Genetics Chapter 9
Outline the steps involved in mapping a bacterial chromosome by cotransformation.
(1) A donor strain is chosen that has a number of prototrophies or antibiotic resistances. (2) The recipient strain's genotype is different from that of the donor. (3) DNA from the donor strain is fragmented and is used to transform the recipient strain. (4) Transformants are selected—for instance, for a prototrophy or antibiotic resistance. (5) Transformants are tested for other genes they acquired in the transformation. (6) Order and distance of transferred genes are determined by comparing cotransformation frequencies.
Outline the steps involved in mapping a bacterial chromosome by conjugation.
(1) Select a donor Hfr strain and a recipient F- strain. The two strains must be different in genotype for several marker genes—for example, leu- and leu+, azir and azs. (2) The two strains are mixed together in a nutrient medium to allow conjugation to begin. After a few minutes, the culture is diluted to prevent new conjugations. (3) At regular intervals, conjugation is interrupted by shearing the mating bacteria away from each other. (4) For each time point, cells that mated are selected—for instance, by requiring that a prototrophy or antibiotic resistance from each parent be present in one cell. (5) Cells that mated are checked for transfer of other genes from the Hfr to the F- strain. (6) Order and distance of transferred genes are determined by comparing the time required for them to transfer from the Hfr to the F- strain.
Using the rII mutant bacteriophage and E. coli strains B and K, Seymour Benzer studied gene structure by both recombination and complementation. Wild-type phages form plaques on strains B or K; rII forms plaques on B only.In one set of experiments, Benzer infected E. coli strain B simultaneously with two different rII mutants. Progeny phages were collected and used to infect strain K, and, occasionally, plaques formed. In a second set of experiments, Benzer infected strain K simultaneously with high amounts of two different rII mutant phages. Occasionally, plaques were observed from this infection. Which set of experiments shows recombination, and which demonstrates complementation? Explain the difference.
(1) The first set of experiments shows recombination, the second complementation. (2) Recombination requires exchange between genomes. In an experiment like this, in which each initial genome contains one mutation, recombination produces completely wild-type DNA (or genomes or chromosomes), along with DNA (or genomes or chromosomes) with the two mutations. (3) In complementation, each mutant genome (or chromosome or DNA) produces a gene product for which the other mutant genome is defective.Section: 9.3
List and describe three different ways that DNA from one bacterium can be transferred into bacterial cells.
(1) Transformation: competent cells take up DNA from the environment. (2) Transduction: DNA is carried into the cell in a virus. (3) Conjugation: DNA from one cell is transferred in a pilus to another cell.
Outline the steps involved in mapping bacterial genes by generalized transduction.
(1) Use two strains of bacteria whose genotypes differ for each gene to be mapped. (2) Phages that have infected one bacterial strain (the donor) are collected and used to infect the other strain. (3) Recipient bacteria with recombinant genotypes are selected. (4) Order and distance of genes are determined by comparing frequencies of cotransduction.
A virulent bacteriophage is used to infect a prototrophic bacterial culture. Phages are collected from the culture and are used to infect a new bacterial strain that has several auxotrophies. After infection, rare prototrophs are found: met+ leu+ 0 met+ pro+ 1 met+ his+ 0 pro+ leu+ 2 pro+ his+ 0 leu+ his+ 1 How are the auxotrophy genes organized on the bacterial chromosome?
(1) met and pro are close to each other. (2) pro and leu are close to each other. (3) leu and his are close to each other. (4) The gene order is met pro leu his.
(a) What is an F' plasmid and how is it formed? (b) Explain how an F' can be used to construct a bacterial strain that is partially diploid. (c) Explain how partial diploid strains can be used to assess interactions between different alleles (e.g., lac + and lac−).
