Exam 3: Bacterial Genetics

Question 1

Describe advantages of using Bacteria as a model system.

Answer 1

1. Reproduction is rapid
2. Many progeny are produced
3. Monoploid (i.e. haploid), so it’s easy to identify recessive mutations because they will present phenotypically
4. Asexual reproduction means it’s easy to get a colony of identical cells
5. Genomes are small
6. Can be grown in the laboratory and take up little space
7. Easy to isolate and manipulate their genes
8. Medical importance: bacteria can cause disease
9. Can be genetically engineered to produce substances of commercial value (human gene for insulin inserted into bacterial cells)
10. Genetically efficient: around 90% of the genome is directly protein coding

Question 2

Contrast theta replication and rolling replication.

Answer 2

Theta: Bidirectional replication of a plasmid; both strands serve as template

Rolling: Unidirectional; one strand serves as template

Question 3

Compare and contrast conjugation, transformation, and
transduction.

Answer 3

Conjugation: Requires a donor and recipient. Genetic information from one bacterium is transferred to another. Recombines at independent locations to become wild-type cells. After conjugation, prototrophs can even result from two auxotrophs.

Transformation: Uptake and incorporation of random DNA molecules. Donor is dead.

Transduction: Involves phages as DNA carriers, similar to transformation.

Question 4

Analyze data from experiments like those of Lederberg and
Tatum, and explain how it indicated that some strains of bacteria can transfer genetic material.

Answer 4

Lederberg and Tatum used two strains of Escherichia coli (E. coli): one strain was auxotrophic (unable to synthesize certain essential nutrients), and the other strain was prototrophic (able to synthesize all essential nutrients).

They mixed these two strains on a growth medium, where the prototrophic strain could grow and support the auxotrophic strain’s growth.

They observed that some auxotrophic cells turned into phototrophic cells over time, indicating that genetic material was being transferred between the strains. This genetic transfer led to the recombination of genetic material and the restoration of the ability to synthesize essential nutrients.

Question 5

Prototrophic strains can survive on

Answer 5

minimal media

(Nutritional components simple; Organic carbon source and inorganic ions)

Question 6

Auxotrophic strains require

Answer 6

supplements

(Amino acids are added as supplements to minimal medium. Colonies have mutation on gene that encodes the synthesis of an essential nutrient.)

Question 7

Replica plating identifies auxotrophs

Answer 7

1. Place bacteria on medium containing leucine (both leu+ and leu- colonies grow)
2. Replica plate the colonies by pressing a velvet surface to the plate
3. Press onto two petri plates, one containing leucine and one not
4. The one lacking leucine will only produce leu- phototrophs. Missing colony will allow areas with auxotrophs to also be isolated.

Question 8

Typically, bacteria cells contain

Answer 8

One circular chromosome and many smaller circular plasmids

Question 9

Define horizontal gene transfer

Answer 9

Transfer of genetic information between related but distinct species. Plays significant role in evolution of bacteria. Can occur within same generation.

Question 10

Define vertical gene transfer

Answer 10

Transfer of genetic information between between members of SAME species generationally.

Question 11

What is the function of the sex pilus?

Answer 11

The sex pilus is a thin, hair-like appendage on the surface of certain bacterial cells, and its main function is to facilitate the transfer of DNA between bacteria.

Question 12

Describe the process of bacterial conjugation

Answer 12

a) Cytoplasmic connection between F+ donor bacterium containing F factor and F- recipient bacterium.

b) A singleDNA strand of the F factor is nicked and replaced via rolling replication.

c) Nicked DNA passes to recipient and is replicated.

d) Recipient is now F+

Episomes can exist independently as plasmids or can integrate (insert) onto the larger chromosome.

Question 13

What is an Hfr cell?

Answer 13

A high frequency recombination (Hfr) cell occurs when the F factor integrates into the bacterial cell. Hfr cells will undergo conjugation more frequently than regular cells.

Question 14

Describe bacterial conjugation in an Hfr cell

Answer 14

The Hfr chromosome which is the F factor along with bacterial genes is nicked and replaced via rolling replication.

The host chromosome can integrate the recipient DNA.

Question 15

Conjugation of an F’ cell and an F- cell will create a ______ cell. Conjugation of an Hfr cell and an F- cell will result in a ______ cell.

