In eukaryotic nuclei, DNA is wound around proteins called histones. About 147 base pairs of DNA wrap around a core of eight histone proteins to form a nucleosome. Chains of nucleosomes coil further to form chromatin. Chromatin exists in two forms.Euchromatin
• loosely packed, with nucleosomes spread apart
• stains lightly
• transcription factors and RNA polymerase can reach the DNA, so genes in euchromatin are actively transcribed.Heterochromatin
• tightly packed, with nucleosomes close together
• stains darkly
• transcription factors and RNA polymerase cannot reach the DNA, so genes in heterochromatin are not transcribed.
Heterochromatin is found at centromeres and at the ends of chromosomes, where there are few genes, and in regions containing genes that the cell has switched off.
Chromatin can change from one form to the other, and this is an important way of controlling gene expression. Epigenetic modifications decide which form a region takes:
• methylation of DNA and removal of acetyl groups from histones favour heterochromatin
• adding acetyl groups to histones favours euchromatin.Different cell types have different regions packed as heterochromatin, so each cell type expresses a different set of genes. For example, the inactivated X chromosome in each cell of a female mammal is condensed into heterochromatin and can be seen as a dark spot called a Barr body.
• loosely packed, with nucleosomes spread apart
• stains lightly
• transcription factors and RNA polymerase can reach the DNA, so genes in euchromatin are actively transcribed.Heterochromatin
• tightly packed, with nucleosomes close together
• stains darkly
• transcription factors and RNA polymerase cannot reach the DNA, so genes in heterochromatin are not transcribed.
Heterochromatin is found at centromeres and at the ends of chromosomes, where there are few genes, and in regions containing genes that the cell has switched off.
Chromatin can change from one form to the other, and this is an important way of controlling gene expression. Epigenetic modifications decide which form a region takes:
• methylation of DNA and removal of acetyl groups from histones favour heterochromatin
• adding acetyl groups to histones favours euchromatin.Different cell types have different regions packed as heterochromatin, so each cell type expresses a different set of genes. For example, the inactivated X chromosome in each cell of a female mammal is condensed into heterochromatin and can be seen as a dark spot called a Barr body.