MPSC 2024 Prelims Paper-I · Question 59 of 100

Correct — option (3), only statements (a) and (c) are relevant. Statement (a) is the structural description Watson and Crick published in 1953: two long chains coiled as a helix about a common central axis. Each chain is a polymer of nucleotides, and the word matters, because a nucleotide is a nitrogenous base joined to a deoxyribose sugar joined to a phosphate group, and it is the phosphate that links one unit to the next through the phosphodiester bonds that form the sugar-phosphate backbone. Statement (c) supplies the chemical rule that holds the two chains together: they are complementary — the paper prints 'complimentary' — so that adenine always faces thymine and cytosine always faces guanine across the helix. That pairing is not arbitrary. A purine is always matched with a pyrimidine, which keeps the width of the helix constant along its length, and the pairs are held by hydrogen bonds, two between adenine and thymine and three between guanine and cytosine, which is why sequences rich in guanine and cytosine take more energy to separate. The rule was foreshadowed by Chargaff's finding that in any DNA sample the amount of adenine equals that of thymine and the amount of guanine equals that of cytosine, and it is what makes the molecule self-templating: separate the two strands and each specifies the other exactly, which is the basis of replication and of transcription. Statements (a) and (c) together therefore give the shape and the rule, which is what the question is asking for. The other two statements each corrupt one of them by a single substituted word.

• (1)Only (a) and (b) is relevant. — This pairs statement (a) with statement (b), and statement (b) fails on one word: it describes the chains as made of nucleoside rather than nucleotide. A nucleoside is only a base joined to a sugar; add a phosphate group and it becomes a nucleotide. Since it is precisely the phosphate that forms the phosphodiester link between one sugar and the next, a chain of nucleosides has nothing to join its units together and no sugar-phosphate backbone at all. The two statements also duplicate each other's content, so this pairing gives the shape of the molecule twice over — once correctly and once with a chemical error — and never states the base-pairing rule.
• (2)Only (b) and (d) is relevant. — This pairs the two defective statements, (b) and (d), and so gets the composition and the pairing rule both wrong. Statement (b) substitutes nucleoside for nucleotide and leaves the chain without the phosphate that links its units. Statement (d) makes two separate errors: it denies that the chains are complementary, which contradicts the entire basis of replication, and it pairs adenine with uracil. Uracil does occur in nucleic acids, but in RNA, where it takes the place of thymine; DNA contains thymine and not uracil, which is one of the two standard differences between the molecules along with the sugar.
• (4)Only (c) and (d) is relevant. — This pairs statement (c), which is correct, with statement (d), which contradicts it directly — one says the chains are complementary and the other says they are not — so the two cannot both be relevant descriptions of the same molecule. Whenever a statement list contains a flat contradiction of this kind it is a signal rather than a difficulty: the pair can be rejected on logic alone, without needing to decide which member is true. Statement (d)'s substitution of uracil for thymine is the further clue, since uracil belongs to RNA.

DNA is a double helix of two polynucleotide chains wound about a common axis. The backbone of each chain is an alternating sequence of deoxyribose sugar and phosphate, joined by phosphodiester bonds between the third and fifth carbons of successive sugars, which gives each chain a direction; the two chains run in opposite directions, or antiparallel. The nitrogenous bases project inward from the backbones and pair across the axis, adenine with thymine through two hydrogen bonds and guanine with cytosine through three. Because a two-ringed purine always pairs with a single-ringed pyrimidine, the distance between the backbones stays uniform, which is what allows a helix of constant diameter to accommodate any sequence. In the common B form the helix is right-handed, with about ten base pairs in each complete turn and a rise of about a third of a nanometre per pair. The structure was proposed by James Watson and Francis Crick in 1953, using the X-ray diffraction evidence produced by Rosalind Franklin and Maurice Wilkins together with Erwin Chargaff's base-composition rules; Watson, Crick and Wilkins received the Nobel Prize in 1962. Its explanatory power lies in the complementarity: each strand carries the full information of the other, so replication is a matter of separating the strands and building the partner of each, and transcription copies one strand into RNA, in which uracil replaces thymine and ribose replaces deoxyribose.

MPSC's biology questions on molecular structure are answered by holding a small number of precise terms apart, and this item is built entirely on two of them: nucleoside against nucleotide, and thymine against uracil. Both are single-word substitutions, and each is planted in exactly one statement, so the whole question reduces to noticing a changed word rather than to recalling anything elaborate about the helix. That is the shape of most statement-list science questions in this paper — statements that are near-duplicates of one another with one term altered — and the reading technique that answers them is to compare the statements against each other before comparing any of them against memory, since the difference between two near-identical statements is exactly where the examiner has put the test. Note also that the paper prints 'complimentary' where 'complementary' is meant, in both of the statements that use the word.

• A nucleotide is a nitrogenous base joined to a sugar and a phosphate group, while a nucleoside is only the base joined to the sugar; the phosphate is what forms the phosphodiester links of the backbone, so DNA chains are chains of nucleotides.
• The two chains of DNA are complementary and antiparallel: adenine pairs with thymine through two hydrogen bonds and guanine with cytosine through three, so each strand specifies the sequence of the other.
• A purine always pairs with a pyrimidine, which keeps the diameter of the helix constant regardless of the base sequence, and Chargaff's rules record the resulting equality of adenine with thymine and guanine with cytosine.
• DNA contains thymine and deoxyribose while RNA contains uracil in place of thymine and ribose in place of deoxyribose, which are the two standard chemical differences between the molecules.
• The double helix was proposed by Watson and Crick in 1953, drawing on the X-ray diffraction work of Rosalind Franklin and Maurice Wilkins and on Chargaff's base-composition rules; Watson, Crick and Wilkins shared the Nobel Prize in 1962.

• Reading past a single substituted word such as nucleoside for nucleotide, which is the entire content of one wrong statement here
• Placing uracil in DNA, when it belongs to RNA and thymine is its DNA counterpart
• Selecting a pair of statements that contradict each other, which can be rejected on logic alone without deciding which is true
• Choosing two statements that say the same thing, which leaves half of what the question asks for unstated

Molecular biology in MPSC papers is tested through statement lists whose members differ by one technical term, and the recurring pairs are nucleoside and nucleotide, purine and pyrimidine, thymine and uracil, ribose and deoxyribose, transcription and translation. The Commission also sets straightforward recall on the discoverers of the double helix and on Chargaff's rules. The efficient preparation is to be able to draw the base pairs with the correct number of hydrogen bonds and to state the two chemical differences between DNA and RNA, since almost every question in this area is answered from those two pieces of knowledge.

No directly related past PYQ was found.

• practice — not a real PYQ

  The difference between a nucleoside and a nucleotide is the presence in the latter of which of the following ?

  • (a)A phosphate group
  • (b)A nitrogenous base
  • (c)A pentose sugar
  • (d)A hydrogen bond

  Answer(a) A phosphate group — a nucleoside is a nitrogenous base joined to a pentose sugar, and adding a phosphate makes it a nucleotide. That phosphate is essential to the structure of a nucleic acid, since the phosphodiester bonds it forms between successive sugars are what join the units into a chain and create the sugar-phosphate backbone.

• practice — not a real PYQ

  In the DNA double helix, guanine pairs with cytosine through how many hydrogen bonds ?

  • (a)One
  • (b)Two
  • (c)Three
  • (d)Four

  Answer(c) Three — guanine and cytosine are held together by three hydrogen bonds while adenine and thymine are held by two, which is why regions rich in guanine and cytosine require more energy to separate and have a higher melting temperature. In both cases a two-ringed purine pairs with a single-ringed pyrimidine, keeping the helix a constant width.