Letters begin
Genetic information flows from DNA through RNA to protein. The 'code of life' lies in the sequence of 'letters' – the four bases, A, C, T and G – that make up DNA. This code is copied and passed on each time a cell divides.
But it has recently become clear that extra information is coded into DNA, on top of the sequence of bases. This 'epigenetic' information is turning out to be essential to life and to affect inheritance.
One of the surprises from the Human Genome Project was the small number of protein-coding genes present. Not the 100 000 many people expected but around 22 000, not much more than in a fly. The key to complexity seems to lie in how genes are used – when and where they are active. The control of gene activity is turning out to be of crucial importance to biology.
One way gene activity is controlled is through modification of DNA or the histone proteins it wraps around. A common example is addition of a methyl chemical group to a C nucleotide. It's a little like adding an accent to a letter, which changes its properties – the way it is pronounced.
Often, epigenetic modification acts to turn a gene off. Crucially, this change can sometimes be inherited, so the gene remains turned off in the next generation. So two people can inherit a gene of identical sequence, but in one it may be active and in the other inactive.
Environmental input
It has recently been found that environmental factors can 'reprogramme' methylation patterns, causing a heritable change in DNA. The male offspring of fathers who smoke, for instance, are more likely to be obese, and this is due to changes in the methylation of chromosomes inherited from their fathers. Similar results have been obtained in places where men chew betel nut.
Perhaps most striking are the results from an isolated Norwegian community, which for many years kept detailed records of its harvests. There is a strong correlation between obesity in children from one generation and poor harvest in their grandparents' time.
When food was scarce, DNA in the grandparents' eggs and sperm was reprogrammed, so offspring were better suited to survive in a harsh environment. This change has been transmitted to the next generation, who have a genotype suited to harsh conditions but ready access to food.
These results, it is suggested, provide a mechanism for organisms to respond to environmental change. Mutation-based evolution would take many generations to shift a phenotype. This epigenetic system enables organisms to respond almost immediately.
Epigenetic changes tend to operate over short time frames. They provide additional mechanisms of gene control and heredity, rather than superseding classical DNA-based genetics.