The DNA double helix structure was discovered by

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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.

Why the other options are wrong — (b) 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) 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) 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.

Concept: 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.

State PCS 2023 Q20 — Science & Technology PYQ with Answer

State PCS 2023 Q20 — Science & Technology PYQ with Answer