Several mechanisms enable heritable cellular memory, in which patterns of gene regulation in a single cell are passed on to progeny cells. List three such mechanisms, briefly describe how each works, and provide a biological instance in which each is important.

Epigenetics refers to the study of heritable phenotypic modifications that do not involve changes in the DNA sequence. Epigenetic pathways are capable of controlling gene expression patterns at transcriptional level which are thought to be inherited between daughter cells. Basically, there are three types of mechanisms that work together to reinforce gene expression patterns (by repressing or activating genes) through epigenetic pathways: non-coding RNAs, DNA methylation and histone modifications. Polycomb (PcG) and trithorax (TrxG) groups of proteins form epigenetic complexes that establish epigenetic modifications on chromatin (i.e., DNA and associated histones), and it is believed that these epigenetic marks can be inherited from a mother cell to their daughters. For example, both PcG and TrxG complexes catalyze the methylation of histone proteins through specific methyltransferase enzymes that establish epigenetic modifications in daughter cells during cell division. Moreover, non-coding RNAs such as, for example, microRNAs (miRNAs) and small interfering RNAs (siRNAs) are known to guide epigenetic complexes to target specific genes, thereby also reinforcing epigenetically inheritable states.