The mammalian genome might have a good reason to hold on to its vast collection of what scientists call junk DNA. Some of this genetic clutter may control gene expression in eggs and the earliest embryos, according to a report in the October Developmental Cell.
About one-third of the total mammalian genome is made of long, repeated stretches of DNA known as retrotransposons. Researchers hypothesize that retrotransposons derived from viruses that infected cells early in animal evolution. Retrotransposons, also called jumping genes, duplicate themselves and insert the copies into random places in DNA. After many generations, thousands of copies of each retrotransposon now reside in the mammalian genome.