STUDY HINTS
A large number of sensitive methods have been developed for measuring the rate of mutation and for isolating and characterizing the range of mutations that occur in all genomes. Novel characteristics of the genetics or the life cycles of many organisms, such as bacteria, viruses, Neurospora, or Drosophila, have been used to focus on different aspects of mutation. The giant polytene chromosomes of Drosophila, for example, have permitted fairly precise mapping of new mutations, using overlapping deletions. Variations in defined media have allowed specific nutritional mutations to be isolated in Neurospora and bacteria, and the relative simplicity of the genetic makeup of bacteria and viruses has been taken advantage of in defining the ways in which mutagenic agents may act upon the genome. The potentials offered by these and other experimental systems are described in your text.
Of particular interest is the genetic repair of DNA. The existence of repair enzymes is an important link to understanding the possible variations in the response of DNA to mutagenic agents. If a certain repair enzyme is defective, as in the human condition xeroderma pigmentosum, the genome is much more sensitive to the action of certain external agents (in this case, ultraviolet radiation). Indeed, variations in genetic repair systems may contribute in a significant way to variations in the responses of different individuals to environmental mutagens.
IMPORTANT TERMS
Ames test
Endonuclease
Exonuclease
Ligase
Mutation frequency
Mutation rate
Muton
Point mutation
Polymerase
Tautomeric shift
Thymine dimer
Transitions
Transversions
PROBLEM SET 15
1. Assume you are studying mutation rate in chickens and find that 19 chicks out of a total of 112,256 hatchlings have a rare dominant condition affecting feather shape. Only three of these chicks have a mother that had that condition. What is the mutation rate for this dominant trait?
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2. In Drosophila, there are strains that have the two X chromosomes attached. These attached-X females also carry a Y chromosome. These strains have many experimental uses, but one of the most powerful is the measurement of the mutation rate for sex-linked visible alleles.
(a) What kinds of offspring would you expect by crossing an attached-X female to a normal male?
(b) How could such a cross be used to measure the mutation rate for sex-linked genes?
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3. Why would it be very difficult to establish that the Hiroshima and Nagasaki atomic bombs have caused gene mutations?
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4. Which would you expect to be higher, forward mutation rate or back mutation rate? Or would these be the same? Please explain your answer.
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5. Consider the following data gathered during a hypothetical experiment to evaluate whether any of three chemicals causes an increase in mutation rate. Three groups of male Drosophila are each fed one of the chemicals; a fourth group is fed sugar water and serves as a control to measure spontaneous mutation. The males are then mated to females carrying a marked X chromosome (e.g., Basc carries the dominant eye shape mutant Bar and inversions to eliminate crossing over between the marked X and the treated X in heterozygous females). Large numbers of heterozygous F1 females are individually mated to males carrying the marked X, and the F2 progeny are examined. The appearance of a new lethal mutation can be detected by the absence of males in the progeny of a given cross. Since F1 females were mated individually, each such cross is the test of a different, single X chromosome. What are the mutation frequencies in each of the following treatments?
|
No. of crosses having wild-type F2 male progeny |
No. of crosses lacking wild-type F2 male progeny |
|
|
Control |
5,379 |
32 |
|
Treatment 1 |
4,901 |
54 |
|
Treatment 2 |
5,249 |
213 |
|
Treatment 3 |
8,243 |
41 |
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6. It is
(a) surprising, or
(b) not surprising, to find out that a substance is a carcinogen for one strain and is harmless for a second, genetically different strain.
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7. Some wavelengths of ultraviolet light are more efficiently absorbed by some molecules than by others, and different molecules have different absorption spectra for ultraviolet radiation. The accompanying figure shows the absorption spectra of DNA and protein, with different wavelengths of ultraviolet radiation. What is your expectation for the curve obtained if we change the ordinate of the graph to read “frequency of mutation”? Explain.
Ultraviolet radiation absorption curves for serum albumin (dotted line) and DNA (solid line). Adapted from A. H. Hollaender, Radiation Biology (New York: McGraw-Hill, 1947).
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8. Please discuss in detail what is going on in the process delineated in the figure.
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9. Ultraviolet radiation in humans can cause
(a) somatic mutations only,
(b) germ-line mutations only, or
(c) both somatic and germ-line mutations.
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10. Other factors being equal, which of the following types of mutations is most likely to be severely deleterious?
(a) frameshift,
(b) transition, or
(c) transversion.
