BPSC 69th CCE Prelims General Studies · Question 20 of 144

Correct — A, James Watson and Francis Crick. On 25 April 1953 the journal Nature carried a paper barely a page long by J. D. Watson and F. H. C. Crick — 'Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid', volume 171, pages 737–738 — setting out the model that still stands. DNA, they proposed, is a double helix: two polynucleotide chains wound round a common axis and running in opposite directions, the sugar-phosphate backbones on the outside and the nitrogenous bases turned inwards, paired across the middle. The pairing was the decisive move. Adenine pairs only with thymine and guanine only with cytosine, which explained in a single stroke Erwin Chargaff's puzzling 1950 finding that in any sample of DNA the amount of A equals the amount of T and the amount of G equals the amount of C. It also explained heredity itself, as the paper's closing sentence noted with deliberate understatement: 'It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.' Pull the two strands apart and each carries the full instructions for rebuilding its partner. The Nobel Prize in Physiology or Medicine for 1962 was awarded jointly, in equal thirds, to Francis Crick, James Watson and Maurice Wilkins, 'for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material'. The model itself, and the paper announcing it, belong to Watson and Crick, which is what this stem asks for.

• (b)Rosalind Franklin and Maurice Wilkins — The most defensible wrong answer on the paper, and the one worth understanding rather than merely rejecting. Franklin and Wilkins worked on DNA by X-ray crystallography at King's College London and produced the evidence the model was built on — Photo 51, the 1952 diffraction image of a DNA fibre taken by Franklin's research student Raymond Gosling, whose X-shaped pattern is the signature of a helix and from which the helix dimensions could be read. Wilkins showed that image to Watson without Franklin's permission, and it proved critical. But Franklin and Wilkins did not propose the double-helix model; they supplied and interpreted the data. Wilkins shared the 1962 Nobel Prize for that contribution. Franklin died on 16 April 1958 at the age of 37, and as Nobel Prizes are not awarded posthumously she could not be included.
• (c)Linus Pauling — Pauling was the most likely man in the world to have solved DNA and he got it wrong. In February 1953, weeks before the Watson–Crick paper, he and Robert Corey published 'A Proposed Structure For The Nucleic Acids' in the Proceedings of the National Academy of Sciences — a TRIPLE helix with the phosphate groups turned inwards, a model that was chemically untenable and was quickly abandoned. Pauling's genuine claims to fame are elsewhere: he worked out the alpha helix in proteins, won the Nobel Prize in Chemistry in 1954 for his work on the nature of the chemical bond, and won the Nobel Peace Prize in 1962 for campaigning against nuclear weapons testing.
• (d)Gregor Mendel — The right subject, the wrong century. Mendel's hybridisation experiments on garden pea plants at Brno, published in 1866 and ignored until their rediscovery in 1900, established the laws of segregation and independent assortment and earned him the title of father of genetics. But he worked entirely at the level of inherited traits and the abstract 'factors' behind them. DNA was not even shown to be the genetic material until the Avery–MacLeod–McCarty experiment of 1944, sixty years after Mendel's death, and its structure came nine years after that.

DNA is a polymer of nucleotides, each nucleotide being a deoxyribose sugar, a phosphate group and one of four nitrogenous bases — the purines adenine and guanine, and the pyrimidines thymine and cytosine. In the Watson–Crick model two such chains run antiparallel and twist into a right-handed helix, joined across the middle by hydrogen bonds between the paired bases: two bonds between adenine and thymine, three between guanine and cytosine, which is why GC-rich DNA is the harder to melt apart. The standard B-form has a diameter of about 2 nanometres, a pitch of 3.4 nanometres and roughly ten base pairs to a turn, so successive base pairs sit 0.34 nanometres apart. The structure's importance is that its shape is its function. Because a purine always faces a pyrimidine, the width of the helix stays constant; because the pairing rules are strict, each strand specifies the other exactly; and because the two strands are only hydrogen-bonded, they can be unzipped without breaking any backbone. Replication, transcription and the whole of the central dogma follow from those three facts.

