STEVE ALLEN / SCIENCE PHOTO LIBRARY STEVE ALLEN / SCIENCE PHOTO LIBRARY
Chromosomes. Computer artwork of paired human chromosomes. Humans have 23 pairs of chromosomes, with one of each pair being inherited from each parent. Each chromosome is made up of two chromatids, which are linked at a region called the centromere. The chromosomes carry the genes that control an individual's growth and development. Genes are lengths of DNA (deoxyribonucleic acid) that contain recipes for the proteins that make up the body. Chromosomes only have an X-shape during replication. At other times the DNA exists as a long thread.
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