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PowerPoint Presentation Materials to accompany Genetics: Analysis and Principles Robert J. Brooker Copyright ©The McGraw-Hill Companies, Inc. Permission.

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Presentation on theme: "PowerPoint Presentation Materials to accompany Genetics: Analysis and Principles Robert J. Brooker Copyright ©The McGraw-Hill Companies, Inc. Permission."— Presentation transcript:

1 PowerPoint Presentation Materials to accompany Genetics: Analysis and Principles Robert J. Brooker Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display CHAPTER 16 GENE MUTION AND DNA REPAIR

2 INTRODUCTION The term mutation refers to a heritable change in the genetic material Mutations provide allelic variations On the positive side, mutations are the foundation for evolutionary change On the negative side, mutations are the cause of many diseases Since mutations can be quite harmful, organisms have developed ways to repair damaged DNA 16-2 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

3 Mutations can be divided into three main types 1. Chromosome mutations Changes in chromosome structure 2. Genome mutations Changes in chromosome number 3. Single-gene mutations Relatively small changes in DNA structure that occur within a particular gene Type 3 will be discussed in this chapter Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16.1 CONSEQUENCES OF MUTATIONS 16-3

4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A point mutation is a change in a single base pair It involves a base substitution Gene Mutations Change the DNA Sequence ’ AACGCTAGATC 3’ 3’ TTGCGATCTAG 5’ 5’ AACGCGAGATC 3’ 3’ TTGCGCTCTAG 5’ A transition is a change of a pyrimidine (C, T) to another pyrimidine or a purine (A, G) to another purine A transversion is a change of a pyrimidine to a purine or vice versa Transitions are more common than transversions

5 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mutations may also involve the addition or deletion of short sequences of DNA Gene Mutations Change the DNA Sequence ’ AACGCTAGATC 3’ 3’ TTGCGATCTAG 5’ 5’ AACGCGC 3’ 3’ TTGCGCG 5’ 5’ AACGCTAGATC 3’ 3’ TTGCGATCTAG 5’ 5’ AACAGTCGCTAGATC 3’ 3’ TTGTCAGCGATCTAG 5’ Deletion of four base pairs Addition of four base pairs

6 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Silent mutations are those base substitutions that do not alter the amino acid sequence of the polypeptide Due to the degeneracy of the genetic code Missense mutations are those base substitutions in which an amino acid change does occur If the substituted amino acids have similar chemistry, the mutation is said to be neutral Gene Mutations Can Alter the Coding Sequence Within a Gene 16-6

7 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gene Mutations Can Alter the Coding Sequence Within a Gene 16-7 Nonsense mutations are those base substitutions that change a normal codon to a termination codon Frameshift mutations involve the addition or deletion of nucleotides in multiples of one or two This shifts the reading frame so that a completely different amino acid sequence occurs downstream from the mutation

8 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16-8

9 Figure 15-1 Copyright © 2006 Pearson Prentice Hall, Inc.

10 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display In a natural population, the wild-type is the most common genotype A forward mutation changes the wild-type genotype into some new variation A reverse mutation has the opposite effect It is also termed a reversion Gene Mutations and Their Effects on Genotype and Phenotype 16-9

11 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display When a mutation alters an organism’s phenotypic characteristics, it is said to be a variant Variants are characterized by their differential ability to survive: Deleterious mutations decrease the chances of survival The most extreme are lethal mutations Beneficial mutations enhance the survival or reproductive success of an organism Conditional mutants: affect the phenotype only under a defined set of conditions 16-10

12 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Second-site mutations: suppressor mutations Intragenic suppressors The second mutant site is within the same gene as the first mutation Intergenic suppressors The second mutant site is in a different gene from the first mutation 16-11

13 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A mutation, may alter the sequence within a promoter Up promoter mutations make the promoter more like the consensus sequence They may increase the rate of transcription Down promoter mutations make the promoter less like the consensus sequence They may decrease the rate of transcription Refer to Table 16.2 for other examples Gene Mutations in Noncoding Sequences 16-12

14 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16-13

15 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display A chromosomal rearrangement may affect a gene because the break occurred in the gene itself A gene may be left intact, but its expression may be altered because of its new location This is termed a position effect There are two common reasons for position effects: 1. Movement to a position next to regulatory sequences Refer to Figure 16.2a 2. Movement to a position in a heterochromatic region Refer to Figure 16.2b AND 16.3 Changes in Chromosome Structure Can Affect Gene Expression 16-19

16 Figure Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Regulatory sequences are often bidirectional

17 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Germ-line mutations are those that occur directly in a sperm or egg cell, or in one of their precursor cells Refer to Figure 16.4a Somatic mutations are those that occur directly in a body cell, or in one of its precursor cells Refer to Figure 16.4b AND 16.5 Mutations Can Occur in Germ-Line or Somatic Cells 16-21

