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DNA: the King's story

DNA structure

Maurice Wilkins and DNA moleculeMaurice Wilkins and DNA moleculeWhat had the King's College London and Cambridge teams discovered?

DNA (deoxyribonucleic acid) is a type of nucleic acid — a large molecule comprising phosphate, sugar and base-pair components rotated around a central axis into long strands.

The basic structure of DNA is relatively simple and repetitive, possessing a common set of characteristics making it an ideal molecule to transmit the genetic code.

DNA consists of two chains, or halves, connected in the middle by nitrogenous bases so as to resemble a molecular ladder coiled around itself: a double helix.

Photography notebookPhotography notebookThe building blocks of the polymeric molecule are called nucleotides and always consist of three parts: a phosphate, a 5-carbon sugar called deoxyribose, and one of four nitrogenous bases, adenine, guanine, cytosine and thymine.

The phosphate and sugar elements are connected together via ester bonds to form the external backbone of the molecule, the 'uprights' of the spiral ladder.

The bases, on the other hand, form the hydrophobic 'rungs' on the inside of the ladder, connecting the two halves. They are joined at one point to the sugar deoxyribose and at another via hydrogen bonds to a base on the parallel half of the DNA molecule.

The bases are of two kinds: purines (adenine and guanine), and pyramidines (cytosine and thymine). Crucially, adenine (A) and thymine (T) can only ever pair up together; guanine (G) and cytosine (C) are similarly uniquely complementary across the axis of the helix.

Wilkins notesWilkins notesThis fact was evidenced by chemical measurement of the ratios of the four bases in sample material undertaken by Erwin Chargaff and others that provided a vital clue to the structure of the molecule and which neatly explained the process of transmission of the genetic code during cell division.

The 'letters' of the genetic code, in the human genome many millions of base pairs long, comprise A, T, G and C. During cell division enzymes split the two strands of DNA revealing the naked code of base letters down the length of each half of the macromolecule.

Loose molecules of A, T, G and C drawn from the surrounding cellular suspension then pair up with their unique 'other halves' to form a replica of the original DNA strand and genetic code.

This basic mechanism also explains the protein synthesis crucial for the construction of cell structures and the functioning of the organism.

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