Mutations are the grist of evolution, and have accumulated in our DNA over time. When populations separate, each group accumulates their own unique set of DNA mutations. Because mutations in a particular sequence of DNA occur at a constant rate, the number of accumulated mutations in that sequence is proportional to the length of time that two groups have been separated. This concept is often known as the "molecular clock." (DNAi Location: Application > Human Origins > Gene geneaolgoy > A molecular clock > DNA mutation)

DNA is under constant attack from reactive chemicals and natural background radiation. Free radicals are the byproducts of normal metabolism in human cells. Seen here as bright particles, they sometimes react with DNA and cause chemical changes. Radiation can also affect DNA. For example, ultraviolet light from the sun can cause harmful chemical changes in the DNA of skin. These changes can lead to kinks in the DNA that prevent genes from being correctly read, or deletions that alter the type of protein produced. Thanks to constant biochemical repair work, most mutations are corrected before that have any effect. But in rare cases mutations can accumulate, and this can give rise to diseases such as cancer.

natural background radiation,harmful chemical changes,dna mutations,cause chemical changes,dna mutation,molecular clock,reactive chemicals,human origins,free radicals,human cells,constant rate,byproducts,grist,rare cases,ultraviolet light,narration,deletions,length of time,metabolism,genes