8 lecture - Mutations Mutagenesis While mutations can occur...

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Mutations Mutagenesis While mutations can occur spontaneously, geneticists stimulate the formation of mutations with mutagenic agents. When the goal is to produce deletions or other rearrangements, X rays or gamma rays are used. When point mutations in a specific gene are desired, chemical mutagens are usually chosen. These mutagens fall into several classes. Base analogs are similar in structure to a base and are incorporated during replication, but they have base pairing interactions that differ from the normal base. 5-Bromouracil is almost identical to thymine and gets incorporated into DNA in place of thymine. Like thymine, it usually pairs with A, but is an alternate isomeric state it pairs with G resulting in an A to G transition. Several mutagens alter the structure of bases resulting in changes in base pairing. Hydroxylating agents add a hydroxyl to bases and result in pairing changes. Hydroxylamine adds a hydroxyl to C; hydroxylated C now pairs with A, resulting in a transition. Alkalating agents add methyl or ethyl groups to bases and change their pairing specificity. EMS adds an ethyl group to G or T, and modified G pairs with T and modified T pairs with G resulting in transitions. Deaminating agents remove amines to result in mispairing. Nitrous acid, for example, deaminates C changing it to U. It also deaminates A to hypoxanthine, a base that pairs with C. Intercalating agents are planar molecules that intercalate into DNA, resulting in a single base pair insertion or deletion. What are the consequences of point mutations? Point mutations can alter the protein of a single gene in several ways. Missense mutations result in single amino acid substitutions. Missense mutations may not affect protein function, or they may cause partial or complete loss of protein function. The effect of missense mutations on protein function depends on which amino acid is changed and the nature of the change. For example, some amino acids, often those on the surface, are relatively insensitive to changes, whereas other amino acids, for example, amino acids at the active site of the enzyme are very sensitive to changes. Some missense mutations are silent because they do not affect the function of the protein; others cause partial or complete inactivation of the proteins function. Nonsense mutations change codons that specify a specific amino acid into a stop codon. This results in a truncated protein that often has no function. In addition, many nonsense mutations also cause the mRNA encoding the truncated protein to become unstable. Most chemically induced mutations are missense and nonsense mutations.
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Frameshift mutations insert or delete one or two nucleotides and shift the reading frame. Splice site mutations change splice donor and acceptor sequences and can cause exons to be deleted or intron sequences to be included in the processed RNA. These changes can causes altered proteins to be synthesized. Finally, mutations in elements required for transcription can affect the ability to properly express a gene. How do mutations disrupt gene function?
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8 lecture - Mutations Mutagenesis While mutations can occur...

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