Analysis of DNA sequences has found over a million Alu elements within each primate genome, many of which are species specific: 5,000 are unique to humans, while 2,300 others are exclusive to chimpanzees. In contrast the orangutan lineage (Sumatran and Bornean orangutans) only has 250 specific Alu.
These tiny pieces of mobile DNA are able to copy themselves using a method similar to retroviruses. But, because this is an inexact process, a segment of 'host' DNA is duplicated at the Alu insertion sites and these footprints, known as 'target site duplications', can be used to 'identify' Alu insertions. Alu elements can be thought of as molecular fossils, and a shared Alu element sequence and location within the genome indicates a common ancestor.
Researchers from Louisiana State University, in collaboration with the Zoological Society of San Diego and the Institute of Systems Biology in Seattle, found a single Alu which is present in great apes, but absent from gibbons and siamang, and so was likely acquired after ancestors of these species diverged. This founder Alu element was found in an intron (non-coding DNA) on chromosome 7.
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