Deck 9: Bacterial and Viral Genetic Systems

Question 1

Which of the following features would NOT be considered an advantage for using organisms such as 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 the genome
D) low cost to maintain and little storage space required
E) genomes being small and readily subjected to genetic manipulation

Answer 1

C

Question 2

The transfer of DNA from a donor cell to a recipient cell through a cytoplasmic connection is called:

A) transformation.
B) transduction.
C) the lysogenic cycle.
D) the lytic cycle.
E) conjugation.

Answer 2

E

Question 3

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 3

not answered

Question 4

Which of the following statements about nutritional requirements and growth of bacteria is NOT true?

A) Culture media developed for bacteria must contain a carbon source and essential elements for the survival of the bacteria.
B) Auxotrophic mutants can grow on Moderate that lacks a 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.

Question 5

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 (genotype and percent of each cotransformant) are shown below. What is the order of the genes? $$\begin{array} { l c } \text { Genotype } & \text { Cotransformants } \\a ^ { + } \text {and } b ^ { + } & 2 \% \\a ^ { + } \text {and } c ^ { + } & 0 \% \\a ^ { + } \text {and } d ^ { + } & 5 \% \\b ^ { + } \text {and } c ^ { + } & 5 \% \\b ^ { + } \text {and } a ^ { + } & 0 \% \\c ^ { + } \text {and } d ^ { + } & 0 \%\end{array}$$

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

Question 6

Which of the following statements about genetic exchange in bacteria is NOT true?

A) In conjugation there has to be a physical connection between the donor cell and the recipient cell.
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.

Question 7

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.

Question 8

How are Hfr strains of bacteria different from F⁺ strains?

A) Cells of F⁺ strains are able to transfer chromosomal genes, whereas cells of Hfr 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.

Question 9

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 ice
E) using high concentrations of DNA

Question 10

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.

Question 11

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 higher risk of giving rise to bacteria with 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 gene to genetically unrelated bacteria.

Question 12

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, which is divided into two compartments by a filter with fine pores, 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

Question 13

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⁺.

Question 14

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 a circular one.
C) In addition to a chromosome, many bacteria possess small extrachromosomal DNA called plasmids.
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.

Question 15

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

A) Hfr
B) F^-
C) F⁺
D) F´
E) F^+/-

Question 16

What are plasmids and what purposes do they serve?

Question 17

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 cell and one F^- cell
E) two Hfr cells

Question 18

Which of the following horizontal gene transfer mechanisms specifically uses time as a basic unit of mapping?

A) transformation
B) crossing over
C) transduction
D) conjugation
E) recombination

Question 19

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

A) autotrophs.
B) heterotrophs.
C) prototrophs.
D) omnitrophs.
E) auxotrophs.

Question 20

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

A) F⁺.
B) F'.
C) F^-.
D) Hfr.

Question 21

In a transformation experiment in the bacterium Streptococcus, the donor has the genotype d⁺e⁺f⁺ (order is not known). The recipient strain is d^-e^-f^- and f⁺ transformants are selected for. The complete genotype is determined for each of the f⁺ transformants and the results are shown below. What is the cotransformation frequency between f and e? $\begin{array} { l c } Genotype&Number~ of ~transformants\\\\a ^ { + } e ^ { + } f ^ { + } & 25 \\ a ^ { + } e ^ { - f ^ { + } } & 4 \\ a ^ { - } e ^ { + } f ^ { + } & 80 \\ a ^ { - } e ^ { - } f ^ { + } & 256 \end{array}$

A) 28.8%
B) 71.2%
C) 7.9%
D) 23.0%
E) 40.6%

Question 22

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

Question 23

A transformation experiment is performed with a donor strain that is resistant to four drugs: A, B, C, and D determined by genes A, B, C, and D. The recipient strain is sensitive to all four drugs. The treated recipient-cell population is divided up and plated on media containing various combinations of the drugs. A cell that is resistant to the added drug(s) will form a colony. The table below shows the results. $$\begin{array} { l l l l } \text { Drug(s) Added } & \text { Number of Colonies } & \text { Drug(s) Added } & \text { Number of Colonies } \\\hline \text { None } & 10,000 & \mathrm { BC } & 51 \\\text { A } & 1,156 & \mathrm { BD } & 49 \\\text { B } & 1,148 & \mathrm { CD } & 786 \\\text { C } & 1,161 & \mathrm { ABC } & 30 \\\text { D } & 1,139 & \mathrm { ABD } & 42 \\\text { AB } & 46 & \mathrm { ACD } & 630 \\\text { AC } & 640 & \mathrm { BCD } & 36 \\\text { AD } & 942 & \text { ABCD } & 30\end{array}$$ One of the genes is quite distant from the other three that are tightly linked. Which gene is distant from the other three?