Answer 15

F’ and F’

Hfr and F-

Question 16

Imprecise excision of the F plasmid
creates an F’ (F prime) cell

Answer 16

Conjugation between an F’ cell and an F- cell results in a “partial diploid” F’ cell

Question 17

Results of conjugation between cells with different F factors

F+ x F-

Hfr x F-

F’ x F-

Answer 17

2 F+ cells

One Hfr cell and one F- cell… no change

2 F’ cells

Question 18

Bacterial Transformation

Answer 18

Transformation is different than conjugation, because in transformation, the donor is dead!

Requires free, naked DNA (from a dead donor cell) and “competent” recipients cells.

Electric shock or heat shock of the competent cells and naked DNA results in uptake of naked DNA into the bacterial cell.

Can change the recipient’s phenotype if donor DNA differs at the alleles that are transformed and integrated.

Question 19

Describe the lytic and lysogenic cycles

Answer 19

Lytic Cycle:
Virus attaches to the host cell’s surface and injects its genetic material into the host cell. New viral particles are assembled from the replicated components within the host cell. As the new viruses are assembled, they burst (lyse) the host cell, destroying it in the process. This releases a large number of newly formed viral particles, which can go on to infect other cells.

Lysogenic Cycle:
Similar to the lytic cycle, the virus attaches to the host cell’s surface and injects its genetic material into the host cell. In the lysogenic cycle, the viral genetic material integrates into the host cell’s genome. This integrated viral DNA is known as a prophage in the case of bacteriophages (viruses that infect bacteria). The host cell divides and replicates with the integrated viral DNA, causing all progeny cells to contain the viral genetic material. At some point, the lysogenic cycle may transition to the lytic cycle.

Question 20

Define a prophage

Answer 20

The genetic material of a bacteriophage, incorporated into the genome of a bacterium and able to produce phages if specifically activated.

Question 21

Define bacterial transduction

Answer 21

Transfer of DNA from one bacterial cell to another, via a phage acting as a vector.

Question 22

Analyze a Davis U-tube experiment. What results of this experiment can support the different transfer mechanisms? Which would allow for transduction, transformation, conjugation, etc.?

Answer 22

-If you break cytoplasmic bridges you’ll stop conjugation, so if there is a filter and it prevents horizontal gene transfer, you can’t have conjugation. If the U-tube setup allows for direct physical contact between the two bacterial strains (no barrier or filter) and genetic transfer occurs, this would support the mechanism of conjugation. Conjugation requires cell-to-cell contact for the transfer of genetic material.

-If one of the bacterial strains in the U-tube is lysed, and its DNA is released into the medium, and the other strain takes up and incorporates the DNA into its genome, this would support the mechanism of transformation.

-Transduction is a mechanism of genetic transfer mediated by bacteriophages (viruses that infect bacteria). If a bacteriophage infects one of the bacterial strains in the U-tube and then transfers genetic material from that strain to the other strain, this would support the mechanism of transduction.

Question 23

Define plaques

Answer 23

A clear area on an otherwise opaque field of bacteria that indicates the inhibition or dissolution of the bacterial cells by some agent, either a virus or an antibiotic.

Question 24

What are the key differences between the lytic and lysogenic cycle?

Answer 24

Outcome:
The lytic cycle leads to the destruction of the host cell, whereas the lysogenic cycle integrates the viral DNA into the host’s genome without immediate cell lysis.

Integration of Genetic Material:
In the lysogenic cycle, the viral DNA becomes a part of the host’s genome, whereas in the lytic cycle, it remains separate from the host genome.

Virulent phages are restricted to lytic infections, while temperate phages can undergo either.

Question 25

Define a complete medium

Answer 25

When amino acids are added as supplements to minimal medium so auxotrophs can survive.

Question 26

F+

F-

Hfr

F’

Answer 26

Present as separate circular DNA
Donor in conjugation

Absent
Recipient in conjugation

Present, integrated into the bacteria’s chromosome
High-frequency donor in conjugation

Present as separate circular DNA carrying some bacterial genes
Donor

Question 27

True or false: Bacterial mutants that require supplemental nutrients in their growth media are called prototrophs

Answer 27

False

Question 28

True or false: Transduction is a form of recombination in bacteria that involves bacteriophages.

Answer 28

True

Question 29

True or false: In general, bacteria are grown in the laboratory in liquid or on semisolid (agar) media

Answer 29

True

Question 30

True or false: Bacteria that are in a particular physiological state to become transformed are called competent

Answer 30

True

Question 31

True or false: A bacteriophage is an auxotrophic type of bacterium

Answer 31

False