Nucleic acids, DNA and RNA

Nucleic acids are large polymers containing three main types of units joined together. These are the nitrogen-containing bases, sugar components and the phosphoric acid part. They occur in all living things and are essential in making proteins and in determining hereditary characteristics. They contain the individual’s genetic code, which dictates the characteristic sequence of amino acids in a protein chain.
Deoxyribose nucleic acid (DNA) and ribonucleic acid (RNA) are compounds found in the nucleus of cells of all living things and have the important roles of telling cells what molecules to synthesize or destroy and when. The base groups are selected from the four nitrogen-containing compounds, ardenine (A), guanine (G), thymine (T) and cytosine (C), and these are joined to the ‘ribose’ (or deoxyribose), a five-carbon carbohydrate or sugar molecule (Figure 5.6).
Phosphoric acid groups are also an essential part of the  tructures of DNA and RNA. The units are joined up to form very long twisted spiralling chains forming inter-twining helixlike structures. DNA is a double helix.

A detailed study of DNA and RNA is outside the scope of this book, but the discovery of the exact structural arrangement was first worked out by Crick,Watson, Wilkins and Rosalind Franklin. The discovery of the structure of DNA makes fascinating reading. The special issue of New Scientist of 15 March 2003 to mark the 50 years since the discovery of the structure of DNA contains many interesting details.1 Did you know that if you extracted all the DNA from your cells and put them end to end, they would stretch to the sun and back 600 times? This is because we have approximately 10 trillion cells in our body and each cell contains thousands of DNA molecules. These cell molecules are under constant chemical and environmental attack and so there is a similar number of repair events to restore these structures.

There are approximately 1020 harmful attacks on the cells of our bodies each day from chemicals, oxidizing free radicals, uv light, cigarette smoke, etc. Unless repair is done quickly, these cells can form deformed structures and cause many molecularbased diseases, including cancers. This is why a constant supply of food in a balanced diet is essential for healthy living. Snack food and slimming diets sometimes lack essential proteins and minerals.

‘Did you hear there was an attempted rape by a masked man on the campus last night? Fred, along with all the other male students, was asked to supply a sample of hair or skin for DNA analysis. What do you suppose that will tell the police?’
Some parts of the sequence of these long chains are characteristic of the person who made them and cannot occur in anyone else in this exact sequence.
Hence, DNA sampling is used to characterize a person. All that is needed is a small piece of hair, a flake of skin or a minute drop of body fluid for analysis to show up this sequence.

The two twisted strands of DNA are held in place by ‘hydrogen bonding’, shown in as lines between the C–G, T–A, etc. groups protruding from the DNA chain. Refer to Chapter 8 to see the mechanism by which hydrogen bonds operate.
Here are two examples showing the hydrogen bonding between O and H atoms and also N and H atoms. Without these hydrogen bonds the helix would untwist and we would fall apart – so make sure you get your DNA in a twist when reading this!