(a) An F' is an F factor that carries one or two genes from the bacterial chromosome. An F' forms when an F factor excises imprecisely from the chromosome, carrying a small part of the chromosome with it. (b) When a cell carrying an F' contacts an F− cell, the F' is copied and transferred to the F− cell. The recipient cell will then possess two copies of the gene or genes that are carried on the F'. (c) It is more difficult to assess interactions between alleles in bacteria because bacteria normally are haploid and the alleles normally do not occur together in a cell or strain. However, F' strains can be used to construct partial diploid strains that allow for these interactions to be tested. For example, a strain can be established that is heterozygous for the lac gene, allowing one to determine the dominance relationship between lac+ and lac−.
(a) Explain the mechanism that leads to rapid evolution of the virus that causes influenza. (b) Distinguish between antigenic drift and antigenic shift, and explain the significance of each to influenza evolution and the occurrence of influenza in humans.
(a) The enzyme that copies the RNA genome of the influenza virus is prone to replication errors, leading to a high rate of mutation and rapid production of new genetic variants upon which natural selection can act. (b) Antigenic drift is the continual change in influenza proteins that result from mutations due to replication errors. Antigenic drift involves fairly small changes in the virus from year to year. Although the changes are not dramatic, they are significant in that most humans are not immune to the new variants and a new flu vaccine must be created each year. Antigenic shift is the production of a fundamentally new type of influenza virus through reassortment that occurs when two or more different strains of virus infect a single cell. Some of the progeny will carry fundamentally new combinations of genetic traits. Typically, very few people will be immune to the new strain, giving rise to the potential for widespread infection or pandemic. Significantly, reassortment is unpredictable, so that vaccines typically are not available until after the new strain has spread throughout the world.
(a) Explain how chromosomal genes are transferred from donors to recipients when cells of an F+ strain are mixed with F− cells. (b) Explain why transfer of chromosomal genes occurs at a higher frequency when cells of an Hfr strain are mixed with F− cells.
(a) Within a few cells of an F+ strain the F factor will integrate into one of several positions of the bacterial chromosome and become Hfr. When the integrated F factor initiates conjugation, genes of the bacterial chromosome will be transferred behind the leading part of the F factor. Once inside the recipient, the transferred chromosomal genes can be recombined into the recipient's chromosome. Transfer of chromosomal genes occurs at only a low frequency because relatively few cells in an F+ strain are Hfr. (b) Almost all of the cells of an Hfr strain contain an F factor that is integrated into the bacterial chromosome. All of these cells have the ability to transfer chromosomal genes to F− cells, so the frequency of transfer is high.
The figure below shows the results of interrupted-mating experiments with three different Hfr strains. What is the order of the genes, starting with C? Hfr strain Order of transfer 1 A, B, E, D, F 2 D, F, C, G, A 3 D, E, B, A, G - C, G, A, D, F, B, E - C, F, D, B, A, E, G - C, B, E, D, F, G, A - C, G, A, B, E, D, F - C, D, F, G, A, B, E
- C, G, A, B, E, D, F
Cotransformation between two genes is more likely if they are - close to one another. - far apart from one another. - both next to the F factor. - both oriented in the same direction. - not located on the same chromosome.
- close to one another.
Which of the following statements about retroviruses is false? All retroviruses contain oncogenes, which can induce the formation of tumors. -All retroviruses contain gag genes whose product forms the viral protein coat. -All retroviruses require pol genes, which are critical for retrotranscription. -All retroviral genomes have gag, pol, and env genes. -Not all RNA viruses are retroviruses.
-All retroviruses contain gag genes whose product forms the viral protein coat.
Two different strains of a mutant phage infected a single bacterium. One phage strain is d-e+ and the other is d+e-. The coinfected phages produced the wild-type phenotype in the bacterium: One phage supplies the wild-type gene product from a d+ allele, and the other supplies the wild-type gene product from the e+ allele. What genetic phenomenon does this demonstrate? -Complementation via transduction event -Co-transformation -Recombination via conjugation -Random mutation -Interrupted transduction
-Complementation via transduction event
Which type of transduction is used to map distances between phage genes? -Generalized transduction -Specialized transduction -Targeted transduction -Random transduction -Discontinuous transduction
-Generalized transduction
Two different strains of a mutant phage infect a single bacterium. One phage strain is d- and the other is e-. Some of the progeny phages are genotype d+e+, and some are d-e-. What genetic phenomenon does this demonstrate? -Complementation -Specialized transduction -Generalized transduction -Recombination
-Recombination
What does the enzyme reverse transcriptase do? -Using the amino acid sequence of a protein as a template, it makes an RNA molecule. -Using RNA as a template, it makes a DNA molecule. -Using RNA as a template, it makes an RNA molecule. -Using DNA as a template, it makes an RNA molecule. -Using DNA as a template, it makes DNA molecule.
