DNA: Structure and function

© Science Museum/Science and Society Picture Library

This section examines how DNA or deoxyribonucleic acid is put together and how the molecule can copy itself perfectly, passing on the fundamental stuff of life.

DNA dissolves in water - and just a few drops of the clear, colourless solution can hold DNA from billions of cells. Look at the following scene - it explores the kind of images you might see when looking at a solution of DNA molecules under a very powerful microscope.

The basic building blocks of DNA are molecules called nucleotides, each of which contains a sugar (deoxyribose), a nucleotide base and an acid group. The terms 'nucleic' and 'nucleotide' are used because these molecules are found in the nucleus of every living cell.

To explain how the building blocks form the double helix structure of DNA, it's useful to think of each of the two long linked strands of the double helix as a ladder. The two sides of each ladder are sugar phosphate backbones, and the rungs of the ladder are pairs of nucleotide bases, composed of different combinations of hydrogen, nitrogen, carbon and oxygen.

Each of the four DNA bases - Adenine, Cytosine, Thymine and Guanine, or A, C, T and G - pairs with only one other: A is always paired with T, and G is always paired with C. This happens because A and T can only form hydrogen bonds at two places, whereas G and C are capable of three hydrogen bond connections. The sequence of these base pairs is unique to every individual. Groups of these sequences make genes. A single gene is represented by a few thousand bases.

In fact, there are two different instances of DNA - mitochondrial DNA and nuclear DNA. Mitochondrial and nuclear DNA are located in different places in the cell. During fertilization, the sperm and egg cell nuclei fuse to form an embryo. The egg cell is very large compared to the sperm, so although the cells' nuclei fuse, the rest of the cell mass in the embryo comes from the egg only. Nuclear DNA is therefore co-inherited but the mitochondrial DNA, which is located outside of the nucleus, is always maternally inherited because all mitochondria in a foetus and later adult are derived from the mitochondria in the mother's egg.

Why is the structure of DNA so important? Apart from the intrinsic beauty of the double helical DNA structure first proposed by Watson and Crick, the structure was of great interest to biologists because it immediately suggested a mechanism whereby DNA could replicate faithfully - one of the great unknown questions of the time. Each DNA strand can act as a template for a separate double helix, so when a cell divides, its DNA physically divides and becomes a template for a new complete DNA molecule.