Ku70, Ku80, DNA-PKcs, Artemis, XRCC4, Cernunnos-XLF, and DNA ligase IV. Ku70 and Ku80 are the most fundamental components of NHEJ. 

They constitute a heterodimer that binds to the DNA ends and recruits DNA-PKcs, which is a protein kinase. DNA-PKcs, in turn, forms a complex with Artemis. Artemis is both a 5’-to-3’ exonuclease and a latent endonuclease that is activated by phosphorylation by DNA-PKcs. 

These nucleolytic activities process the broken ends and prepare them for ligation. Ligase IV performs ligation in a complex with XRCC4 and Cernunnos-XLF.

TLS Online TPP Program

#Id: 4017

#Unit 2. Cellular Organization

Chromosomes occupy chromosome territories in the nucleus and are not entangled with each other.

Heterochromatic regions, silenced genes, and gene-sparse regions of chromosomes are typically localized to the nuclear periphery.

TLS Online TPP Program

#Id: 4020

#Unit 2. Cellular Organization

Local genome alteration, termed kataegis involves activation-induced deaminase (AID)

Chromothripsis entire chromosomes or large parts thereof are shattered (thripsis in Greek means “shattering”) and stitched together again in what appears to be a random manner, leading to dozens or sometimes even hundreds of rearrangements.

TLS Online TPP Program

#Id: 4021

#Unit 2. Cellular Organization

Active genes are often found at the borders of chromosome territories, and active genes from several chromosomes may cluster in inter-chromosomal territories that are enriched in transcription machinery.

TLS Online TPP Program

#Id: 4023

#Unit 2. Cellular Organization

In most regions, the fibers are much less densely packed than in the mitotic chromosome. This material is called euchromatin. It has a relatively dispersed appearance in the nucleus and occupies most of the nuclear region.

TLS Online TPP Program

#Id: 4024

#Unit 2. Cellular Organization

Some regions of chromatin are very densely packed with fibers, displaying a condition comparable to that of the chromosome at mitosis. This material is called heterochromatin.

TLS Online TPP Program

#Id: 4033

#Unit 2. Cellular Organization

In some cases, the various heterochromatic regions, especially those associated with centromeres, aggregate into a densely staining chromocenter.

The common form of heterochromatin that always remains heterochromatic is called constitutive heterochromatin. 

It is permanently condensed. 

It replicates late in S phase and has a reduced frequency of genetic recombination relative to euchromatic gene rich areas of the genome.