Key Concept 14.2 described how certain viruses called retroviruses can insert their genetic material into the host cell’s genome. Such insertions happen at random, and if one occurs within a gene, it can cause a loss-of-function mutation in that gene. In many cases, the viral DNA remains in the host genome and is passed on from one generation to the next. When this happens the virus is called an endogenous retrovirus. Endogenous retroviruses are common—in fact, they make up 5 to 8 percent of the human genome.

Another form of DNA, called a transposon or transposable element, can also insert itself into genes and cause mutations. As you will see in Chapter 17, transposons are widespread in both prokaryotic and eukaryotic genomes. A transposon is a DNA sequence of a few hundred to a few thousand base pairs that can move from one position in the genome to another. It usually carries genes that encode the enzymes needed for this movement. Some transposons remove themselves from their positions in the genome and then insert themselves into other sites (the “cut and paste” mode of transposition). These transposons do not always excise cleanly, but leave behind short sequences of a few base pairs that become permanent mutations in the affected genes. Other transposons first replicate themselves, and then the new copies are inserted into new sites in the genome (the “copy and paste” mode). A sequence of genomic DNA is sometimes carried along with the transposon DNA when it moves, and gene duplication occurs. As you’ll see, gene duplication plays an important role in evolution.

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