of unknown function. Forty to forty-five percent of the genome is made up of short repeat sequences, some of which have known functions such as stabilizing chromosome structure, inactivating one of the two X chromosomes, and the satellite DNA found at the chromosome centromeres and telomeres, which play a role in chromosome structure, duplication, and cell division. There are also "pseudogene" regions which are thought to be remnants of real genes in which one copy has been inactivated. They provide a record of evolutionary history.
In 2001, it was estimated that the human genome contains between 30,000 and 40,000 genes, but more recent estimates put the figure at less than 30,000 (NCBI, 2004). Another theory which is no longer valid is the one gene-one protein theory. Of the roughly one percent of the genome that is expressed, 40 percent can be spliced to produce multiple proteins from a single gene, i.e. cutting and pasting of the mRNA transcript into various combinations of mature mRNA for translation. Scientists believe
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