DNA

DNA (deoxyribonucleic acid) is the genetic information found in the nuclei of most organisms. It is arranged into structures called chromosomes. The structure of DNA was identified as being a 'double-helix' by Watson and Crick in 1953.

DNA is composed of 4 bases; the purines: Adenine (A) and Thymine (T) and the pyrimidines: Guanine (G) and Cytosine (C). These form complementary bases pairs of AT and GC. DNA also contains a phosphate group connected to a deoxyribose sugar.

Structure of DNA

DNA strands are primarily composed of three repeating units:

• 2-Deoxyribose sugar - A five carbon sugar (pentose) similar to that of Ribose sugar found in RNA. Its chemical formula is C₅H₁₀O₄

• Phosphate group - Contains one phosphorus atom, bonded to 4 oxygens. Forms a phosphodiester bond, which connects 2 deoxyribose sugars together resulting in the formation of a chain
• A base - One from Adenine, Cytosine, Guanine or Thymine. This forms a side chain branching from the 2-deoxyribose sugar (the 2-Deoxyribose sugar/Phosphate group region is regarded as the 'backbone' of DNA strands)

In order to produce a double stranded DNA structure, interactions occur between complementary bases.

The complementary base pairs in DNA interact with one another via hydrogen bonds:

• A-T interactions consist of 2 intermolecular hydrogen bonds
• G-C interactions consist of 3 intermolecular hydrogen bonds

These interactions form bridges between two DNA chains, thus creating a double stranded 'ladder' shaped structure.

Despite many other theories, In 1953 James Watson and Frances Crick discovered the true structure of a double stranded DNA molecule to be a 'Double Helix'. This was solved as a result of 'stick-and-ball' models they created, along with utilising the work of fellow scientists Rosalind Franklin and Maurice Wilkins on X-ray crystallography . The X-ray diffraction photographs obtained from DNA fibres, displayed a unique X-shape, which illustrates a Helical stucture, although they indicated a repeating structure of 3.4Å apart