4Regulation of Gene Expression
A bacteria Escherichia coli has approximately 4,300 genes, while human cells have around 26,000. These genes are not always expressed, and their expression is regulated. Those essential for the sustenance and multiplication of cells are collectively referred to as housekeeping genes.Unicellular prokaryotes such as E. coli and multicellular eukaryotes such as humans have similar mechanisms in gene expression control. Multicellular eukaryotes also have genes associated with cell differentiation. A eukaryote-specific structure known as chromatin plays an important role in gene regulation for such function. Heterochromatin is a tightly packed form of chromatin in which gene expression is limited, while euchromatin - a loosely packed form of chromatin - has genes that are under active transcription. Chromatin structure also affects gene expressions, by changing its structure according to variable cell environment.