-Using RNA as a template, it makes a DNA molecule.
The life cycle of virulent phages that always kill their host cell and never become inactive prophages would be the -lethal cycle. -lytic cycle. -temperate cycle. -strict cycle. -lysogenic cycle.
-lytic cycle.
You perform interrupted conjugation using an a+b+c+d+l+m+n+o+ Hfr strain and an F- strain that is a-b-c-d-l-m-n-o-. You observe the following genes transferred together in order from last to first: n+a+c+m+ o+m+c+a+n+ o+b+d+l+n+ What is the map order of the genes?
-m-c-a-n-l-d-b-o-.
Using the rII mutant bacteriophage and E. coli strains B and K, Seymour Benzer studied gene structure by both recombination and complementation. Wild-type phages form plaques on strains B or K; rII forms plaques on B only. In one set of experiments, Benzer infected E. coli strain B simultaneously with two different rII mutants. Progeny phages were collected and used to infect strain K, and, occasionally, plaques formed. In a second set of experiments, Benzer infected strain K simultaneously with high amounts of two different rII mutant phages. Occasionally, plaques were observed from this infection. If progeny were collected from the plaques on strain K in the first experiment and then were used in low numbers to infect strain K, what proportion of the infection events (low or high) would produce plaques? Why?
A high proportion of the infections would lead to a plaque. These plaques on strain K are formed by a wild-type recombinant phage. Their genomes (or chromosomes) contain no mutations. Therefore, virtually every infection would lead to a plaque, unless a phage had acquired a rare, new mutation.
Using the rII mutant bacteriophage and E. coli strains B and K, Seymour Benzer studied gene structure by both recombination and complementation. Wild-type phages form plaques on strains B or K; rII forms plaques on B only. In one set of experiments, Benzer infected E. coli strain B simultaneously with two different rII mutants. Progeny phages were collected and used to infect strain K, and, occasionally, plaques formed. In a second set of experiments, Benzer infected strain K simultaneously with high amounts of two different rII mutant phages. Occasionally, plaques were observed from this infection. If progeny were collected from the plaques on strain K in the second experiment and then were used in low numbers to infect strain K, what proportion of the infection events (low or high) would produce plaques? Why?
A low proportion of the infections would lead to a plaque. These plaques on strain K formed from the complementation of gene products produced within the infected bacterial cell. Thus, each infected cell contained both phage types, but each phage genome still contains the original mutation. If packaged into a phage, the phages would be mutant and unable to infect strain K on their own. Therefore, virtually no infection would lead to a plaque, unless recombinant phages had formed in the initial infection.
Both retroviruses and lysogenic bacteriophages employ a mechanism that allows them to be replicated and passed from cell to cell without producing viruses. What is the common mechanism that these two very different viruses use?
Both integrate their viral genome into a host genome, so that when the cell DNA is replicated, the viral genetic information is replicated as well.
What is the difference between specialized and generalized transduction?
Generalized transduction transfers fragments from anywhere in the chromosome; specialized transduction transfers only DNA that is adjacent to the prophage insertion site.
Explain the significance of horizontal gene transfer to bacterial evolution and to our ability to discern relationships between different groups of bacteria.
