chromosome

Genetic archival material is condensed into chromosomes (Left - click to enlarge image).

1. one of two sister chromatids
2. centromere (kinetocore)
p. short 'p' arm of chromatid
q. long 'q' arm of chromatid

A chromosome contains a long strand of deoxyribonucleic acid containing genes, regulatory sequences, and non-coding sequences of nucleotides, in association with proteins. The full chromosomal complement of a cell comprises the genome, which is the complete hereditary information of an organism contained within macromolecules of archival DNA .

The multiple nuclear chromosomes of eukaryotes comprise long helical strands of DNA wrapped around structural proteins called histones – this composite material is termed chromatin (diagram). Each eukaryotic chromosome comprises one or two (sister) chromatids (1), each with a kinetochore (2) for attachment to a microtubule of the spindle apparatus during cell division. Chromatids have a long (q) and a short (p) arm attached to the centromere (2). Sister chromatids attach to each other, or to the spindle apparatus, by means of special proteins and DNA base sequences in the kinetochore region.

Chromosomes (Gk. 'colored bodies') are most visible during metaphase (tem), and least condensed (dispersed) when participating in expression (tem, tem2) such as occurs in cells with large undifferentiated nuclei (colored tem cancer cell, fluorescence microscopy of cancer, stem cells, immature cell with oncogene (black dots) ).

Prokaryotes mostly possess one or two* chromosomes termed nucleoids (tem). Prokaryotes lack a membrane enclosed nucleus and their DNA is usually contained in circular structures located within the cytosol, but may be organized as linear strands that are typically attached to the plasma membrane. Plasmids are small circular, extrachromosomal genetic elements that can be transmitted from one bacterium to another through pili during conjugation. (more)

Chromatin (DNA plus histone protein) exists in two basic forms:
1. Euchromatin, from which DNA is being actively transcribed (expressed) into RNA for ultimate translation into polypeptide and protein molecules.
2. Heterochromatin, which consists of either:
a. Facultative heterochromatin, which is sometimes expressed.
b. Constitutive heterochromatin, which is located around the centromere and usually contains repetitive sequences, and which is never expressed.

*for example, Vibrio cholerae and Deinococcus radiodurans