If all cells are created from the same genetic material, why are there so many different cell types? Listen to Sriharsa Pradhan, Senior Scientist, RNA Biology at NEB, as he describes how DNA is methylated and how this affects the path of reading the DNA code the same way an obstruction would derail a train off its tracks.
Epigenetics is a phenomenon. Especially if you're looking into a multicellular organism like a human or a mouse, you'll see every single cell have the same genetic component, but each cell has a different function. Now if you look into the cellular nucleus or where the genetic material is stored, this genetic materials in different cells have undergone different kind of modification.
Essentially, your genetic material DNA wrapped around the histone molecule and makes the chromatin, which is essentially constituting your genes. Now these genes are transcriptionally on or off based on the epigenetic program that is imposed on them. Now if you look into the epigenetic program, what is on the genes, you will see predominately their methylation of DNA and methylation acetylation of histone molecule and that are also ribosylation and some other histone modification takes place, but for simplicity factor, DNA methylation is one of the primary epigenetic molecule that is sitting on the DNA.
In mammalian DNA it is essentially 5-methylcytosine where the cytosine residue gets converted to 5-methylcytosine by a set of enzymes which are named as DNA cytosine-5 methyltransferase. There are three enzymes and these are essential in development and they put the epigenetic pattern onto the DNA. It's a very complicated process. It starts right from the fertilization til the mammal is adult mammal is taken birth.
When we talk about methylation tag, basically think about a situation where a train is going on a railway track. You put a big stone on the track and when the train hits the big stone, it can get derailed. The train doesn't go from destination A to B. Think about methylation tag very similar to a train track where a big metal molecule is sitting on the track and the proteins that are reading this DNA molecule, suddenly they're reading through and see this big gigantic methylation tag and they get confused. They fall off. As a result, the DNA to RNA, or the messenger that is supposed to be synthesized by reading the DNA, that's never made. If there is no messenger, there is no protein. By putting methylation, essentially you are silencing that particular gene and the protein is not produced.