Horizontal gene transfer is the transfer of genes from one type of bacteria to another through conjugation, transformation, or (less likely) transduction. In some cases, genes can be transferred horizontally between bacteria that are not closely related. Horizontal gene transfer allows bacteria of different types to exchange genetic variation and to acquire new genetic traits that can be tested by natural selection. Some evidence suggests that horizontal gene transfer is extensive in nature. For example, about 17% of the E. coli genome is thought to have come from other types of bacteria. As a result, some bacteriologists question whether concepts of species even apply in bacteria, given that a particular bacterium can contain parts of a variety of genomes from different groups. Horizontal gene transfer complicates molecular analysis of relationships between different types of bacteria because such analyses assume that the genes being compared have been acquired by each group through vertical gene transfer (inheritance). To the extent that assumption is violated by horizontal gene transfers, molecular analysis of relationships is made much more difficult. In reality, such analyses are valid only for that part of the genome that has been transferred vertically by inheritance. In other words, the history of the group is not reflected perfectly in the history of its genome.
What are plasmids and what purposes do they serve?
Plasmids are small, circular, extra-chromosomal DNA molecules found naturally in bacteria. They carry extra genes and can transfer these genes from one bacterial cell to another. They are also used extensively in genetic engineering.
What causes an F- cell to be converted to F+?
The F factor plasmid in an F+ cell is replicated, and one copy is transferred to the F- cell through conjugation.
What causes an F- cell to be converted to Hfr in the presence of F+ cells?
The F- cell must first receive an F factor plasmid by conjugation with an F+ cell. Once inside the recipient cell, the F plasmid can integrate into the bacterial chromosome, converting the cell to Hfr.
HIV has a high mutation rate. What causes this, and how might this be advantageous to the virus?
The reverse transcriptase of HIV has an unusually high error rate, so that many mutations occur as the HIV genome is converted from RNA to DNA. A high mutation rate, though potentially creating nonfunctional viral proteins, also leads to rapid evolution of the virus, allowing it continually to adapt to new threatening conditions even within one host.
Two phage phenotypes are controlled by the genes a and b. In a mapping experiment, a culture of bacteria is infected simultaneously with an a-b+ strain and an a+b- strain. When plaques are analyzed, five out of 1000 have the a+b+ or a-b- phenotype. Based on the information, how far apart are genes a and b?
They are 0.5 map units apart.
How does a virulent phage differ from a temperate phage?
Virulent phages reproduce using only the lytic cycle, whereas temperate phages can use the lytic or lysogenic cycle.
You perform interrupted-mating experiments on three Hfr strains (A, B, and C). Genes are transferred (from last to first) in the following order from each strain: strain A, thi-his-gal- lac-pro; strain B, azi-leu-thr-thi-his; strain C, lac-gal-his-thi-thr. How are the F factors in these strains oriented? a. A and B are oriented in the same direction. b. B and C are oriented in the same direction. c. A and C are oriented in the same direction. d. All of them are oriented in the same direction. e. It cannot be determined from the information given.
a. A and B are oriented in the same direction
Which of the following statements about antibiotic resistance in bacteria is NOT true? a. Antibiotic resistance cannot be conferred by conjugation as conjugation only affects the fertility of bacteria. b. The antibiotic resistance gene can be transmitted to bacteria via transformation or transduction. c. Environments where antibiotics are frequently used such as hospitals are under the higher risk of developing antibiotic resistance. d. Antibiotic resistance often originates from the microbes that produce antibiotics for their own survival. e. The plasmid containing the antibiotic resistance gene can pass the genes to genetically unrelated bacteria.
a. Antibiotic resistance cannot be conferred by conjugation as conjugation only affects the fertility of bacteria.
leu- bacteria are mixed in a flask with leu+ bacteria, and soon all bacteria are leu+. However, if the leu- cells are on one side of a U-tube and the leu+ cells are on the other, the leu- cells do not become prototrophic. Which process is likely to produce this observed result? a. Conjugation b. Transduction c. Transformation d. Reciprocal translocation e. Transfection
a. Conjugation
In order to better understand arginine biosynthesis in bacteria, a microbial geneticist might first isolate mutant bacterial strains. a. What characteristics must these mutant bacteria have? b. Outline a strategy for isolating such mutants. c. List three possible methods for mapping the genetic location of the mutations in these strains.
