an esterified phosphate group. A nucleoside contains the base and sugar but lacks the phosphate component. The Reason is also factually true: the five named compounds are nitrogen-containing heterocyclic bases. However, that statement does not explain why phosphate changes the classification of a base–sugar unit. The explanatory distinction depends specifically on component composition, not on the heterocyclic character shared by the bases. Both parts are valid independently, but the Reason does not supply the criterion that separates nucleosides from nucleotides. The missing explanatory link would have been that addition of phosphate to a nucleoside produces a nucleotide. Base ring classification is a separate fact and therefore cannot explain the Assertion.
106. A nucleotide is treated so that its phosphate group is removed without breaking the bond between its nitrogenous base and sugar. The remaining product is classified as:
ⓐ. a nucleoside
ⓑ. a free nitrogenous base
ⓒ. a different nucleotide
ⓓ. a fatty acid
Correct Answer: a nucleoside
Explanation: A nucleotide contains three component types: a nitrogenous base, a sugar and phosphate. The treatment removes only the phosphate group and explicitly preserves the base–sugar connection. The remaining molecule contains the two components required for a nucleoside. It would become a free nitrogenous base only if the sugar were also removed from the base. The change does not produce another nucleotide, since the component that distinguishes a nucleotide from a nucleoside has been lost. This prediction follows from subtracting one defined component while preserving the other two, making the final classification uniquely dependent on the biological context.
107. Study the component records below.
| Compound | Nitrogenous base | Sugar | Phosphate |
|---|
| P | Present | Absent | Absent |
| Q | Present | Present | Absent |
| R | Present | Present | Present |
| S | Absent | Present | Present |
Which ordered set represents a nitrogenous base, a nucleoside and a nucleotide, respectively?
ⓐ. Q, R and S
ⓑ. S, Q and P
ⓒ. R, P and Q
ⓓ. P, Q and R
Correct Answer: P, Q and R
Explanation: Compound P contains only the nitrogenous-base component, so it represents a free base. Compound Q adds a sugar to the base but has no phosphate, fulfilling the definition of a nucleoside. Compound R contains the base, sugar and phosphate and is a nucleotide. Compound S lacks a nitrogenous base and cannot belong to any of the three requested categories despite containing sugar and phosphate. The ordered relation must preserve the increasing component sequence: \[ \text{base}\rightarrow\text{base+sugar}\rightarrow\text{base+sugar+phosphate} \] Reading all three component columns is necessary; the presence of a sugar or phosphate alone cannot establish nucleotide identity. Interpreting the result requires noting that a phosphate group distinguishes a nucleotide from its corresponding nucleoside, while the base alone lacks the pentose. The reasoning rests on the fact that a nitrogenous base joined to a pentose forms a nucleoside, and addition of phosphate forms a nucleotide.
108. Let \(B\) represent a nitrogenous base, \(S\) a sugar, \(P\) a phosphate group, \(N_s\) a nucleoside and \(N_t\) a nucleotide. Consider the relation:
\[
B\xrightarrow{+S}N_s\xrightarrow{+P}N_t
\]
Which interpretation is valid?
ⓐ. \(N_s\) is a nucleotide, while \(N_t\) is a free base.
ⓑ. \(N_s\) is a free base, while \(N_t\) is a nucleoside.
ⓒ. \(N_s\) is a nucleoside, while \(N_t\) is a nucleotide.
ⓓ. \(N_s\) is a nucleotide, while \(N_t\) is a nucleoside.
Correct Answer: \(N_s\) is a nucleoside, while \(N_t\) is a nucleotide.
Explanation: The first transformation attaches a sugar to a nitrogenous base. A molecule containing these two components is a nucleoside, so \(N_s\) has that identity. The second transformation adds phosphate to the base–sugar unit. The resulting three-component molecule is a nucleotide, represented by \(N_t\). The arrows describe a component-building relation rather than conversion between unrelated molecular classes. Reversing the identities would ignore the phosphate criterion, while calling either product a free base would discard an attached component. The symbolic sequence compactly expresses the nesting of structural complexity from base to nucleoside to nucleotide. The symbols encode composition rather than a reaction mechanism: \(N_s\) contains B and S, while \(N_t\) retains both and gains P. The first addition changes a free base into a nucleoside, and the second changes that nucleoside into a nucleotide without altering base identity. Composition is conserved across the second step except for the added phosphate group.
109. Compound X contains a nitrogenous base attached to a sugar. Compound Y contains the same base–sugar unit together with an esterified phosphate group and can serve as a building unit of a nucleic acid. How should X and Y be classified?
ⓐ. X is a nucleoside, while Y is a nucleotide.
ⓑ. X is a base, while Y is a nucleoside.
ⓒ. X is a nucleotide, while Y is a nucleoside.
