AP BIOLOGY

Detailed explanation-1: -Mutations result either from errors in DNA replication or from the damaging effects of mutagens, such as chemicals and radiation, which react with DNA and change the structures of individual nucleotides. All cells possess DNA-repair enzymes that attempt to minimize the number of mutations that occur (Section 14.2).

Detailed explanation-2: -A mutation is a change in the DNA sequence of an organism. Mutations can result from errors in DNA replication during cell division, exposure to mutagens or a viral infection.

Detailed explanation-3: -Random, unpredictable DNA “mistakes” account for nearly two-thirds of the mutations in cancers. These mutations occur as a result of DNA copying errors made when normal cells divide. What causes such random DNA copying errors? Human cells are constantly regenerating.

Detailed explanation-4: -Incorrectly paired nucleotides that still remain following mismatch repair become permanent mutations after the next cell division. This is because once such mistakes are established, the cell no longer recognizes them as errors.

Detailed explanation-5: -A gene variant is a permanent change in the DNA sequence that makes up a gene. This type of genetic change used to be known as a gene mutation, but because changes in DNA do not always cause disease, it is thought that gene variant is a more accurate term.