What are genetics and epigenetics?
Genetics and epigenetics are two, closely-related branches of biology. The former is much more well known and consists of studying how hereditary characteristics are passed on from generation to generation. DNA is the molecule in which genetic information is stored in a 4-letter code: A, C, G and T. The combination of letters is not a matter of chance. On the contrary, they combine in a precise manner because, in living things, the 4 letters make up the code for generating the instructions (genes) for synthesising all proteins. This DNA is in the nucleus of the cells linked to the histones around which DNA wraps itself, as if it were a spool of thread.
Epigenetics, on the other hand, is not so well-known and the analogy of an orchestra and a music score is commonly used in order to explain it. Genetics is the score in a musical composition. The score includes all the instructions for playing a piece of music. That is, the human genome stores the information (20-25,000 genes) needed for synthesising all the proteins in our organism.
Epigenetics, for its part, is how each orchestra interprets the score. This means that a particular section of the music might be played with particular intensity whilst other parts might not even be played. In biology, the analogy means that not all genes are expressed with the same degree of intensity in all cells and each one is the expression of a set of the different genes we need in order to function. For example, the proteins a skin cell needs in order to work are different to those required by the liver. Epigenetics is responsible for deciding and determining which genes are expressed in each cell.
How does it do it?
It is achieved by joining small molecules together and modifying DNA or the histones. The most commonly studied epigenetic modification is DNA methylation. That is, the addition of a molecule called methyl to the C letters in DNA. This union impedes gene expression.
Epigenetics plays an essential role in the embryo because it determines the genes that will be expressed and at what point during development this will happen. All the cells in an embryo have the same genetic information passed on from the parents. Epigenetic modifications are the signs that determine which genes are expressed in each cell so that the embryo develops correctly.