 Have you ever wondered why your arms grow out of your shoulders and not out of somewhere else? Or why your hair grows out of your head and not out of your fingertips? Well the answer to these questions is epigenetics. So what is epigenetics? It is the study of how behaviors and the environment impact the expression of genes, but not the genetic sequence itself. So how is this done? The influence of epigenetics can occur via the following mechanisms. DNA methylation and histone modification. It can also occur through other mechanisms, but today we'll just explore these two. And these processes are very natural and normal and are probably going on in your body right now. They are there to help control which genes are expressed and which ones aren't in every cell. So let's talk about DNA methylation. This is the process in which methyl groups are added to the nucleotide bases of DNA and below in this table there is an example. If you were to do some physical exercise you expose your muscle cells to a certain level of stress and this in turn leads to DNA methylation which regulates genes involved in production of more muscle fibers. This is an example of how a behavior and an environment can change the way that a gene is expressed. Histone modification however is where acetyl or methyl groups are added to the histones of chromatin and remember that histones are proteins around which chromatin are wrapped. And again in this table there is an example. If you were to drink some green tea you expose some of your cells to EGCG and this is a chemical found in green tea. And this chemical leads to the acetylation of some histones making tumor suppressor genes more accessible for transcription and these genes are involved in preventing the development of tumors. So you may be asking how has gene expression changed? As said before acetylation and methylation do not change DNA sequences themselves rather they either silence or promote gene activity by adding a chemical group to a sequence or a histone and to the right we can see the chemical structure of a methyl group. Now let's talk about the relationship between epigenetics and disease because things can go wrong with epigenetic changes that is genes can be incorrectly turned on or turned off. So environmental factors can make an individual more susceptible to disease. For instance, viral molecules can weaken the immune system by changing histones and this allows it to replicate and live with it inside its host. Another example is that increased DNA methylation of specific genes can result in decreased expression of BRCA1 and this is a gene that prevents breast cancer. So essentially increased DNA methylation of specific genes can contribute to breast cancer. In summary, epigenetics is a study of how behaviors and the environment can change genes. We learned about two mechanisms in which genes can be regulated. There's DNA methylation, the process in which methyl groups are added to nucleotide bases, and histone modification where acetyl or methyl groups are added to histones.