11.5 Mutations  (Page 7/14)

Key concepts and summary

• A mutation is a heritable change in DNA. A mutation may lead to a change in the amino-acid sequence of a protein, possibly affecting its function.
• A point mutation affects a single base pair. A point mutation may cause a silent mutation if the mRNA codon codes for the same amino acid, a missense mutation if the mRNA codon codes for a different amino acid, or a nonsense mutation if the mRNA codon becomes a stop codon.
• Missense mutations may retain function, depending on the chemistry of the new amino acid and its location in the protein. Nonsense mutations produce truncated and frequently nonfunctional proteins.
• A frameshift mutation results from an insertion or deletion of a number of nucleotides that is not a multiple of three. The change in reading frame alters every amino acid after the point of the mutation and results in a nonfunctional protein.
• Spontaneous mutations occur through DNA replication errors, whereas induced mutations occur through exposure to a mutagen .
• Mutagenic agents are frequently carcinogenic but not always. However, nearly all carcinogens are mutagenic.
• Chemical mutagens include base analogs and chemicals that modify existing bases. In both cases, mutations are introduced after several rounds of DNA replication.
• Ionizing radiation, such as X-rays and γ-rays, leads to breakage of the phosphodiester backbone of DNA and can also chemically modify bases to alter their base-pairing rules.
• Nonionizing radiation like ultraviolet light may introduce pyrimidine (thymine) dimers, which, during DNA replication and transcription, may introduce frameshift or point mutations.
• Cells have mechanisms to repair naturally occurring mutations. DNA polymerase has proofreading activity. Mismatch repair is a process to repair incorrectly incorporated bases after DNA replication has been completed.
• Pyrimidine dimers can also be repaired. In nucleotide excision repair (dark repair) , enzymes recognize the distortion introduced by the pyrimidine dimer and replace the damaged strand with the correct bases, using the undamaged DNA strand as a template. Bacteria and other organisms may also use direct repair , in which the photolyase enzyme, in the presence of visible light, breaks apart the pyrimidines.
• Through comparison of growth on the complete plate and lack of growth on media lacking specific nutrients, specific loss-of-function mutants called auxotrophs can be identified.
• The Ames test is an inexpensive method that uses auxotrophic bacteria to measure mutagenicity of a chemical compound. Mutagenicity is an indicator of carcinogenic potential.

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