25Gene Mutation and DNA Repair

25Gene Mutation and DNA Repair - Gene Mutation and DNA...

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1 Gene Mutation and DNA Repair A mutation is a heritable change in the genetic material. Consequences of Mutation General Information Chromosome mutations are changes in chromosome structure, while genome mutations are changes in chromosome number. Single-gene mutations are relatively small changes in chromosome structure that occur within a particular gene. Gene mutations are molecular changes in the DNA sequence of a gene. A gene mutation represents a permanent change in the DNA sequence. Point mutations change a single base within the DNA. Base substitutions involve a change from one base to another. Transitions involve a change from a pyrimidine to another pyrimidine (e.g., T to C) or a purine to another purine (e.g., A to G). Transversions involve a change from a pyrimidine to a purine, or vice-versa. Besides base substitutions, a single base pair or multiple base pairs may be deleted from the DNA or added to the DNA. Gene mutations can alter the coding sequence within a gene. Mutations within the coding sequence of a structural gene may have a number of effects on the structure of the encoded polypeptide (Table 16.1). Silent mutations do not alter the amino acid sequence of the polypeptide. Missense mutations change the amino acid sequence of the polypeptide. Sickle-cell anemia is an example of a missense mutation in humans (Figure 16.1). Nonsense mutations change a normal codon to a stop codon. Nonsense mutations in bacterial operators may inhibit the operation of downstream genes. This is called polarity.
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2 Frameshift mutations involve the addition or deletion of a number of nucleotides that are not divisible by three. This produces a completely new amino acid sequence downstream of the mutation. In general, mutations tend to produce polypeptides that have reduced, rather than enhanced, function; however: Silent mutations do not alter the sequence of amino acids in the polypeptide and, therefore, have no effect. Neutral mutations alter the amino acid sequence, but do not affect protein function. A mutation may, in fact, be advantageous and selected for; mutation is the raw material of evolution. Gene mutations are also given names that describe how they affect the wild-type genotype and phenotype. The term wild-type is used to describe the most common genotypes in a population. When a specific mutation is rare in a population, it is known as a mutant allele. Reverse mutations, also called reversions, change a variant back to a common, wild-type sequence. Mutations are also named according to their effect on the phenotype. A mutation that alters the phenotype is called a variant. Deleterious mutations decrease the chances of survival. Lethal mutations result in the death of the cell or organism. Beneficial mutations enhance the survival or reproductive success of an organism.
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This note was uploaded on 05/19/2011 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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25Gene Mutation and DNA Repair - Gene Mutation and DNA...

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