Human chromosomes. Coloured scanning electron micrograph (SEM) of two human chromosomes. Chromo- somes are mainly composed of DNA (deoxyribonucleic acid), which carries the genetic code in the form of genes (lengths of DNA with specific functions). These chromosomes are seen during the early stages of cell division. Each chromosome has replicated to form two identical strands (chromatids), linked by a region known as the centromere. Humans have 46 chromosomes, made up of 23 pairs. Each pair comprises one chromosome from each parent. The 46 chromosomes are found in nearly all cells, and their DNA controls the individual's growth and development. Magnification unknown.
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