ⓓ. X is a nucleoside, while Y is a free base.
Correct Answer: X is a nucleoside, while Y is a nucleotide.
Explanation: Compound X has the two components that define a nucleoside: a nitrogenous base and an attached sugar. Compound Y retains those components and adds an esterified phosphate group, giving the complete nucleotide composition. Its ability to serve as a nucleic-acid building unit is consistent with that classification. The functional evidence supports the component evidence rather than replacing it. X cannot be a free base since a sugar is already attached, and Y cannot remain a nucleoside after phosphate addition. The pair illustrates how one added component changes both the chemical name and the molecule's relation to nucleic-acid construction.
110. Match each nucleoside with its corresponding nucleotide name. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Adenosine | \(1\). Adenylic acid |
| Q. Guanosine | \(2\). Guanylic acid |
| R. Uridine | \(3\). Uridylic acid |
| S. Cytidine | \(4\). Cytidylic acid |
ⓐ. P-\(1\), Q-\(2\), R-\(3\), S-\(4\)
ⓑ. P-\(2\), Q-\(1\), R-\(4\), S-\(3\)
ⓒ. P-\(1\), Q-\(3\), R-\(2\), S-\(4\)
ⓓ. P-\(4\), Q-\(2\), R-\(3\), S-\(1\)
Correct Answer: P-\(1\), Q-\(2\), R-\(3\), S-\(4\)
Explanation: Adenosine is the nucleoside corresponding to adenylic acid, while guanosine corresponds to guanylic acid. Uridine and cytidine similarly correspond to uridylic acid and cytidylic acid. Each nucleotide name represents the phosphate-containing form related to its named nucleoside. The correct mapping follows the retained base identity as phosphate is added to the sugar component. This relation is not a random vocabulary pairing: the nucleoside provides the base–sugar unit, and its corresponding nucleotide contains that unit plus phosphate. Maintaining the name root allows each pair to be recognised without confusing one nitrogenous-base family with another. The molecular observations indicate that a phosphate group distinguishes a nucleotide from its corresponding nucleoside, while the base alone lacks the pentose. The evidence supports only the conclusion that a nucleotide is more than a base name: its identity includes the attached pentose and phosphate.
111. Consider the following statements about named nucleosides and nucleotides.
I. Adenosine is a nucleoside.
II. Guanylic acid is a nucleoside.
III. Thymidine is a nucleoside.
IV. Cytidylic acid is a nucleotide.
ⓐ. I and II only
ⓑ. II, III and IV only
ⓒ. I, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, III and IV only
Explanation: Adenosine and thymidine are named nucleosides, each consisting of a nitrogenous base attached to a sugar. Cytidylic acid is the corresponding phosphate-containing nucleotide form related to cytidine. Guanylic acid is also a nucleotide, not a nucleoside, so Statement II is the incorrect member of the set. The distinction can be checked through both naming and composition: the listed “-osine” or “-idine” forms here represent nucleosides, while the corresponding “-ylic acid” forms represent nucleotides. Statements I, III and IV preserve this relation across different bases without treating every similarly rooted name as the same molecular category. The names are correctly interpreted only after composition is understood: adenosine and thymidine are nucleosides, whereas the corresponding acid forms contain phosphate.
112. A sample contains adenosine and adenylic acid. Chemical analysis shows that only adenylic acid contains an esterified phosphate group. What is their correct relationship?
ⓐ. Both are nitrogenous bases with no attached sugar.
ⓑ. Adenosine is a nucleoside, while adenylic acid is its corresponding nucleotide.
ⓒ. Adenosine is a nucleotide, while adenylic acid is its corresponding nucleoside.
ⓓ. Both are nucleotides that differ only in their nitrogenous bases.
Correct Answer: Adenosine is a nucleoside, while adenylic acid is its corresponding nucleotide.
Explanation: Adenosine contains the adenine-related base–sugar combination and is classified as a nucleoside. Adenylic acid contains the same basic base–sugar unit together with an esterified phosphate group, making it the corresponding nucleotide. The supplied phosphate evidence provides the decisive classification boundary. Both compounds retain the same base identity, so their difference is not a change from one nitrogenous base to another. Nor can adenosine be called a free base, since it already includes sugar. The relationship is one of component addition: phosphate converts the nucleoside form into the nucleotide form while preserving the associated base family.
113. Examine the records below.
| Compound | Recorded components | Recorded category |
|---|
| P. Adenosine | Base and sugar | Nucleoside |
| Q. Guanylic acid | Base, sugar and phosphate | Nucleotide |
| R. Thymidine | Base, sugar and phosphate | Nucleoside |
| S. Cytidylic acid | Base, sugar and phosphate | Nucleotide |
Which record requires correction?
ⓐ. P should be recorded as a nucleotide containing phosphate.
