cerevisiae), telomeres, centromeres, and origins of replication have all been defined using genetics and have been cloned. When assembled they can be grown as a small chromosome in bacteria and form a vector capable of incor- vector carrier porating up to a million bases of other DNA as a chromosome in yeast (a

This technology has been used to investigate the properties of yeast chromosomes but has been most extensively used in the early phases of genome mapping projects. By cloning complete representations of the human genome into large YACs, the order of these YACs could be deduced by a number of methods and overlapping ones assembled conceptually into a representation of regions of the human genome. These are useful for finding genes from information about the inheritance of genetic diseases. They have also been useful for testing the function of genes in mice. Because of the size of YACs, they frequently contain all of the DNA needed to control the expression of genes with the correct developmental and tissue specificity. When injected into developing mouse eggs, they can fully correct mutations. More recently YACs have been largely supplanted by BACs, because the latter are easier to manipulate and prepare in the laboratory.

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