A threadlike structure, several of which can be seen in the nuclei of eukaryotic cells during cell division. They are composed of *
chromatin and hence carry the genetic information. The chromatin becomes condensed prior to cell division, which renders the chromosomes visible under the light microscope. The chromosome is divided along its length into two identical strands, the *
chromatids, joined at some point by the *
centromere. The chromosome thus has four arms, the length of these depending upon the position of the centromere. The number, size, and shape of the chromosomes as seen at prophase are generally characteristic of a species (see
karyotype).
During interphase the chromosomes uncoil into long narrow threads of *
DNA about 2 nm in diameter. These bear beadlike structures, the *
nucleosomes, which are highly organized aggregations of histones and DNA. The complex coiling and recoiling of the DNA and histones that occurs prior to cell division considerably reduces the length of the chromosome and increases the diameter to between 10 and 30 nm. Bacterial and viral 'chromosomes' are much simpler. There is only one per cell and they consist solely of a single or double strand of DNA or RNA without any associated histones. They do not become condensed and hence are only visible using an electron microscope. See also
homologous chromosomes ,
B-chromosome.
