Chromosomal

A chromosome is a DNA molecule with part or all of the genetic material (genome) of an organism.

Prokaryotes usually have one single circular chromosome, whereas most eukaryotes are diploid, like humans. Chromosomes in eukaryotes are composed of chromatin fiber. Chromatin fiber is made of nucleosomes. A nucleosome is a histone octamer with part of a longer DNA strand attached to and wrapped around it. Chromatin fiber, together with associated proteins is known as chromatin. Chromatin is present in most cells, with a few exceptions, for example, red blood cells. Occurring only in the nucleus of eukaryotic cells, chromatin contains the vast majority of DNA, except for a inherited maternally, which is found in the .

Chromosomes are normally visible under a light microscope only when the cell is undergoing the metaphase of cell division. Before this happens every chromosome is copied once (S phase), and the copy is joined to the original by a centromere resulting in an X-shaped structure. The original chromosome and the copy are now called sister chromatids. During metaphase, when a chromosome is in its most condensed state, the X-shape structure is called a metaphase chromosome. In this highly condensed form chromosomes are easiest to distinguish and study.

In prokaryotic cells, chromatin occurs free-floating in cytoplasm, as these cells lack organelles and a defined nucleus. Bacteria also lack histones. The main information-carrying macromolecule is a single piece of coiled double-helix DNA, containing many genes, regulatory elements and other noncoding DNA. The DNA-bound macromolecules are proteins that serve to package the DNA and control its functions. Chromosomes vary widely between different organisms. Some species such as certain bacteria also contain plasmids or other extrachromosomal DNA. These are circular structures in the cytoplasm that contain cellular DNA and play a role in horizontal gene transfer.

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