Phenotype Development, Gene Expression

Regulation of Gene Expression#

Virtually any step of gene expression can be modulated, from transcriptional initiation, to RNA processing, and to the post-translational modification of a protein. Often, one gene regulator controls another, and so on, in a gene regulatory network.

Epigenetics#

Any process affecting the expression of genes that does not involve a change to the sequence of the gene itself. The genes themselves might be identical but the way they are expressed or read out is changed.

Mechanism#

One way a gene can be turned on or off is by chemical changes to the genome itself. The most common such alteration is methylation of a DNA base.
A second common means of regulating transcription places chemical markers not on the double helix itself but on the histone proteins around which it is wound in the chromosomes.
Other modifications weaken the binding of the DNA wound around the histone, as if loosening a coil of thread on a bobbin.

Is There an Evolutionary Epigenetics? #

On the one hand, it does not appear to directly impact inheritance. The gametes—eggs and sperm—have mechanisms that protect their genomes from much epigenetic alteration, and strip away more or less all of it anyway as these cells mature into the state that takes part in reproduction. At the stage of gamete maturation called the primordial germ cell, epigenetic marks in the genome are largely erased.

On the other hand, some epigenetic modification survive that process, though, and thereby be inherited. There is good evidence that this can happen, especially in plants—but it is probably rare in mammals. There is no good reason to believe that inherited epigenetic marks on the genome can last for more than a generation or two before they are washed away.

Persistent non-genetic changes#

See Persistent non-genetic changes.