This is a scientist-and-achievement question, and it is answered by knowing what each name actually did rather than by recognising which sounds most familiar. Set the four against the discovery in question. Mendel goes first: he was doing statistics on pea plants in the 1860s, generations before anyone knew what a gene was made of, so he cannot have discovered a molecular structure. Pauling is the sophisticated trap, because he is a genuine structural chemist and was working on DNA at exactly the right moment — his triple-helix model of February 1953 was simply wrong. That leaves the King's College pair against the Cambridge pair, and the distinction the question is testing is between producing evidence and building the model that explains it. Franklin and Wilkins made the X-ray diffraction images; Watson and Crick assembled the double-helix model and published it. Both halves are essential to the history, and it is worth being able to say so, but the stem asks who discovered the structure. Note also that option (b) has a factual attraction that makes it dangerous — Wilkins really did share the Nobel Prize with Watson and Crick, so a candidate who half-remembers three names on a Nobel citation can talk themselves into it. The safeguard is to remember which three: Crick, Watson and Wilkins, not Franklin.

• Watson and Crick's paper 'Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid' appeared in Nature volume 171, pages 737–738, on 25 April 1953, and closed with the line 'It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.'
• The Nobel Prize in Physiology or Medicine 1962 was awarded jointly, one-third each, to Francis Crick, James Watson and Maurice Wilkins 'for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material'.
• Photo 51 was taken in 1952 at King's College London by Raymond Gosling, Rosalind Franklin's doctoral student, in John Randall's laboratory; it was the 51st diffraction photograph Gosling had made, and Wilkins showed it to Watson without Franklin's knowledge. Franklin, born 25 July 1920, died on 16 April 1958 aged 37, and Nobel Prizes are not awarded posthumously.
• Chargaff's rules, published in 1950, established that in any DNA the quantity of adenine equals that of thymine and the quantity of guanine equals that of cytosine — the observation the base-pairing model explained.
• Geometry of the B-form double helix: right-handed, two antiparallel strands, diameter about 2 nm, pitch 3.4 nm, roughly 10 base pairs per turn and 0.34 nm between successive base pairs; adenine–thymine pairs are held by two hydrogen bonds and guanine–cytosine pairs by three.

• Crediting Rosalind Franklin and Maurice Wilkins with the discovery of the structure: their X-ray diffraction work supplied the critical evidence, but the double-helix model was proposed by Watson and Crick
• Assuming Linus Pauling had no DNA model — he published one in February 1953, a triple helix, and it was wrong; his Nobel Prizes were for the chemical bond in 1954 and for peace in 1962
• Mixing up who shared the 1962 Nobel Prize in Physiology or Medicine: it was Crick, Watson and Wilkins, and Franklin was ineligible because she had died in 1958

BPSC asks this as a flat attribution question — a discovery in the stem and four scientists in the options, decided purely on whether the candidate can pair a name with an achievement, and the 69th paper carries several such one-line science-history items. UPSC prefers to come at the same knowledge sideways: naming a laureate and asking which field he worked in, matching a list of scientists against a list of achievements, or asking which structural feature of DNA actually makes faithful copying possible — a question you cannot answer by memorising names alone.

• practice — not a real PYQ

  The Nobel Prize in Physiology or Medicine, 1962, for discoveries concerning the molecular structure of nucleic acids was shared by

  • (a)Watson, Crick and Franklin
  • (b)Watson, Crick and Wilkins
  • (c)Watson, Crick and Chargaff
  • (d)Watson, Crick and Pauling

  Answer(b) Watson, Crick and Wilkins — the prize was awarded jointly in equal thirds to Francis Crick, James Watson and Maurice Wilkins; Rosalind Franklin had died in 1958 and Nobel Prizes are not awarded posthumously.

• practice — not a real PYQ

  According to Chargaff's rules, which of the following holds true for a double-stranded DNA molecule?

  • (a)Adenine equals guanine and thymine equals cytosine
  • (b)Adenine equals thymine and guanine equals cytosine
  • (c)Adenine equals cytosine and guanine equals thymine
  • (d)All four bases are present in equal amounts

  Answer(b) Adenine equals thymine and guanine equals cytosine — each purine pairs with a specific pyrimidine across the helix, A with T by two hydrogen bonds and G with C by three, which is exactly the regularity Chargaff had measured in 1950 and the base-pairing model went on to explain.