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ANSWERS TO PROBLEM SET 15
1. Since three of the chicks can be assumed to have inherited the condition from their mothers, there are only 16 new mutations for the feather shape in this sample. Furthermore, since this feather shape is dominant, either of the two alleles could mutate to produce the phenotype. We must, therefore, consider all available alleles (except those in the chicks inheriting a preexisting mutant) when determining mutation rate. For 112,253 chicks (112,256 minus the 3 chicks we are discounting), there are 16 mutations occurring in 224,506 gametes, giving a mutation rate of 16/224,506 = 7.13 × 10−5.
2.
(a) The attached-X method in Drosophila melanogaster is designed to detect newly arisen X-linked mutations in a male. The attached-X female has a neutral Y chromosome. She produces two types of eggs, as indicated in the following diagram.
(b) The male produces sperm containing either an X or a Y chromosome. Since we know from the male’s phenotype that he carries no lethal or visible mutation on his X chromosome, any of the sons that carry such a mutant will be affected. Visible mutations are readily detected, and with appropriate care some semilethals might also be picked up. This type of inheritance pattern, in which the X chromosome is transmitted from father to son, is called patroclinous transmission.
X* denotes a tested X chromosome. The patroclinous males (boxed) are scored for newly arisen visible mutations.
3. Most gene mutations are recessive. Of those that are dominant, at least some would be lost because of lethality during development. For the more common recessive mutant to be detected within one generation of the exposure, it would have to occur in the X chromosome of the oocyte, where it could then be detected if it eventually gave rise to a son. For early detection of an autosomal recessive trait, both parents would have to have the same gene mutate independently, and a child would have to get both the (probably single) mutated ovum and the one sperm (or one of a small proportion of sperm) that carry this mutation. This requires a very large number of unlikely events. Most recessive autosomal mutants will be transmitted to a child of the exposed person and then down through subsequent generations to a number of people. Eventually, two people carrying this mutant could marry and be expected to produce an affected child, with a probability of 25 percent. It would not be possible to identify this child as a direct victim of the bombs, however, since mutations of the same gene also occur spontaneously or as a result of the influence of other environmental mutagens.
4. Forward mutation rate is higher than back mutation rate. There can be thousands of base pairs present in a single gene, and the mutations in many of these can change the gene product to yield a mutant phenotype. Yet, once the gene has mutated, there are only a limited number of changes that can occur to “repair” the gene product and return it to normal. At the extreme of considering any nucleotide change a mutation, whether it changes the protein or not, there might be literally thousands of ways to mutate a gene from “normal” to a mutant form, but only one way to return a given mutant form to normal. By analogy, “It is relatively easy to kick the TV set and break it, but not as likely that another random kick to the same set will fix the break.”
5. Since males have only one X chromosome and the treated males are alive, their X chromosome must initially carry no lethal mutation. The experimental design assumes that a mutagenic chemical eaten by the male can increase the genetic errors that occur during sperm formation. The cross is diagramed, with the treated X chromosome marked with an asterisk.
The mutation rate for each treatment is given by the number of crosses lacking wild-type males (indicated by the box in the diagram) divided by the total number of chromosomes tested: Control, 32/5,411 = 0.59 percent new lethal mutations; 1, 54/4,955 = 1.09 percent; 2, 213/5,462 = 3.90 percent; 3, 41/8,284 = 0.49 percent. Thus, chemicals 1 and 2 appear to have elevated mutation rate, but chemical 3 did not.
6.
(b) Not surprising. Genetically determined physical or biochemical differences in the way in which chemicals are absorbed and turned over in the body can lead to major variations in response. Indeed, this is one of the basic problems studied in the medical field of pharmacogenetics.
7. Since ultraviolet radiation is a mutagen, and since DNA is the genetic material, you might expect that the curve for the efficiency of different wavelengths of ultraviolet (UV) radiation in inducing mutation will resemble the solid-line curve (DNA) rather than the dotted-line curve (protein). This has been shown to be the case. It is also interesting to note that germicidal ultraviolet lamps emit radiation at about 2,600 angstroms, where DNA shows maximum absorption.
8. The process diagramed is the repair of thymine dimers, a form of UV-induced mutation.
(1) The DNA is hit by ultraviolet light.
(2) A thymine dimer is formed.
(3) The repair begins with a special endonuclease cutting the phosphate–sugar backbone.
(4) Exonucleases then remove six or seven nucleotides, including the dimer.
(5) The deleted portion is refilled by DNA polymerase, using the complementary strand as a template.
(6) The gaps are closed by ligase (joining enzyme).
9.
(a) Somatic mutations only; ultraviolet light affects only those compounds that can absorb it. The rays are too long to penetrate more than a few cell layers or so.
10.
(a) Frameshifts; they can alter many amino acids in the protein and cause early chain termination.