Chromatin Condensation Process

Chromatin Structure

• Chromatin is the complex of DNA, chromosomal proteins, and other chromosome constituents isolated from nuclei.
• There are two major classes of proteins in chromatin: histones (H1, H2A, H2B, H3, and H4) and nonhistone chromosomal proteins.

Three Levels of DNA Packaging

• The first level of condensation involves packaging DNA as a negative supercoil into nucleosomes, to produce the 11-nm-diameter interphase chromatin fiber.
• This involves an octamer of histone molecules, two each of histones H2a, H2b, H3, and H4.
• The second level of condensation involves an additional folding or supercoiling of the 11-nm nucleosome fiber, to produce the 30-nm chromatin fiber. Histone H1 is involved in this process.
• The third level of condensation involves the separation of segments of the giant DNA molecules present in eukaryotic chromosomes into independently supercoiled domains or loops.

Centromeres and Telomeres

• Centromeres are constricted regions that can be recognized as a specific DNA sequence that is repeated many times, frequently in long tandem arrays.
• Each centromere of human chromosomes, for example, contains 5000 to 15,000 copies of a 171 base-pair-long sequence called the alpha (aliphoid) satellite sequence.
• Telomeres have unique properties and provide at least three important functions: preventing deoxyribonucleases from degrading the ends of the linear DNA molecules, preventing fusion of the ends with other DNA molecules, and facilitating replication of the ends of the linear DNA molecules without loss of material.
• Telomeres usually contain 500 to 3000 TTAGGG repeats and gradually shorten with age in normal (noncancerous) human somatic cells.

Chromosome Structure in Eukaryotes

• Eukaryotic chromosomes contain huge molecules of DNA that are highly condensed during mitosis and meiosis.
• The centromeres and telomeres of eukaryotic chromosomes have unique structures.
• Most eukaryotes are diploid, having two complete sets of genes, one from each parent.

Repetitive DNA

• Eukaryotic genomes contain repetitive DNA sequences, sometimes as many as a million times.
• DNA containing such repeated sequences is a major component (15 to 80 percent) of eukaryotic genomes.