Epigenetic cellular memory
How cell fates are established and how identities of different cell types are maintained during development of multi-cellular eukaryotes are questions of extreme biological significance at the heart of development. A single-celled zygote undergoes many rounds of mitotic divisions which ultimately lead to generation of over 200 different specialized cell types in human body during development. Although, each cell type contains same basic genetic information (DNA), yet their identity is different from one another which are maintained throughout development. It is known that differential gene expression programs lead to different cell lineages and each cell type remembers its identity due to maintenance of cell type specific gene expression program referred to as transcriptional cellular memory. Transcriptional memory involves changes in the chromatin state of lineage specific genes; changes that can persist through DNA replication and mitosis, which means they are inherited from mother to daughter cells. Such heritable changes are called epigenetic modifications and can be covalent marks on DNA and/or histones, and therefore would not alter the basic genetic information in a cell. However, epigenetic changes may either activate or silence the expression of lineage specific genes and set the stage for differential gene expression among different cell type. This explains how cells with same DNA can acquire different identity which is maintained through epigenetic inheritance during development. In Drosophila melanogaster (fruit flies), genetic analyses have uncovered two groups of genes, the Polycomb Group (PcG) and the trithorax Group (trxG), responsible for maintaining gene expression patterns stably and heritably. Importantly, PcG and trxG proteins are evolutionary conserved and most of our knowledge about their function was pioneered from studies in Drosophila. Molecular analysis showed that many of the proteins encoded by the PcG and trxG act in large complexes, and modify the local properties of chromatin to maintain transcriptional repression (PcG) or activation (TrxG) of their target genes. My lecture will primarily focus on introducing epigenetics, transcriptional cellular memory and how they affect our development.