What is epigenetics?

If you have looked up the term in the glossary, you will have read that epigenetics refers to:

“Heritable changes in gene activity that are not caused by changes in the DNA sequence.”

If you are new to the subject, this may sound like an alien language.

Let’s start with the DNA sequence:

All living organisms contain DNA within their cells. DNA is the molecule that carries the information the cell needs for its development, growth and function. So, in short, all the DNA molecules in an organism can be viewed as the instruction manual to build that organism.

Surely you have seen pictures or images depicting the chemical structure of DNA, even if you don’t now much about it. It’s all over the place and many biotechnology companies use it in their logos. DNA is elegantly arranged in a double helix. Much of the molecule’s structure is chemically identical in all DNA molecules, whether it is DNA in humans or in any other living thing. But four small molecules, each one represented by a letter (A, T, C or G), are arranged along the length of the DNA string in a set order. The order of these letters is what defines the DNA sequence and this order differs between organisms.

Now, the DNA sequence is pretty much the same in every individual of the same species. For instance, we humans share an almost identical sequence, called the genome. You may have heard about the Human Genome Project, which scientists completed in 2004. What they achieved was the reading of all the letters in the human DNA sequence – around 3 billion letters!

If we share the DNA sequence, why are we so different? Noticed that I said “almost identical”. This is because there is a very small part of the DNA sequence that is unique to every one of us – in fact, only 0.01% of the DNA sequence is responsible for the incredible variability we see in humans.

A simple explanation for this amazing fact is that the part of the DNA sequence, remember I said is 3 billion letters roughly, with a slightly different order is the collection of our genes. And these genes account only for about 1% of the entire DNA sequence. The rest of the sequence can be a bit different too, but the changes that are obvious to us, the ones we can see in our different looks and those less obvious like the ones driving how our bodies function, are in our genes.

Going back to the definition of epigenetics…

Hopefully you have a better understanding of what the DNA sequence is. Simply put, a sequence of four letters in a specific order describing biological instructions.

As I explained above, differences in the order of the letters within the genes make us different, at different levels (appearance or physiology).

If a change in the DNA sequence happens, this is called a mutation. Once a mutation occurs, for instance one letter changes to another letter, the mutation is set and now the sequence is different. We inherit our DNA from mum and dad, each contributing 50% of their DNA, so any change in the parents’ DNA may be passed on to the offspring. The changes in the DNA are heritable.

But in epigenetics we are talking about heritable changes that are not changes in the DNA sequence. How is this possible?

Not all the genes are active in all the cells. Think about it… All the cells in our body share the same DNA. But a cell in your brain (a neuron) is very different to a cell in your muscles. How is this possible? The neuron has a set of genes that are active (on) and others that are inactive (off), and the muscle cell has a different set of active and inactive genes. The neuron is expressing the genes it needs to look and work as a neuron. And the muscle is expressing the “muscle genes”. Some genes in a specific type of cell need to be always on (expressed) and others off (repressed) so that the cell functions properly. But others can be switched on or off depending on different circumstances (stage of development, time of the day, environmental factors…).

To sum up, the on/off switching changes the activity of genes but does not change their DNA sequence. Go back to the definition of epigenetics and you will see that we are almost there…

There is another small detail to consider. The word heritable. In fact, these changes in the way the genes are expressed can be long term and passed on to the offspring. Not all the changes are heritable but some are indeed inherited throughout several generations.

One important difference of epigenetic changes as opposed to DNA changes is that the former are relatively easy to be reversed or induced.

The expression or repression of genes can be carried out through different mechanisms orchestrated by a variety of molecules. These molecules with their many different ways to affect how genes behave are many times responding to environmental factors. Here is a simple example: if you burn your finger, the cells in the area will activate the genes needed to deal with the damage. The molecules carrying out the job are “regulating gene expression.”

This is a short term response to a short term environmental change. But there are other more stable epigenetic changes, like some linked to lifestyle habits, for example.

In the section “Epigenetic Mechanisms” you will find how epigenetics work in more detail.

To know more about Genetics and Epigenetics, check out this fantastic resource from the University of Utah.