genome, all the genetic content contained within an organism. An organism’s genome is made up of molecules of deoxyribonucleic acid (DNA) that form long strands that are tightly wound into chromosomes, which are found in the nucleus of eukaryotic organisms and in the cytoplasm of prokaryotic organisms. Chromosomes that are unique to certain organelles within a cell, such as mitochondria or chloroplasts, are also considered a part of an organism’s genome. A genome includes all the coding regions (regions that are translated into molecules of protein) of DNA that form discrete genes, as well as all the noncoding stretches of DNA that are often found on the areas of chromosomes between genes. The sequence, structure, and chemical modifications of DNA not only provide the instructions needed to express the information held within the genome but also provide the genome with the capability to replicate, repair, package, and otherwise maintain itself. The human genome contains between 20,000 and 25,000 genes within its three billion base pairs of DNA, which form the 46 chromosomes found in a human cell. In contrast, Nanoarchaeum equitans, a parasitic prokaryote in the domain Archaea, has one of the smallest known genomes, consisting of 552 genes and 490,885 base pairs of DNA. The study of the structure, function, and inheritance of genomes is called genomics. Genomics is useful for identifying genes, determining gene function, and understanding the evolution of organisms.
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