18 Figure Therefore, the mutation can be passed on to future generations The size of the patch will depend on the timing of the mutation The earlier the mutation, the larger the patch An individual who has somatic regions that are genotypically different from each other is called a genetic mosaic Therefore, the mutation cannot be passed on to future generations

19 Mutations can occur spontaneously or be induced Spontaneous mutations Result from abnormalities in cellular/biological processes Errors in DNA replication, for example Induced mutations Caused by environmental agents Agents that are known to alter DNA structure are termed mutagens These can be chemical or physical agents Refer to Table 16.4 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16.2 OCCURRENCE AND CAUSES OF MUTATION 16-23

20 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 16-24

21 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Are mutations spontaneous occurrences or causally related to environmental conditions? This is a question that biologists have asked themselves for a long time Jean Baptiste Lamarck Proposed that physiological events (e.g. use and disuse) determine whether traits are passed along to offspring Charles Darwin Proposed that genetic variation occurs by chance Natural selection results in better-adapted organisms Spontaneous Mutations Are Random Events 16-25

22 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display These two opposing theories of the 19th century were tested in bacteria in the 1940s and 1950s Salvadore Luria and Max Delbruck studied the resistance of E. coli to bacteriophage T1 ton r (T one resistance) They wondered if ton r is due to spontaneous mutations or to a physiological adaptation that occurs at a low rate? The physiological adaptation theory predicts that the number of ton r bacteria is essentially constant in different bacterial populations The spontaneous mutation theory predicts that the number of ton r bacteria will fluctuate in different bacterial populations Their test therefore became known as the fluctuation test 16-26

23 16-27 The Luria-Delbruck fluctuation test Figure 16.6 E.. coli is grown in the absence of T1 phages 20 million cells each Plates containing T1 phages Great fluctuation in the number of ton r coloniesRelatively even distribution of ton r colonies Many ton r bacteria Mutation occurred at an early stage of population growth, before T1 exposure No ton r bacteria Spontaneous mutation did not occur Several independent ton r mutations occurred during different stages These are mixed together in a big flask to give an average value of ton r cells

24 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Joshua and Ester Lederberg were also interested in the relation between mutations and the environment At that time (1950s), there were two new theories Directed mutation theory Selected conditions could promote the formation of specific mutations allowing the organism to survive This was consistent with Lamarck’s viewpoint Random mutation theory Environmental factors simply select for the survival of those individuals that happen to possess beneficial mutations This was consistent with Darwin’s viewpoint Random Mutations Can Give an Organism a Survival Advantage 16-28

25 16-29 Figure 16.7 Replica plating A few ton r colonies were observed at the same location on both plates!!! This indicates that mutations conferring ton r occurred randomly on the primary (nonselective plate) The presence of T1 in the secondary plates simply selected for previously occurring ton r mutants This supports the random mutation theory The Lederbergs developed a technique to distinguish between these two theories

26 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The term mutation rate is the likelihood that a gene will be altered by a new mutation It is commonly expressed as the number of new mutations in a given gene per generation It is in the range of to per generation The mutation rate for a given gene is not constant It can be increased by the presence of mutagens Mutation rates vary substantially between species and even within different strains of the same species Mutation Rates and Frequencies 16-30

27 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Within the same individual, some genes mutate at a much higher rate than other genes Some genes are larger than others This provides a greater chance for mutation Some genes have locations within the chromosome that make them more susceptible to mutation These are termed hot spots Note: Hot spots can be also found within a single gene Refer to Figure 6.20 Mutation Rates and Frequencies 16-31

28 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display Figure 6.20 Contain many mutations at exactly the same site within the gene

29 Table 15-2 Copyright © 2006 Pearson Prentice Hall, Inc.

30 Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display The mutation frequency for a gene is the number of mutant genes divided by the total number of genes in a population If 1 million bacteria were plated and 10 were mutant The mutation frequency would be 1 in 100,000 or The mutation frequency depends not only on the mutation rate, but also on the Timing of the mutation Likelihood that the mutation will be passed on to future generations Mutation Rates and Frequencies 16-33

31 Figure 15-8 Copyright © 2006 Pearson Prentice Hall, Inc.

32 Figure 15-9 Copyright © 2006 Pearson Prentice Hall, Inc.

33 16-73 Figure Nucleotide Excision Repair

34 16-81 Figure DNA strands A and C have the same sequence DNA strands B and D have the same sequence Note: Recombinational repair occurs while the two DNA copies are being made Recombination during DNA replication

35 16-82 Figure The gap has been repaired; but the thymine dimer remains


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