The same, but different…
by Jim Caryl
Epigenetics is a term that is being bandied around quite a bit in the biological literature these days. It is not a new term, but in its current definition the term is used to term heritable changes in gene expression that occur without changes to the DNA.
So what does this actually mean? Well, most people will be familiar with the fact that DNA provides the blue print for how an organism is put together, and that over time mutations in the DNA can change certain properties of what that organism looks like; or they may result in a genetic disease, such as cystic fibrosis or sickle cell anaemia.
However, how do we explain the phenomenon wherein sets of identical Human twins, whom share identical DNA, one twin can develop schizophrenia, pancreatic cancer or diabetes, whilst the other remains unaffected? If we were interpreting their development on the basis of their DNA sequence only, then we have a conundrum.
The answer is that DNA is involved in a dynamic, interpretative process. For example, you may buy a new computer, and in this computer there is a graphics chip that is controlled by a piece of software called “firmware”. This software tries to get the most out of the hardware. Every so often, a new piece of firmware is released, and sometimes it can revolutionise the function of that graphics chip. The chip hasn’t changed, but the software has. This is not a perfect analogy, but what I want to convey is that sometimes the hardware doesn’t need to change; sometimes you can just change the way it’s used.
Thus, the sequence of bases of DNA does not necessarily have to be altered for a new effect to be seen in the resulting organism; some changes can occur by epigenetic processes. There are several different types of epigenetic process, and these differ depending on whether we are speaking about high organisms, such as Humans, or single-celled organisms, such as bacteria.
At the simplest level, one such epigenetic change might be a process call methylation; in this, a chemical group is literally tacked onto the DNA at a certain sequence, which can result in a change of gene expression. If a gene is seen as a piece of DNA that results in a functional product, then we can start to see how changing the level at which this product is produced can have an effect.
One of the recent and interesting findings about such epigenetic changes is that they too can be inherited, leading to questions about the nature of “genetic memory”, the idea that the lifestyle lead by your grandparents can have had a direct effect on the way that your DNA expresses its instructions. In the example of the twins, once identical twin embryos have separated, each cell division can result in the accumulation of an increasing number of these epigenetic changes, adding up to quite a difference over a life-time. Thus even things that are the same can be different.