a. Each mutant strain must be auxotrophic for the amino acid arginine. This characteristic of mutant strains can be used to determine the number of genes, the locations of genes, and the functions of genes that encode proteins used for arginine biosynthesis. b. Bacteria would first be plated on complete media (i.e., media containing arginine and other essential nutrients). Next, bacteria would be replica-plated both to Petri plates containing media that lacked arginine and to duplicate plates containing media that contained arginine. Analysis of the growth of the strains on these two types of media would reveal auxotrophs: Those that grow on media containing arginine but do not grow on media lacking arginine. c. Three methods are (1) interrupted conjugation, (2) cotransformation, and (3) generalized transduction.
A bacterial cell transfers chromosomal genes to F- cells, but it rarely causes them to become F+. The bacterial cell is a. Hfr. b. lysogenic. c. auxtrophic. d. lytic.e. F+
a. Hfr.
You are studying a new phage that infects H. pylori. You have isolated two mutant strains of the phage, each producing a different plaque phenotype due to a specific mutation: rough (r) and big (b). You co-infect H. pylori with both strains by adding a mixture of phages to a culture of cells. You collect the cell lysate containing progeny phages; plate diluted phages on a lawn of H. pylori cells; and observe 970 rough plaques, 890 big plaques, 0 rough and big plaques, and 500 normal, wild-type plaques. What is the recombination frequency between the r locus and the b locus? Can r and b be different alleles of the same locus? How can you explain the results?
a. No recombinant plaques were observed, so the recombination frequency is 0. b. No, the 500 wild-type plaques are a result of complementation between the two phage strains in doubly infected cells. c. The genes could be overlapping so that no recombination occurs between them.
You are using phages to map three toxin-production genes (R, Y, and G) in a new bacterium. You grow phages in a strain of the bacteria that produces all three toxins (R+, Y+, G+), isolate the phage, and then infect a second bacterial strain that cannot produce any of the toxins (R-, Y-, G-). The recipient bacteria are then grown on colorimetric media (media that change color in response to toxin presence) to see which toxin genes are transferred together by the phage. The data are as follows: What kind of mapping is this called? Which gene is in the middle? Which of the outside genes is closer to the middle gene?
a. Transduction b. Y c. R
Which of the following statements about bacterial genome is NOT true? a.All bacteria contain a single circular double stranded DNA as their genome. b. Some bacteria may have linear chromosomes instead of circular one. c. In addition to chromosome, many bacteria possess small extrachromosomal DNA called plasmid. d.Each plasmid contains an origin of replication that allows independent replication for its maintenance. e. The F factor, which is important for bacterial conjugation is found as a circular episome of E.coli.
a.All bacteria contain a single circular double stranded DNA as their genome.
Which of the following statements about nutritional requirement and growth of bacteria is NOT true? a. Culture media developed for bacteria must contain carbon source and essential elements for the survival of the bacteria. b. Auxotrophic mutants can grow on medium that lack carbon source because they can synthesize their own nutrients. c. Each bacterium has specific nutritional needs and conditions for successful cultivation. d. Prototrophic bacterial strains can grow on minimal media. e. The growth rate of bacteria on specific media can be assessed by the number and size of bacterial colonies.
b. Auxotrophic mutants can grow on medium that lack carbon source because they can synthesize their own nutrients.
When the F integrates into the E. coli chromosome, the result is an _______ strain. a. Hfr b. F- c. F+ d. F' e. F+/-
b. F-
Bacterial cells containing an F plasmid that has acquired bacterial chromosomal genes are called a. F+. b. F′. c. F-. d. Hfr.
b. F′.
What is the result of conjugation between F' and F- cells? a. One F+ cells b. Two F' cells c. Two F+ cells d. One Hfr and one F- cells e. Two Hfr cells
b. Two F' cells
The figure below shows a partial chromosome map of an E. coli Hfr strain. Each mark equals 10 minutes. If transfer of genes begins at "*" and goes in the direction of the arrow, which of the predicted results from this map is highly likely observed? a. gal will be the first and ton will be the last gene to be transferred. b. lac and azi will rarely be transferred together. c. Ten minutes after transfer of ton, azi will be transferred. d. It would take 30 minutes to transfer all of the genes that are shown. e. All the chromosomal genes will be transferred by the end.
b. lac and azi will rarely be transferred together.