A) A
B) B
C) C
D) D
E) A or C

Question 24

Which of the following statements is FALSE?

A) When Hfr cells are crossed with F^- cells, most of the recipient cells remain F^-.
B) The F plasmid or factor is capable of integrating into the bacterial chromosome at several different sites.
C) F⁺ cells are normally capable of undergoing conjugation with F^- and Hfr recipient cells.
D) F⁺ cells can convert F^- cells to F⁺ upon conjugation.
E) Hfr cells are capable of transferring part of the bacterial chromosome to recipient cells upon conjugation.

Question 25

Integrated, inactive phage DNA is called a:

A) progeny.
B) prophage.
C) transformant.
D) transductant.
E) conjugate.

Question 26

You are mapping five bacterial genes using cotransformation frequencies. The genes are designated A, B, C, D, and E. You perform a series of crosses and measure the percent cotransformation between all pairs of genes. The results are shown in the table below. Percent contransformation between gene pairs
$\begin{array} { l l l l l l } & A & B & C & D & E \\ A & - & & & & \\ B & 55 & - & & & \\ C & 14 & 32 & - & & \\ D & 80 & 65 & 22 & - & \\ E & 45 & 25 & 4 & 34 & - \end{array}$ What is the CORRECT order for the five genes?

A) A B C D E
B) C E A B D
C) D A B E C
D) B E A C D
E) E A D B C

Question 27

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

Question 28

What causes an F^- cell to be converted to F⁺?

Question 29

You cross an Hfr strain of ala⁺ his^- val⁺ genotype with an F^- strain of ala^- his⁺ val^- genotype and wish to recover F^- ala⁺ his⁺val^- recombinants. Which of the following media should you use that will allow the recombinants to grow and form colonies but not allow growth of the two parental strains?

A) Minimal Moderate with histidine and valine but no alanine
B) Minimal Moderate with valine but no alanine or histidine
C) Minimal Moderate with histidine and alanine but no valine
D) Minimal Moderate with no histidine, alanine, or valine
E) Minimal Moderate with alanine but no histidine or valine

Question 30

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

Question 31

(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.

Question 32

Explain the significance of horizontal gene transfer to bacterial evolution and to our ability to discern relationships between different groups of bacteria.

Question 33

(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^-).

Question 34

A transformation experiment is performed with a donor strain that is resistant to four drugs, A, B, C, and D, determined by genes A, B, C, and D. The recipient strain is sensitive to all four drugs. The treated recipient-cell population is divided up and plated on media containing various combinations of the drugs. A cell that is resistant to the added drug(s) will form a colony. The table below shows the results. $$\begin{array} { l l l l } \text { Drug(s) Added } & \text { Number of Colonies } & \text { Drug(s) Added } & \text { Number of Colonies } \\\hline \text { None } & 10,000 & \mathrm { BC } & 51 \\\text { A } & 1,156 & \mathrm { BD } & 49 \\\text { B } & 1,148 & \mathrm { CD } & 786 \\\text { C } & 1,161 & \mathrm { ABC } & 30 \\\text { D } & 1,139 & \mathrm { ABD } & 42 \\\text { AB } & 46 & \mathrm { ACD } & 630 \\\text { AC } & 640 & \mathrm { BCD } & 36 \\\text { AD } & 942 & \text { ABCD } & 30\end{array}$$ One of the genes is quite distant from the other three that are tightly linked. What is the probable order of the three tightly linked genes?

A) DAC
B) ACB
C) BDC
D) DCA
E) ADC

Question 35

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

A) conjugation.
B) induction.
C) transformation.
D) transduction.
E) infection.

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?

Question 37

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 MOST likely to be 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.

Question 38

The table below shows the results of interrupted-mating experiments with three different Hfr strains. What is the order of the genes, starting with C? $$\begin{array} { | c c | } \hline \text { Hfr strain } & \text { Order of transfer } \\\hline 1 & A , B , E , D , F \\\hline 2 & D , F , C , G , A \\\hline 3 & D , E , B , A , G \\\hline\end{array}$$

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

Question 39

HIV belongs to a group of viruses called _____ viruses.

A) dsDNA
B) ssDNA
C) ssRNA-RT
D) dsDNA-RT
E) ssRNA

Question 40

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

Question 41

To MOST readily demonstrate transformation of bacteria in the laboratory one could extract DNA from:

A) an auxotroph and add it to prototrophic cells.
B) arg^- cells and add it to arg⁺ cells.
C) strep^s (sensitive) and add it to strep^r (resistant) cells.
D) arg⁺ cells and add it to arg^- cells.
E) both arg^- and arg⁺ cells, mix them, and select for recombinants.

