chromosome

noun

(Gr. khrôma: color; sôma: body) A thread-like structure in the nucleus of a cell, containing a linear sequence of genes, which determine hereditarily cell structure and function. A thread-like structure in the nucleus of eukaryotic cells composed of DNA and protein. Chromosomes carry the genetic information of the cell in their DNA, organized into linear arrangements of genes. The locations of genes on particular chromosomes and their relative positions along the chromosome can be mapped by classic breeding experiments, or by more recent techniques for probing specific DNA sequences ( See DNA probe). Chromosomes can be observed directly in cells only while they are dividing. Each species has a constant chromosome number; humans have 23 pairs of chromosomes. One of the threadlike structures that carry the genetic information (see gene) of living organisms and are found in the nuclei of their cells. Chromosomes consist of a central axis of DNA with associated RNA and proteins. Before cell division, the long filamentous threads contract and thicken and each chromosome can be seen as two identical threads (chromatids) joined at the centromere. During cell division the chromatids separate to become the daughter chromosomes (see mitosis). Chromosome number is characteristic of a species. For example, a normal human body cell has 46 chromosomes comprising 22 matched pairs (called autosomes) and two sex chromosomes. A human sperm or egg cell has half this number of chromosomes (see meiosis). Abnormal numbers or parts of chromosomes often lead to abnormalities in the individual concerned. Down's syndrome is caused by the presence of an extra number 21 chromosome.