Chapter 7 Anatomy and function of a gene: Dissection Through Mutation
Mutations are heritable changes in base sequences that modify the information content of DNA.
Wild-type allele: One allele that dictates the most commonly found phenotype in a population Forward mutation: change of a wild-type allele to a mutant allele. Reverse mutation or reversion: A mutant allele to wild type
Fig. 7.2 Mutations classified by the effect on DNA (nonhomologous)
Fig. 7.3 Rates of spontaneous mutation albino brown Average spontaneous rate: 2-12 X10-6
General conclusions about spontaneous mutations: Mutations affecting phenotype occur very rarely. Different genes mutate at different rate. The rate of forward mutation is almost always higher than the rate of reversion.
Fig. 7.6 How natural processes can change the DNA DNA replication introduces random bases A or G hydrolysis
How natural processes can change the DNA irradiation
Mistakes during DNA replication can also alter genetic information DNA polymerase’s proofreading function
Fig. 7.10 Unequal crossing-over and transposons can rearrange DNA
Fig. 7.11 Mutagens Induce Mutations Exposure to X ray increases the mutation rate in Drosophila The greater the X-ray does, the greater the frequency of mutations
Mutagens Any physical or chemical agent that raises the frequency of mutations above the spontaneous rate
Fig. 7.12a1 How mutagens alter DNA Base analogs
Fig. 7.12b1 A T A
Fig. 7.12c1 How mutagens alter DNA Intercalating bases
DNA repair mechanisms Base excision repair removes damaged bases. Nucleotide excision repair corrects damaged nucleotides. Methyl-directed mismatch repair corrects mistakes in replication. Repair of double strand breaks by nonhomologous end-joining.
Fig. 7.7 Mutations in Nucleotide Excision repair Skin lesion
Fig. 7.13 Ames test identifies potential carcinogens
Fig. 7.14 Eye color mutations produce a variety of phenotype WT
Complementation test distinguishes mutations in different genes from mutations in the same genes.
How recombination within a gene could generate a wild-type allele
Fig. 7.17a12 What mutations tell us about gene structure Bacteriophage T4
Fig. 7.17a34 Bacteriophage T4 plaques
Fig. 7.17b12 Phenotypic properties of rII- mutants
Fig. 7.17c2 Mutations 1 and 2 are recessive to WT
Fig. 7.17d2 Revertants are extremely rare
Conclusion about the rII mutant experiment: A gene consists of different parts that can each mutate. Recombination between different mutable sites in the same gene can generate a normal, wild-type allele. 3. A gene performs its normal function only if all of its components are wild-type.
What is the arrangement of nucleotides in a gene-in a continuous row or dispersed in precise patterns around the genome? Do the various mutations alter many different nucleotides or only a small subset within each gene?
1. The number of mutable sites in the rII region is very close to the number of the nucleotides estimated to be in this region: mutation can arise from the change of a single nucleotide. (2) linear recombination map: A gene is composed of a continuous linear sequence of nucleotide pairs. (3) Positions of mutations in rIIA did not overlap those of rIIB gene: nucleotide sequence of these two genes are separate and distinct.
A gene is a linear set of nucleotide pairs, located within a discrete region of a chromosome, that serve as a unit of function.
How genotype correlates with phenotype Mutations can affect phenotype by: Altering the amino-acid composition Altering the amount of protein
Types of mutations Null mutants: abolish the function of a protein, deletion of a entire gene Hypomorphic mutations: proteins with much less amount, or protein with weak but detectable activity.
Fig. 7.25 Why some mutant alleles are recessive
Fig. 7.26 Incomplete dominance can arise when phenotypes is in proportion to the amount of functional protein
Fig. 7.27 Why some mutants are dominant
Neomorphic mutations: generate novel phenotypes antennapedia Protein with a new function Protein expressed at inappropriate time or space (ectopic expression)