STRUCTURE AND FUNCTION OF CANCER DNA
The genetic code is stored in human cells in 23 pairs of chromosomes. Each chromosome comprises a very long molecule of DNA containing the genes, which are interspersed with spacer sequences. Each gene is flanked by regions of DNA that control when a particular gene is switched on or off. For example, the gene coding for the protein myosin, a key component of muscle cells, will be switched on where needed – in muscle – but off in other tissues where it is not required, such as nerve cells. The network of on and off switches is clearly critical to regulation of the behaviour of cells, and study of these controls is a major feature of cancer research – if the controls do not work, cells can grow in an unregulated fashion, as occurs in cancers.
To understand how cells carry out all these functions, it is necessary to understand a bit more about the structure of DNA and how the code embedded in the DNA molecule is translated into the end product that is the functioning organism. DNA is an abbreviation for deoxyribose nucleic acid. It had been known for some time before the famous discovery of its double helical structure by Crick and Watson in 1953 that DNA contained the genetic code. The DNA molecule is a long spine of two alternating building blocks – a sugar (called deoxyribose) and a phosphate group linked to four molecules named adenine, guanine, cytosine, and thymine (abbreviated to A, G, C, T) and referred to as bases. These bases are arranged along the spine of the DNA molecule and form two complementary pairs, A with T and C with G, that can bond to each other. The double helical nature of DNA results from one strand (the positive or sense strand) being matched by a complementary antisense strand with an A paired with every T, C with every G, and so on. The A-T and C-G bonds thus provide the ‘glue’ that maintains the double helical structure of the DNA strands. The complementary nature of the bond process means that if the two strands are pulled apart and each used as a template for two new strands, the result is two identical copies of the first DNA molecule.
This inherent property of DNA, whereby it can make identical copies of itself, is one of the fundamental properties of all life on Earth. The structure of DNA is very tightly conserved across the whole spectrum from the simplest to the most complex. The fidelity of the duplication process is also extremely high. The error rate is so low that it takes many generations to accumulate significant differences – the rate of genetic ‘drift’ – and is one of the bases for evolutionary biology. Coming back to the genetic books in the library: each time a cell divides, a complete set of the 23 pairs of volumes with their 21,000 genes (‘pages’ of information) must be ‘typed’ by the cell. From time to time, a comma, letter, or full stop will be mistyped. Mostly, as in a book, this will not alter the meaning, but sometimes, changes will be critical, with consequent alteration to how the daughter cell carrying the change (called a mutation) functions. Parenthetically, the number of small random differences can be tracked across the evolutionary tree to allow estimates of when a given pair of species diverged from each other.