Epigenetics refers to the study of heritable changes in our genome that occur without altering the DNA or genetic code.
The basics of gene expression
Our DNA is tightly packaged into the nucleus of every cell, and carries our unique genetic code. Each gene occupies a specific region of DNA so that in effect, our genes are positioned one after another along the DNA.
Basically, in the nucleus the genetic code of our genes is 'read' and the code is transcribed into mRNA. This mRNA then leaves the nucleus and enters the cytoplasm of the cell where each mRNA molecule is translated into a specific protein. Ultimately it is these proteins that carry out the functions in the cell. Proteins make up the cell membrane, form enzymes, they transport glucose, help the cell to grow and also perform tasks like assisting the cell to replicate and divide.
In order for a cell to survive and ultimately for the proper functioning of an organism, individual genes need to be turned on or "expressed" at specific times and with a specific magnitude. Not every cell has a requirement for every gene all the time. For example, genes that control the formation of keratin for hair are turned off in tissues such as the liver and heart but are turned on in the cells of the hair follicle. Some genes are always expressed, however other genes are expressed only in times of need. Also, some genes are expressed at a very high level while others are expressed only at a low level, depending on the needs of the organism.
Genetics vs Epigenetics
While genetics refers to the gene sequence, or DNA code, epigenetics refers to all the other factors that control how and when each gene is expressed. The epigenetic machinery that controls gene expression is extremely complex and there are aspects that are not yet fully understood.
Epigenetic controls
In order to fit all our genes and DNA into the nucleus, it needs to be tightly packaged. Humans have over 20 000 protein encoding genes and these are packaged into 23 pairs of chromosomes. In order to package the DNA effectively, the DNA molecule is wound tightly around protein structures called histones. Together, the histones and DNA are referred to as the chromatin. Epigenetics can affect the expression of DNA by two main mechanisms: by modifying the DNA or by modifying the chromatin.