Is DNA (or RNA) sequence the only way information can be
encoded and passed on from generation to generation?
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
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.
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'
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
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.