Question

What modifies the packing of DNA around histones and determines the accessibility of transcriptional molecules to target genes?

Answer 1

The answer to your query is as follows:

Nucleosomes are the specific units in which the DNA in chromatin is packed around the different types of histone proteins.

A nucleosome is made up of 147 bp of DNA that is packed around an octomeric core of histone proteins. It consists of two H3-H4 histone dimers that are surrounded by two H2A-H2B dimers. Nucleosome spacing plays a critical role in determining the chromatin structure. The chromatin structure is divided into two types namely heterochromatin and euchromatin.

Thus, the chromatin structure and the accessibility of transcriptional molecules to target genes are regulated by the modifications of both the DNA and histone tails.

Research has shown that two important tags or epigenetic modifications like DNA methylation and histone modification changes the DNA accessibility and chromatin structure. Thus, play a very important role in regulating the patterns of gene expression.

The modifications can be the result of any exogenous influences or can be the result of environmental alterations of phenotypes. The histone modifications include two important mechanisms namely, methylation and acetylation.

Post-translational modification of histone tails like lysine acetylation and lysine methylation influences the nucleosome spacing in open structure. It is thus accessible to various nuclear factors that will affect the patterns of gene expression. Maintenance of the inactive chromatin is also affected by specific histone lysine modifications.

Thus, histones can be post-translationally modified and can help in restructuring the chromatin and packaging of DNA in many ways like:

• Phosphorylation
• Ubiquitination
• Acetylation
• Methylation

For example, Histone acetylation that occurs at the ε-amino group of lysine residues in H3 and H4 tails of the histones is consistently associated in promoting the process of transcription.

Hope it helps!