Describe the various mechanisms by which expression of genes are controlled by the cell's environment.

One of the mechanisms within a cell’s own environment that plays a part in controlling the expression of genes is histone acetylation and de-acetylation. These are the processes by which the lysine residues within the N-terminal tail protruding from the histone core of the nucleosome are acetylated and de-acetylated (i.e. an acetyl functionalgroup is either transferred from one molecule to the other, or removed from a molecule). Normally, the lysine residues bear a positive charge that can bind to the negatively charged DNA to form a condensed structure which inhibits the transcription of that gene. However, acetylation removes the positive charge on the histones, thereby decreasing the interaction of the N termini of histones with the negatively charged phosphate groups of DNA. Consequently, the condensed chromatin is transformed into a more relaxed structure that is associated with greater levels of gene transcription
DNA methylation is another mechanism in a cell’s environment which has been observed to play a role in mediating gene expression, and is itself also affected by environmental factors. Evidence of this has been found in studies that elucidate how methylation near gene promoters varies considerably depending on cell type, with more methylation of promoters correlating with low or no transcription. It is also important to note that many environmental factors, such as pollution, diet, temperature or stress can affect methylation. An example of this can be seen in field mice. Their coat colour is either dark brown or yellow, depending on the methylation of the gene for this trait. When methylated, the coat colour is brown. When not methylated, the gene is active and the coat colour is yellow. Exposing mice during pregnancy to a diet high in folic acid (which contains the methyl group), increased the proportion of dark mice in their litters.The control of post-translational phosphorylation of transcription factors by ATP is also another mechanism used by the cell to either activate or inhibit the transcription factor in turning on gene expression.