Question 42

In E. coli, a P1 generalized transduction was carried out using a donor strain of ara⁺ leu⁺ ilvH⁺ genotype. The recipient strain was ara^- leu^- ilvH^-. 381 ara⁺ transductants were first recovered and then the remaining genotypes of these 381 were determined and the results are shown below: $$\begin{array} { l } \mathrm { ara } ^ { + } \text {leu}^-{ ilvH }^- \quad = 32 \\\mathrm { ara } ^ { + } \mathrm { leu } ^ { + } i \mathrm { lv } \mathrm { H } ^ { - } = 9 \\\mathrm { ara } ^ { + } \operatorname { leu }^- i \mathrm { lv } \mathrm { H } ^ { + } = 0 \\\mathrm { ara } ^ { + } \mathrm { leu } ^ { + } \mathrm { llv } \mathrm { H } ^ { + } \quad = 340 \\\end{array}$$ Which gene is in the middle?

A) ara
B) leu
C) ilvH
D) ara or ilvH
E) leu or ara

Question 43

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

A) lethal
B) lytic
C) temperate
D) strict
E) lysogenic

Question 44

The human immunodeficiency virus (HIV) is what type of virus?

A) a DNA virus that can infect a type of human blood cell
B) a double-stranded RNA virus that can recombine with the genomes of similar viruses from other species
C) a single-stranded RNA virus that can undergo reverse transcription
D) a single-stranded DNA virus that uses reverse transcriptase to make a provirus
E) a double-stranded DNA virus that can infect both humans and chimpanzees simultaneously

Question 45

A recipient gal^- bio^- attl^- strain of E. coli is transduced by P1 phage from a donor wild-type gal⁺ bio⁺ attl⁺ strain. (attl^- is a mutant which can't form a l prophage because the site for l integration is missing or defective.) gal⁺ transductants are selected for by plating the recipient cells on a Moderate with galactose as the sole carbon or energy source. Replica plating and testing for lysogenic ability give the complete genotypes of 106 transductants: Class 1: $\quad \mathrm { gal } ^ { + } \mathrm { bio } ^ { - } \mathrm { attl } ^ { - } \quad 71$
Class 2: $\quad \mathrm { gal } ^ { + } \mathrm { bio } ^ { + }attl^-$ $\quad 0$
Class 3: $\quad\mathrm { gal } ^ { + }bio^-$ $a t t l ^ { + } \quad 9$
Class 4: $\quad \mathrm { gal } ^ { + } \mathrm { bio } ^ { + } \mathrm { attl } ^ { + } \quad 26$ What is the frequency of cotransduction between gal and attl?

A) 33%
B) 25%
C) 8.5%
D) 67%
E) 46%

Question 46

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 a DNA molecule.

Question 47

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

Question 48

Which explanation is one of the reasons for the ability of the influenza virus to evolve rapidly?

A) Its single-stranded DNA genome is sensitive to environmental factors that create mutations.
B) The enzyme that replicates its RNA genome is prone to make mistakes that result in mutations.
C) Its ability to integrate into the host genome allows it to remain in host cells for long periods of time.
D) It can convert its RNA genome into a DNA copy by using the enzyme reverse transcriptase.
E) It has the ability to replicate its genome both within the host cell and outside of it.

Question 49

A wild-type strain (h⁺r⁺) of phage T2 was mixed with a double mutant (hr) for host range (h) and rapid lysis (r) and used with a high multiplicity of infection to infect E. coli B. The progeny from the cross were titered (counted) on a mixture of E. coli B and E. coli B/2 strains. The following plaques were scored: hr,212; h⁺r,114; hr⁺,106; h⁺r⁺,188
What is the approximate recombination frequency for these two loci?

A) 35%
B) 24%
C) 14%
D) 11%
E) 65%

Question 50

Two different strains of a mutant phage infected a single bacterium. One phage strain is d^-e⁺ and the other is d⁺e^-. The phages go through the lytic cycle and produce progeny. What will be the recombinant genotypes in the progeny?

A) d^-e⁺ and d⁺e^-
B) d⁺e⁺ and d^-e^-
C) d^-e⁺ and d⁺e⁺
D) d⁺e⁺ and d⁺e^-
E) d^-e⁺ and d^-e^-

Question 51

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.
E) Not all RNA viruses are retroviruses.

Question 52

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
E) conjugation

Question 53

A mapping study was done with the E. coli genes cheA, cheB, eda, and supD using P1 generalized transduction. The frequency of cotransduction was determined for pairs of genes as shown below. $$\begin{array} { l c } \text { Markers } & \% \text { Cotransduction } \\\text { che A-eda } & 15 \\\text { cheA-supD } & 5 \\\text { cheB-eda } & 28 \\\text { cheB- } \sup D & 2.7 \\\text { eda-sup } & 0\end{array}$$ What is the MOST probable order for the four genes?

