Gene mutation

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Functional consequences of base changes We first f consider what happens when a mutation arises in a polypeptide coding part of a gene. Depending on the consequences, single-base substitutions are classified into: Silent or synonymous mutations: the mutation changes one codon for an amino acid into another codon for that same amino acid. Missense mutations: the codon for one amino acid is replaced by a codon for another amino acid. Nonsense mutations: the codon for one amino acid is replaced by a translation termination (stop) codon. The severity of the effect of missense and nonsense mutations on the polypeptide may differ. If f a missense mutation causes the substitution of a chemically similar amino acid (conservative substitution), then it is likely that the alteration will have a less-severe effect on the protein's structure and function. Alternatively, chemically different amino acid substitutions, called nonconservative substitutions, are more likely to produce severe changes in protein structure and function. Nonsense mutations will lead to the premature termination of translation. Thus, they have a considerable effect on protein function. Unless they occur very close to the 3’ end of the open reading frame, so that only a partly functional truncated polypeptide is produced, nonsense mutations will produce inactive protein products. Genetica per Scienze Naturali a.a. 08-09 prof S. Presciuttini
Functional consequences of frameshift mutations On the other hand, single-base additions or deletions have consequences on polypeptide sequence that extend far beyond the site of the mutation itself, like nonsense mutations. Because the sequence of mRNA is "read" by the translational apparatus in groups of three base pairs (codons), the addition or deletion of a single base pair of DNA will change the reading frame starting from the location of the addition or deletion and extending through to the carboxy terminal of the protein. Hence, these lesions are called frameshift mutations. These mutations cause the entire amino acid sequence translationally downstream of the mutant site to bear no relation to the original amino acid sequence. Thus, frameshift mutations typically exhibit complete loss of normal protein structure and function. Genetica per Scienze Naturali a.a. 08-09 prof S. Presciuttini
Page 1 and 2: MUTATIONS Questo documento è pubbl
Page 3 and 4: Basic terminology about gene mutati
Page 5 and 6: Germinal mutations Somatic mutation
Page 7: Point mutations: base substitutions
Page 11 and 12: Mutations in non-coding regions No
Page 13 and 14: Errors in DNA replication Mispairi
Page 15: From mispairs to mutations (a) A gu

Gene mutation

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