ⓑ. Q should be recorded as a nucleoside lacking phosphate.
ⓒ. S should be recorded as a free nitrogenous base.
ⓓ. R should contain base and sugar without phosphate.
Correct Answer: R should contain base and sugar without phosphate.
Explanation: Thymidine is a nucleoside, so its recorded category is appropriate, but the component entry is not. A nucleoside contains a nitrogenous base and sugar without the phosphate group that defines a nucleotide. The R row should list only base and sugar. The other rows are internally consistent: adenosine is a base–sugar nucleoside, while guanylic acid and cytidylic acid are phosphate-containing nucleotides. The task requires checking whether each name, component set and category agree within the same row. R alone contains a contradiction between nucleoside classification and a three-component nucleotide composition. This relation is supported by the observation that a phosphate group distinguishes a nucleotide from its corresponding nucleoside, while the base alone lacks the pentose. Record R is the inconsistent entry: thymidine lacks phosphate, whereas adding phosphate would produce a nucleotide rather than the stated nucleoside.
114. Uridine undergoes esterification with a phosphate group while its base–sugar connection remains unchanged. The product formed is:
ⓐ. uridylic acid
ⓑ. uracil
ⓒ. cytidine
ⓓ. thymidylic acid
Correct Answer: uridylic acid
Explanation: Uridine is the nucleoside formed from the uracil-related base and a sugar. Adding an esterified phosphate group converts this two-component nucleoside into its corresponding nucleotide. The nucleotide name associated with uridine is uridylic acid. Uracil would represent the free nitrogenous base after removal of sugar rather than phosphate addition. Cytidine belongs to a different base family, while thymidylic acid contains a thymine-related unit. Since the treatment preserves the original base–sugar connection and adds only phosphate, the name root remains associated with uridine and the molecular category changes from nucleoside to nucleotide.
115. Evaluate the following statements about nucleotides and nucleic acids.
I. DNA and RNA consist of nucleotide building units.
II. Nucleosides lacking phosphate are the complete repeating units of nucleic acids.
III. Nucleic acids are associated with genetic-material roles.
IV. A nucleotide contains a nitrogenous base, sugar and phosphate.
ⓐ. I and II only
ⓑ. I, III and IV only
ⓒ. II, III and IV only
ⓓ. I, II, III and IV
Correct Answer: I, III and IV only
Explanation: DNA and RNA are nucleic acids constructed from nucleotide units, making Statement I valid. A nucleotide contains a nitrogenous base, a pentose sugar and phosphate, so Statement IV states the required component plan. Nucleic acids carry genetic-material roles in this biological context, supporting Statement III. A nucleoside lacks phosphate and is not the complete nucleotide building unit described for these polymers. Statement II removes an essential component and misidentifies the monomeric unit. The accepted combination links molecular composition with polymer organisation and biological role rather than treating base, nucleoside and nucleotide as interchangeable terms. Statements I, III and IV preserve the nucleotide-to-nucleic-acid relation, while Statement II incorrectly substitutes a phosphate-free nucleoside for the repeating nucleotide unit.
116. Assertion: DNA and RNA are polymers formed from nucleosides that lack phosphate.
Reason: Their building units are nucleotides containing a nitrogenous base, a sugar and phosphate.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not correctly explain Assertion
ⓒ. Assertion is true and Reason is false; therefore Reason cannot explain Assertion
ⓓ. Assertion is false and Reason is true; therefore Reason cannot explain Assertion
Correct Answer: Assertion is false and Reason is true; therefore Reason cannot explain Assertion
Explanation: The Assertion is false since DNA and RNA are built from nucleotides, not phosphate-free nucleosides. The Reason correctly states the three-component structure of a nucleotide: nitrogenous base, sugar and phosphate. Phosphate is part of the nucleotide units contributing to nucleic-acid polymers, so its absence changes the molecular category to a nucleoside. The Reason directly identifies the factual error in the Assertion. The distinction connects monomer composition with polymer identity and prevents the base–sugar intermediate from being mistaken for the complete nucleic-acid building unit. For this classification, a nitrogenous base joined to a pentose forms a nucleoside, and addition of phosphate forms a nucleotide. The conclusion rests on the observation that purines and pyrimidines classify nitrogenous bases by ring structure, whereas DNA–RNA distinctions also involve the pentose sugar and the occurrence of thymine or uracil.
