Chapter 9

Question 1

Bacterial mutants that require supplemental nutrients in their growth media are called

a. autotrophs.
b. heterotrophs.
c. prototrophs.
d. omnitrophs.
e. auxotrophs.

Answer 1

e. auxotrophs.

Question 2

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.

Answer 2

b. Auxotrophic mutants can grow on medium that lack carbon source because they can synthesize their own nutrients.

Question 3

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.

Answer 3

c. prototrophs.

Question 4

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

Answer 4

c. Complete absence of recombination, which maintains the integrity of genome

Question 5

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.

Answer 5

a. All bacteria contain a single circular double stranded DNA as their genome.

Question 6

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

Answer 6

b. Two F’ cells

Question 7

Bacterial cells containing an F plasmid that has acquired bacterial chromosomal genes are called

a. F+.
b. F′.
c. F–.
d. Hfr.

Answer 7

b. F′.

Question 8

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+

Answer 8

a. Hfr.

Question 9

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.

Answer 9

c. Interrupted conjugation results in the production of Hfr strains.

Question 10

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

Answer 10

d. Chilling on the ice

Question 11

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

Answer 11

d. Conjugation

Question 12

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.

Answer 12

e. conjugation.

Question 13

When the F integrates into the E. coli chromosome, the result is an _______ strain.

a. Hfr
b. F–
c. F+
d. F’
e. F+/–

Answer 13

b. F–

Question 14

Cotransformation between two genes is more likely if they are

a. close to one another.
b. far apart from one another.
c. both next to the F factor.
d. both oriented in the same direction.
e. not located on the same chromosome.

Answer 14

a. close to one another.

Question 15

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.

Answer 15

a. Antibiotic resistance cannot be conferred by conjugation as conjugation only affects the fertility of bacteria.

Question 16

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

Answer 16

a. Conjugation

Question 17

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.

Answer 17

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.

Question 18

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.

Answer 18

a. A and B are oriented in the same direction.

Question 19

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

Answer 19

c. c b a d

Question 20

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?

Her strain Order of transfer
1 A, B, E, D, F
2 D, F, C, G, A
3 D, E, B, A, G

a. C, G, A, D, F, B, E
b. C, F, D, B, A, E, G
c. C, B, E, D, F, G, A
d. C, G, A, B, E, D, F
e. C, D, F, G, A, B, E

Answer 20

d. C, G, A, B, E, D, F

Question 21

The process of transferring DNA from one bacterium to another through a bacteriophage is

a. conjugation.
b. induction.
c. transformation.
d. transduction.
e. infection.

Answer 21

d. transduction.

Question 22

Integrated, inactive phage DNA is called a

a. progeny.
b. prophage.
c. transformant.
d. transductant.
e. conjugate.

Answer 22

b. prophage.

Question 23

HIV belongs to a group of viruses called

a. dsDNA viruses.
b. ssDNA viruses.
c. ssRNA-RT viruses.
d. dsDNA-RT viruses.
e. ssRNA viruses.

Answer 23

c. ssRNA-RT viruses.

Question 24

The life cycle of virulent phages that always kill their host cell and never become inactive prophages would be the

a. lethal cycle.
b. lytic cycle.
c. temperate cycle.
d. strict cycle.
e. lysogenic cycle.

Answer 24

b. lytic cycle.

Question 25

Which type of transduction is used to map distances between phage genes? a. Generalized transduction b. Specialized transduction c. Targeted transduction d. Random transduction e. Discontinuous transduction

Answer 25

a. Generalized transduction

Question 26

What does the enzyme reverse transcriptase do? a. Using the amino acid sequence of a protein as a template, it makes an RNA molecule. b. Using RNA as a template, it makes a DNA molecule. c. Using RNA as a template, it makes an RNA molecule. d. Using DNA as a template, it makes an RNA molecule. e. Using DNA as a template, it makes DNA molecule.

Answer 26

b. Using RNA as a template, it makes a DNA molecule.

Question 27

Which of the following statements about retroviruses is false? a. All retroviruses contain oncogenes, which can induce the formation of tumors. b. All retroviruses contain gag genes whose product forms the viral protein coat. c. All retroviruses require pol genes, which are critical for retrotranscription. d. All retroviral genomes have gag, pol, and env genes. Not all RNA viruses are retroviruses.

Answer 27

a. All retroviruses contain oncogenes, which can induce the formation of tumors.

Question 28

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? a. Complementation b. Specialized transduction c. Generalized transduction d. Recombination

Answer 28

d. Recombination

Question 29

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? a. Complementation via transduction event b. Co-transformation c. Recombination via conjugation d. Random mutation e. Interrupted transduction

Answer 29

a. Complementation via transduction event

Question 30

What are plasmids and what purposes do they serve?

Answer 30

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.

Question 31

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.

Answer 31

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.

Question 32

What causes an F– cell to be converted to F+?

Answer 32

The F factor plasmid in an F+ cell is replicated, and one copy is transferred to the F– cell through conjugation.

Question 33

What causes an F– cell to be converted to Hfr in the presence of F+ cells?

Answer 33

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.

Question 34

Outline the steps involved in mapping a bacterial chromosome by conjugation.

Answer 34

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. 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. At regular intervals, conjugation is interrupted by shearing the mating bacteria away from each other. 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. Cells that mated are checked for transfer of other genes from the Hfr to the F– strain. Order and distance of transferred genes are determined by comparing the time required for them to transfer from the Hfr to the F– strain.

Question 35

Outline the steps involved in mapping a bacterial chromosome by cotransformation.

Answer 35

A donor strain is chosen that has a number of prototrophies or antibiotic resistances. The recipient strain’s genotype is different from that of the donor. DNA from the donor strain is fragmented and is used to transform the recipient strain. Transformants are selected—for instance, for a prototrophy or antibiotic resistance. Transformants are tested for other genes they acquired in the transformation. Order and distance of transferred genes are determined by comparing cotransformation frequencies.

Question 36

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?

Answer 36

The map is circular: -m-c-a-n-l-d-b-o-.

Question 37

(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.

Answer 37

(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.

Question 38

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−).

Answer 38

(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−.

Question 39

How does a virulent phage differ from a temperate phage?

Answer 39

Virulent phages reproduce using only the lytic cycle, whereas temperate phages can use the lytic or lysogenic cycle.

Question 40

A retrovirus has an RNA genome but integrates into the DNA chromosome of a host cell. Explain how it does this.

Answer 40

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.

Question 41

List and describe three different ways that DNA from one bacterium can be transferred into bacterial cells.

Answer 41

Transformation: competent cells take up DNA from the environment. Transduction: DNA is carried into the cell in a virus. Conjugation: DNA from one cell is transferred in a pilus to another cell.

Question 42

What is the difference between specialized and generalized transduction?

Answer 42

Generalized transduction transfers fragments from anywhere in the chromosome; specialized transduction transfers only DNA that is adjacent to the prophage insertion site.