Question

What role does chromatin structure play in regulating gene expression? Explain fully.

Answer 1

The DNA bound with Histone proteins is called chromatin. The histone proteins are namely H1, H2A, H2B, H3 and H4. Chromatin is present only in eukaryotes.

Chromatin plays an imporant role in gene expression. Chromatin regulates gene expression by:

1. DNA methylation - This alters the chromatin structure and thereby transcriptional activity
2. Histone Acetylation-Deactylation - Histone acetylation results in more open chromatin structure and thus giving access to RNA polymerase for transcription. Deacetylation results in closed chromatin and results in transcriptional inactivity.
3. Histone Methylation-Demethylation - Histone methylation at certain positions is associated with regions of transcriptionally silenced chromatin, whereas methylation at other positions is associated with transcriptionally active regions of DNA
4. Histone ubiquitination - Histones can be modified by addition of a protein called Ubiquitin. When ubiquitylated, H2A is associated with repression of transcription. Whereas when histone H2B is ubiquitylated, it enhances transcription.
5. Histone Phosphorylation - Phosphorylation of Histones catalysed by phosphatases increases transcriptional activity.
6. Histone O-GlcNAcylation - O-GlcNAcylation of histones results in increase of transcriptional activity