Most people have heard of DNA, the blue print of life, the chemical instructions inside each of your cells that make you you. Identical twins have the exact same DNA so why then don’t they always look identical? They have different fingerprints, different diseases, different likes and dislikes. These differences are attributed to the “nature vs nurture” phenomenon, but what if I told you there was a little more to it than that?
Let’s start on a smaller scale. Take you for example, all of the cells inside your body have a copy of the exact same DNA sequence. But if every cell has the same set of instructions, what makes a skin cell a skin cell or a red blood cell a red blood cell? The answer is epigenetics. If DNA is the instruction manual, epigenetics is the lecturer telling you which bits to read and which bits to ignore. So although the manual has the instructions for every cell type, each cell only reads the parts relevant to itself.
As well as allowing the production of different cell types, epigenetics is important as it gives the cells in your body the ability to express different proteins in response to different environmental factors. Smoking for example alters your epigenetic profile as does diet, stress and alcohol to name but a few. The epigenetic profile of your cells also changes with age but despite all of these changes, the actual DNA sequence remains exactly same. The only thing that changes is how the sequence is read and expressed. This explains why identical twins with the exact same “blue print” can look very different.
So how does it work? Well epigenetics is enforced by 3 main mechanisms. DNA methylation, histone modification and non-coding RNA associated gene silencing. As scary as these titles may sound, I’m going to break down each one to hopefully give you a better picture of how this “alternative reading” really works.
First of all, DNA methylation is simply the addition of a methyl group to your DNA sequence (A methyl group is just a chemical group). The sequence itself doesn’t actually change but the addition of a methyl group tells your cells not to read that particular gene.
Image adapted from “Introduction to DNA Methylation.” http://www.sigmaaldrich.com/technical-documents/articles/biofiles/introduction-to-dna-methylation.html
The next method works by making certain parts of your DNA more or less accessible. If it was laid out in a straight line, the DNA from a single cell would be approximately 2m long. If I laid out all of the DNA from all of my cells it would stretch from my computer desk here in the UK to the sun and back about 70 times. That’s a lot of DNA yet somehow that 2m of genetic code manages to pack itself into a tiny cell that’s smaller than the eye can see. This is thanks to histones.
Imagine your DNA as string wrapped around lots of little spools (the spools being the histones). These are then twisted to compact them further into chromosomes allowing all of your DNA to fit into to such a tiny space.
Image adapted from “DNA: The Genetic Material.” https://www.studyblue.com/notes/note/n/dna-the-genetic-material/deck/4784988
When your body needs to read a certain gene that section is unwound to make it accessible. Think of it like reading a book. You can only read the pages that are open at that particular time, the rest is hidden away. This is the same for DNA. Genes that are read all the time are packaged less tightly and those that aren’t are packaged really densely. This packaging however can be changed my modifying the histones that the DNA is wrapped around to turn different genes on or off (histone modification).
The final, most “sciencey” sounding method is known as non-coding RNA associated gene silencing. This is where small RNA sequences (RNA is like DNA’s cousin only shorter lived and less robust) interfere with the reading or expression of target genes thereby switching them off (a.k.a silencing them).
These methods are being used by your cells all of the time to control which of your genes are expressed for normal, everyday function. They are also employed in response to environmental stimuli (e.g. smoking and diet) or in response to life events where there are big changes in your body such as puberty or pregnancy. These epigenetic marks are constantly changing and can have a big impact on our health and well-being. When this epigenetic control goes wrong for example it can lead to diseases such as certain types of cancer. This is why one twin may have arthritis for example but the other doesn’t; due to differences in their epigenome. These marks can also be passed on to our children and our children’s children. This is where the popular headline “You are what your grandmother ate” came from. (If you haven’t heard this title before, please follow the link and have a read!)
The field of epigenetics is still in its infancy and scientists are working hard to unravel all of the elements that govern our epigenetic profile. That being said, epigenetics is already being explored as a target for new drugs and treatments against a variety of diseases including cancer and autoimmune diseases. So although the study of epigenetics is still in its early days, the future of this field is certainly a bright one.
And that is why, in my opinion, there is no such thing as identical!
If you want to know more, check out this cool YouTube video by SciShow: