Question

Which statement is true with respect to eukaryotic chromatin?

A> histone post-translational modifications are lost

b>None of the the other selections are true statements

c> Euchromatin represents regions with hypoacetylation of histones that leads to densely packed nucleosomes and promotes transcription

D> Euchromatin represents regions with hyperacetylation of histones that leads to loosely packed nucleosomes and promotes transcription

Answer 1

Answer: Chromatin is a complex where DNA is wrapped around histone proteins that forms chromosomes in the nucleus. The basic unit of chromatin is nucleosome (DNA + protein). Histone has lysine residues at the N-terminal tail that undergo acetylation and deacetylation to regulate gene expression and replication. DNA is negatively charged due to phosphate groups and hence it is wrapped tightly against histone proteins that has positively charged aminoacids such as lysine. Post-translation modifications such as acetylayion of lysine residues on the histone proteins by histone acetylransferases removes the positive charges at the N-termini of histone and reduce the interaction between the negatively chraged of DNA with the N-terminal of histone. This leads to loosely packed nucleosomes and hence the DNA is more accesible for transcription machinery. Relaxed and transcriptionally active DNA is called as euchromatin. Hence, euchromatin represents regions with hyperacetylation of histones that leads to loosely packed nucleosomes and promotes transcription and Option D is the correct answer. As the acetylation of histone is a post-translational modification, option A is an incorrect answer. Regions with hypoacetylation of histones that leads to densely packed nucleosomes is called as heterochromatin and it inhibits transcription. Hence, Option C is incorrect answer.