A) cheA-supD-eda-cheB
B) cheA-eda-cheB-supD
C) supD-cheA-eda-cheB
D) supD-cheA-cheB-eda
E) cheB-cheA-eda-supD

Question 54

In E. coli, a P1 generalized transduction was carried out using a donor strain of ara⁺ leu⁺ ilvH⁺ genotype. The recipient strain was ara^- leu^- ilvH^-. 381 ara⁺ transductants were first recovered and then the remaining genotypes of these 381 were determined and the results are shown below: $$\begin{array} { l } \mathrm { ara } ^ { + } \text {leu}^-{ ilvH }^- \quad = 32 \\\mathrm { ara } ^ { + } \mathrm { leu } ^ { + } i \mathrm { lv } \mathrm { H } ^ { - } = 9 \\\mathrm { ara } ^ { + } \operatorname { leu }^- i \mathrm { lv } \mathrm { H } ^ { + } = 0 \\\mathrm { ara } ^ { + } \mathrm { leu } ^ { + } \mathrm { llv } \mathrm { H } ^ { + } \quad = 340 \\\end{array}$$ What is the cotransduction frequency between ara and leu?

A) 8.4%
B) 58%
C) 68%
D) 92%
E) 2.4%

Question 55

Generalized transduction by phage P1 of four E. coli genes (a, b, c, and d) shows the following percentages of cotransduction: $$\begin{array} { l l l r } a - b & 51 \% & b - c & 2 \% \\b - d & 12 \% & a - d & 40 \% \\a - c & 19 \% & c - d & 79 \%\end{array}$$ What is the CORRECT order of the four genes?

A) a b c d
B) b a d c
C) d a c b
D) d c a b
E) b d a c

Question 56

Some retroviruses contain genes that can stimulate cell division and cause tumors in host cells. Such genes are called:

A) oncogenes.
B) lytic genes.
C) prophage genes.
D) capsid genes.
E) envelope genes.

Question 57

Plaques produced by the temperate phage lambda on E. coli are turbid, since some bacteria remain viable within the plaque area. Plaques produced by the virulent phage T4 are usually clear. Which statement is the BEST explanation for this difference?

A) Lambda can propogate via the lytic or lysogenic cycle.
B) T4 carries a smaller DNA genome than lambda.
C) Recombination occurs more frequently between lambda genomes than T4 genomes.
D) Lambda often carries a portion of the sex factor.
E) T4 will not infect E. coli cells that are growing.

Question 58

A new strain of H1N1 influenza virus that can infect humans consists of RNA parts from which three strains of influenza virus?

A) swine, human, and chimpanzee
B) bird, human, and monkey
C) human, bird, and chimpanzee
D) human, bird, and swine
E) human, monkey, and chimpanzee

Question 59

Which of the following statements concerning the role of reverse transcriptase in the life cycle of certain RNA viruses is FALSE?

A) It is produced from the pol gene.
B) It makes DNA from an RNA template.
C) It is essential for producing a provirus.
D) It is part of the life cycle of retroviruses.
E) It is very accurate and leads to a low mutation rate.

Question 60

Two mutations that affect plaque morphology in a particular phage (a^- and b^-) have been isolated. Phages carrying both mutations (a^-b^-) are mixed with wild-type phage (a⁺b⁺) and added to a culture of bacterial cells. Following infection and lysis, samples of the phage lysate are collected and cultured on bacterial cells. The following numbers of plaques are observed: $$\begin{array}{l}\begin{array} { l r } \text { Plaque Phenotype}&{ Number }\\a ^ { + } b ^ { + } & 2043 \\a ^ { + } b & 320 \\a ^ { - } b ^ { + } & 357 \\a^- b ^ { - } & 2134\end{array}\end{array}$$ What is the frequency of recombination between the a and b genes?

A) 7%
B) 14%
C) 42%
D) 28%
E) 36%

Question 61

HIV has a high mutation rate. What causes this, and how might this be advantageous to the virus?

Question 62

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:

a. What kind of mapping is this called?
b. Which gene is in the middle?
c. Which of the outside genes is closer to the middle gene?

Question 63

Outline the steps involved in mapping bacterial genes by generalized transduction.

Question 64

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

Question 65

What is the difference between specialized and generalized transduction?

Question 66

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?

Question 67

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 0 normal, wild-type plaques.
a. What is the recombination frequency between the r locus and the b locus?
b. How can you explain the results?

Question 68

How does a virulent phage differ from a temperate phage?

Question 69

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: How are the auxotrophy genes organized on the bacterial chromosome?

Question 70

(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.

Question 71

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?