Epigenetics! You've heard of it, but what do you know about it?

Before I get there though, I need to introduce you to chromatin!

We all know about chromosomes which we instantly see in our minds as the highly compacted 'X' formations they take on during cell division, but chromosomes are actually made up of a supercomplex of DNA and protein that we refer to as chromatin.

Chromatin is made of millions of nucleosomes which themselves are composed of 146 bases of DNA that are coiled 1.67 times around a core protein complex called a histone.

Now, chromatin can exist in a cell in two different formats: euchromatin (open and accessible for expression) and heterochromatin (closed and not accessible for expression).

"But what controls whether a region of the genome is accessible for expression or not?"

That's the epigenome!

In Greek, Epi means 'above' or 'before' and epigenetics is why regular old genetics isn't super straightforward.

While the raw sequence of the bases within our genome is important, when, where and how much of the different parts of a genome are expressed determines cellular function.

And what controls expression is whether the cellular machinery that converts DNA to RNA has access to the genome.

Epigenetics is the study of all of the non-DNA sequence alterations that impact gene expression.

The two most common of these changes that we know about involve DNA methylation and histone modification.

Methylation: Cytosine, one of the DNA bases, can be modified with the addition of a methyl group (CH3) to the C5 position. This acts as a repressor of transcription, meaning, this mark can prevent the conversion of DNA into RNA because methylation makes the area unrecognizable.

Histone Modifications: histones, the protein core of nucleosomes that make up chromatin, can also be modified. Histone acetylation (COCH3) is associated with open chromatin and the methylation of histones closes or compacts it.

But don't worry, this gets even more complicated, because the addition of those regulatory marks to DNA and histones can be controlled by cis and trans acting elements!

Cis acting elements: regions of non-coding DNA that regulate gene expression ie binding sites for proteins.

Trans acting elements: proteins that bind to DNA to regulate gene expression.

In the context of epigenetics, cis acting DNA elements are recognized and recruit trans-acting proteins that can modify DNA and histones to regulate the expression of the genome. Cis and trans acting elements aren't limited to epigenetic controls of expression though - this is just one of their many functions.

Now I remember why I avoid writing about epigenetics.

Because, it gets extra complicated!

How these elements function, what regions of the genome are open and closed, and which genes are expressed IS DIFFERENT in every single cell in our bodies.

Sequencing to the rescue?