Chromosomes (a) are the darkly stained microscopic bodies in the nucleus which contain genetic information (genes) that determine inheritance characteristics
of an organism. A chromosome basically consists of a core of DNA (deoxyribonucleic acid) surrounded by a jacket of basic protein called histone. Bacteria and blue-greens have no nuclei or chromosomes, but instead, have a DNA-bearing structure called a nucleoid. Some of their DNA may be in circular forms called plasmids (b).

Chromosome Structure

Chromosomes are composed of chromatin, which can be seen microscopically with the use of stains such as the Feulgen reagent or acetocarmine. Chromosomes are best observed during mitotic or meiotic division, when the chromatin is condensed and chromosomes visible. Chromosomes have a simple external structure. A chromosome (a) has a small body called a centromere (c) with “arms” (d) on either side. The part of the centrome where spindle fibers are attached is called a kinetochore. Small segments, almost circular, that occur near the ends of a chromosome are called satellites (e).
During mitosis and meiosis, the chromosome replicates into two strands called chromatids (f) attached together at the centromere. Chiasma (crossing-over)
is when chromatids (f) overlap, break, and exchange segments with other chromatids (g) during meiosis. It may involve 2, 3 or all 4 chromatids. This can give rise to genetic variability or mutations.

Chromosome Number

Chromosome number is characteristic for the majority of vegetative (somatic) cells of an organism. This is called the 2n or diploid number.
Organisms whose somatic cells contain multiple sets of chromosomes are called polyploids. These occur as a result of hybridization between sexually compatible parents with different chromosome numbers. Polyploidy is widely evident in plants, but is rare in animals.