Molecular biologists have known for some time that the genetic code residing in an organism’s DNA is not the only determinant of the organism’s characteristics (its “phenotype“). Identical twin animals and humans, and clones of plants, have the same DNA as their counterparts – yet they are not identical to each other. Various factors can alter how the same DNA is expressed in corresponding cells of twins and clones. The study of these factors makes up the science of epigenetics.
One of the most important epigenetic mechanisms is DNA methylation, which is a chemical modification to individual DNA nucleotides. It is usually the addition of a methyl group to a cytosine base that is adjacent to a guanine base. In humans up to 90% of cytosine-guanine pairs may be methylated. Methylation usually has the effect of silencing genes if it occurs in a gene’s promoter region.
Methylation patterns can be inherited in successive generations of a plant or animal. However, not too surprisingly, small changes of the pattern can occur, since the normal DNA copying process doesn’t automatically reproduce the methylation. Nevertheless, once a mutation of the pattern has occurred in an egg or sperm cell, it can be passed on to later generations. An obvious question is about how the rate of such epigenetic mutations compares with that of mutations in the DNA itself.
New research has answered that question for Arabidopsis thaliana, a form of cress that is a popular model organism for plant biologists. As it happens, mutations of methylation sites occur a whole lot more often than DNA mutations – about 5 orders of magnitude more often.
The researchers discovered that as many as a few thousand methylation sites on the plants’ DNA were altered each generation. Although this represents a small proportion of the potentially six million methylation sites estimated to exist on Arabidopsis DNA, it dwarfs the rate of spontaneous change seen at the DNA sequence level by about five orders of magnitude.
This suggests that the epigenetic code of plants – and other organisms, by extension – is far more fluid than their genetic code.
Even more surprising was the extent to which some of these changes turned genes on or off. A number of plant genes that underwent heritable changes in methylation also experienced substantial alterations in their expression – the process by which genes control cellular function through protein production.