Which of the following facts would NOT be considered as an advantage for using bacteria and viruses for genetic studies? a. Rapid reproduction and high progeny number b. Haploid genome for expressing mutations c. Complete absence of recombination, which maintains the integrity of genome d. Low cost to maintain and little storage space required e. Genomes being small and readily subjected to genetic manipulation
c. Complete absence of recombination, which maintains the integrity of genome
Which of the following statements about genetic exchange in bacteria is NOT true? a. In some viruses, the DNA that encodes one gene product can overlap with DNA that encodes a different gene product. b. Plasmids do not have to integrate into the host cell chromosome in order to be replicated. c. Interrupted conjugation results in the production of Hfr strains. d. The order of gene transfer is not the same for different Hfr strains. e. Antibiotic resistance can be transferred from one bacterial cell to another by conjugation.
c. Interrupted conjugation results in the production of Hfr strains.
How are Hfr strains of bacteria different from F+ strains? a. Cells of Hfr strains are able to transfer chromosomal genes, whereas cells of F+ strains cannot. b. Cells of Hfr strains cannot initiate conjugation with F− cells. c. The F factor is integrated into the bacterial chromosome in all or most cells of an Hfr strain but in only a few cells in an F+ strain. d. Cells of Hfr strains carry F' plasmids, whereas F+ cells do not. e. Cells of Hfr strains can initiate conjugation with F+ cells or other Hfr cells.
c. The F factor is integrated into the bacterial chromosome in all or most cells of an Hfr strain but in only a few cells in an F+ strain.
A bacterium of genotype a+b+c+d+ is the donor in a cotransformation mapping. The recipient is a-b-c-d-. Data from the transformed cells are shown below. What is the order of the genes? a+ and b+ 2 a+ and c+ 0 a+ and d+ 5 b+ and c+ 5 b+ and d+ 0 c+ and d+ 0 a. a c b d b. a d c b c. c b a d d. c a d b e. b c d a
c. c b a d
Bacterial strains that can produce all the necessary compounds and therefore grow on minimal media are called a. autotrophs. b. heterotrophs. c. prototrophs. d. omnitrophs. e. auxotrophs.
c. prototrophs.
Which of the following will have the least influence on the efficiency of transformation in E. coli bacteria? a. Calcium chloride treatment b. Heat shock c. Electrical field d. Chilling on the ice e. The amount of foreign DNA
d. Chilling on the ice
Which of the following horizontal gene transfer mechanisms would specifically use time as a basic unit of mapping? a. Transformation b. Crossing-over c. Transduction d. Conjugation e. Recombination
d. Conjugation
The process of transferring DNA from one bacterium to another through a bacteriophage is a. conjugation. b. induction. c. transformation. d. transduction. e. infection.
d. transduction.
Bacterial mutants that require supplemental nutrients in their growth media are called a. autotrophs. b. heterotrophs. c. prototrophs. d. omnitrophs. e. auxotrophs.
e. auxotrophs.
The transfer of DNA from a donor cell to a recipient cell through a cytoplasmic connection is called a. transformation. b.transduction. c. lysogenic cycle. d. lytic cycle. e. conjugation.
e. conjugation.
Integrated, inactive phage DNA is called a a. progeny. b. prophage. c. transformant. d. transductant. e. conjugate.
prophage
HIV belongs to a group of viruses called - dsDNA viruses. - ssDNA viruses. - ssRNA-RT viruses. - dsDNA-RT viruses. - ssRNA viruses.
ssRNA-RT viruses.
A retrovirus has an RNA genome but integrates into the DNA chromosome of a host cell. Explain how it does this.
the retrovirus genome encodes a reverse transcriptase enzyme. RTase makes a DNA copy of the viral RNA genome. The DNA copy integrates into the host cell's DNA.