117. Match each chemical level with its defining description. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Nitrogenous base | \(1\). Nitrogen-containing heterocyclic compound |
| Q. Nucleoside | \(2\). Base joined to sugar |
| R. Nucleotide | \(3\). Base, sugar and phosphate |
| S. Nucleic acid | \(4\). Polymer of nucleotide units |
ⓐ. P-\(2\), Q-\(1\), R-\(4\), S-\(3\)
ⓑ. P-\(1\), Q-\(3\), R-\(2\), S-\(4\)
ⓒ. P-\(1\), Q-\(2\), R-\(3\), S-\(4\)
ⓓ. P-\(3\), Q-\(2\), R-\(4\), S-\(1\)
Correct Answer: P-\(1\), Q-\(2\), R-\(3\), S-\(4\)
Explanation: A nitrogenous base is the heterocyclic nitrogen-containing component at the first chemical level. Joining it to a sugar produces a nucleoside. Addition of phosphate creates a nucleotide containing all three components. Repetition of nucleotide units then produces a nucleic-acid polymer. The mapping follows an increasing hierarchy of molecular assembly: \[ \text{base}\rightarrow\text{nucleoside}\rightarrow\text{nucleotide}\rightarrow\text{nucleic acid} \] Each level retains components from the previous level while adding a new structural relation. Keeping these levels distinct is essential for identifying both small nitrogenous compounds and the larger genetic-material polymers constructed from them. Component addition fixes the hierarchy: sugar converts a base into a nucleoside, phosphate converts that nucleoside into a nucleotide, and polymerisation produces nucleic acid. The important distinction is that omitting or adding one component changes the chemical category even when the nitrogenous base remains the same.
118. A cell is supplied only with molecules containing a nitrogenous base attached to sugar, but none of the supplied molecules carries phosphate. Which conclusion is best supported?
ⓐ. The supplied molecules are nucleosides rather than complete nucleotide units for DNA or RNA.
ⓑ. The supplied molecules are nucleotides ready for nucleic-acid assembly without modification.
ⓒ. The supplied molecules are free bases since sugar does not affect classification.
ⓓ. The supplied molecules are phospholipids lacking only fatty-acid components.
Correct Answer: The supplied molecules are nucleosides rather than complete nucleotide units for DNA or RNA.
Explanation: A base joined to sugar forms a nucleoside. The stated molecules lack phosphate, so they do not contain the full base–sugar–phosphate composition required for nucleotide classification. DNA and RNA are built from nucleotides, making the supplied nucleosides incomplete as the described nucleotide units unless phosphate is added. They are not free bases since the sugar remains attached, and their component pattern has no relation to phospholipid classification. The changed condition links molecular composition with polymer construction: absence of phosphate affects both the chemical name of each supplied molecule and its readiness to function as a complete nucleic-acid building unit. Without phosphate, the supplied units stop at the nucleoside stage and cannot themselves serve as complete nucleotide monomers.
119. Consider the following statements about primary metabolites.
I. They include commonly occurring biomolecules such as amino acids and sugars.
II. Their roles in normal physiological processes are generally identifiable.
III. They are defined mainly by specialised occurrence in a few organisms.
IV. They contribute directly to routine cellular activities.
ⓐ. I and III only
ⓑ. II and IV only
ⓒ. I, II and IV only
ⓓ. I, II, III and IV
Correct Answer: I, II and IV only
Explanation: Primary metabolites are common cellular compounds with recognisable roles in normal physiology. Amino acids contribute to protein formation and other routine processes, while sugars participate in cellular energy and material relations. Statements I, II and IV fit this description. Specialised or restricted occurrence is more characteristic of secondary metabolites, making Statement III invalid. The word primary does not refer to molecular size or the order in which a compound was discovered. It indicates a close connection with the ordinary physiological functioning of cells. A suitable classification must consider both how commonly the substance occurs and whether its role in normal cellular activity is identifiable. The functional evidence shows that DNA and RNA are polynucleotides, so complete hydrolysis yields nucleotide-related components rather than amino acids, fatty acids or monosaccharides.
120. A survey detects amino acids and simple sugars in many tissues, where they participate in routine cellular processes. A coloured compound appears only in certain flower tissues and is not required for the same general processes. The most appropriate classification is:
ⓐ. amino acids are secondary metabolites, while sugars and the coloured compound are primary metabolites
ⓑ. amino acids and sugars are primary metabolites, while the coloured compound is a secondary metabolite
ⓒ. all three are primary metabolites since each is a carbon compound
ⓓ. all three are secondary metabolites since each can be isolated from living tissue
Correct Answer: amino acids and sugars are primary metabolites, while the coloured compound is a secondary metabolite
Explanation: The amino acids and sugars are widely detected and have identifiable roles in normal physiological processes, which supports their classification as primary metabolites. The coloured compound has a more specialised distribution in particular flower tissues and fits the secondary-metabolite pattern. Being a carbon compound isolated from living tissue establishes biomolecule status but does not by itself determine whether a substance is primary or secondary. The decisive evidence combines occurrence with physiological role. Primary metabolites are associated with routine cellular functioning, whereas secondary metabolites commonly show specialised production or distribution and may serve ecological or other particular roles.
