Question

2. Describe condensation, including where, when and how it occurs. (Your answer should include gene, chromatin, chromosome, interphase, mitotic phase, gene inactivation, chromatin remodeling, histone covalent modifications, inheritance, methylation, acetylation, phosphorylation).

Answer 1

Packaging of DNA into chromosomes --

In G0 and interphase stage of a cell , DNA exists as a nucleoprotein complex called chromatin , during M- phase chromatin is converted into more condensed form termed as chromosomes.Each chromosomes in anaphase stage of cell cycle consists of a single , enormously long linear DNA molecule associated with proteins that fold and pack the fine DNA thread into more compact structure.

Histone modification --

ALL of the histone protein are chemically modified , chemical modification of histones are associated with structural changes that occur at the time of replication and transcription. There are at least 8 distinct types of modifications found on histones [ such as acetylation, methylation, phosphorylation, sumoylation , ubiquitylation, ADP ribosylation ] . Three most common types of chemical modifications are -

1. Acetylation - each of the histone proteins making up the nucleosome core contains a flexible amino terminus of 11-37 extending from the fixed structure of the nucleosomes.These N- termini are called histone tails . acetyl groups are added to the lysine amino acids in the histone tail of each of the core molecules . enzymes that acetylate histones are called histone acety transferases.

2. Methylation - Many lysine and arginine amino acids at the N - terminus of histone undergo methylation. methylation at lysine or arginines may be one of three different forms; mono , di or trimethyl for lysines and mono - or di for arginine , it is catalysed by methyl transferase.

3. Phosphorylation - Although information about phosphorylation is limited however it is clear that they too can influence chromatin structure and have a significant impact on cellular activity . example- phosphorylation of histone H3 and H1 has been associated with the formation of metaphase chromosomes.