Chromatin

Chromatin is a complex of DNA and proteins. It is responsible for the characteristic basophilia of the nucleus. The densely staining material is highly condensed chromatin called heterochromatin and the lightly staining material is a dispersed form called euchromatin.

The proteins of chromatin include 5 basic proteins called histones and others called nonhistone proteins.

Euchromatin is indicative of active chromatin, i.e. chromatin that is stretched out so that the genetic information in the DNA can be read and transcribed.

The smallest units of chromatin structure are macromolecular complexes of DNA and histones called nucleosomes.

In dividing somatic (mitotic) cells, chromatin is condensed and organized into discrete bodies called chromosomes. Each chromosome is formed by two chromatids that are joined together at a point called the centromere.

With the exception of the mature gametes, the egg and sperm, human cells contain 46 chromosomes organized as 23 homologous pairs. Twenty-two of the pairs have identical chromosomes and are called autosomes. There is one pair of sex chromosomes, designated X and Y. The chromosomal number, 46, is found in most of somatic cells of the body and is called the diploid (2n) number. The mature sex cell, egg and sperm, as a result of meiosis have only 23 chromosomes, the haploid (1n) number as well as the haploid (1n) amount of DNA.

Karyotypes are chromosome pairs sorted according to their morphology. In females, only X chromosome (either of the two) is used by each cell, the inactive X is often visible as a clump of heterochromatin, termed sex chromatin, or the Barr body can be used to identify the sex of a fetus.

Дата добавления: 2015-07-25