Chapter 3: Bacterial Genetics

Question 0

Transformation - defn

Answer 0

competent cells have the ability to take up naked DNA from the environment (such as released from bacteria during lysis). DNA can then be incorporated into the recipient’s genome if there is enough homology between strands (so transformation is usually seen between closely related or same species)

Question 1

What are 4 ways in which bacteria are able to exchange genes?

Answer 1

1) transformation
2) transduction
3) conjugation
4) transposition

Question 2

Transduction - defn

Answer 2

Bacteriophage (a virus that infects bacteria) carries a piece of bacterial DNA to another

Question 3

What are the two types of transduction?

Answer 3

generalized

specialized

Question 4

Describe the process of generalized transduction.

Answer 4

a “packaging” event.

1) A lytic/virulent phage infects the bacterium. Phage DNA is transcribed, replicated, and translated into capsids and enzymes.
2) Cleavage of bacterial DNA, but some pieces left intact.
3) Capsids are translated and packed. Parts of bacterial DNA may become packaged in viral capsid.
4) Cell lysis occurs, liberating phages to infect another bacterium and transfer bacterial genes.

Question 5

What type of phage is associated with generalized transduction?

Answer 5

Lytic (virulent) phage

Question 6

What type of phage is associated with specialized transduction?

Answer 6

Lysogenic (temperate) phage

Question 7

Describe the process of specialized transduction.

Answer 7

= an “excision” event

1) Lysogenic (temperate) phage infects bacterium. Phage DNA incorporates into bacterial chromosome (now called a prophage).
2) Normally the prophage just waits doing nothing, but can eventually become active.
3) Prophage DNA, if becomes active, is spliced out of the bacterial chromosome and replicated, translated, and packaged into a capsid. Some flanking bacterial genes may be excised with it and packaged with the phage DNA.
3) Cell lysis occurs, liberating phages to infect another bacterium and transfer bacterial genes.

Question 8

Genes for which 5 bacterial toxins are encoded in a lysogenic phage?

Answer 8

ABCDE

1) ShigA-like toxin
2) Botulinum toxin (certain strains)
3) Cholera toxin
4) Diphtheria toxin
5) Erythrogenic toxin of Strep. pyogenes

Question 9

Conjugation - defn

Answer 9

DNA is transferred directly by cell to cell contact, and can occur between unrelated bacteria

Question 10

Which mechanism of bacterial genetic exchange is the major mechanism for the transfer of antibiotic resistance?

Answer 10

conjugation

Question 11

What does the F plasmid encode?

Answer 11

enzymes and proteins that form the sex pilus

Question 12

The bacterial sex pilus is encoded by what? And is responsible for what process?

Answer 12

F plasmid; conjugation

Question 13

What process is the F plasmid important for?

Answer 13

conjugation

Question 14

What are F+ cells?

Answer 14

Bacteria that carry F plasmids

Question 15

What are F- cells?

Answer 15

Bacteria without the F plasmid

Question 16

Describe the process of F+ x F- conjugation.

Answer 16

1) F+ plasmid contains genes to encode formation of the sex pilus, which protrudes from the cell surface of the donor F+ bacteria
2) Sex pilus binds to and penetrates the cell membrane of the recipient F- bacteria, forming a conjugal bridge.
3) Nuclease breaks off one strand of the F+ plasmid DNA, and this single strand passes through the sex pilus to the F- bacterium
4) Conjugal bridge breaks down. DNA replication occurs so the single strand is now double stranded F plasmid circular DNA. The recipient F- cell is now F+.
.

Question 17

What is transferred across the F+ x F- sex pilus / conjugal bridge? Does chromosomal DNA get transferred?

Answer 17

F plasmid is transferred. No transfer of chromosomal genes.

Question 18

What is Hfr?

Answer 18

High frequency of chromosomal recombination. F plasmid can become integrated into the bacterial chromosome (as in lysogenic specialized transduction)

Question 19

Describe the process of Hfr x F- conjugation.

Answer 19

1) Formation of sex pilus and transfer of F+ plasmid to F- cell.
2) F plasmid becomes incorporated into bacterial chromosomal DNA of recipient cell - recipient is now called an Hfr cell.
3) Integrated F plasmid is excised but contains some flanking chromosomal DNA - these new plasmids are F’ plasmids. F’ plasmids are transferred across conjugal bridge.
4) (Alternatively the entire circular bacterial chromosome + integrated F plasmid can undergo replication and be transferred from the Hfr to the recipient cell.)

Question 20

What is transferred across the Hfr x F- sex pilus / conjugal bridge? Does chromosomal DNA get transferred?

Answer 20

Transfer of both plasmid and chromosomal genes.

Question 21

What are transposons?

Answer 21

mobile genetic elements, segments of DNA that can “jump” (via excision and integration) from one location to another.
Can transfer genes from plasmid to bacterial chromosome to DNA of phages and vice versa.

Question 22

Describe the process of transposition.

Answer 22

1) Transposons are integrated into the DNA of phages, plasmids, bacterial chromosomes. They do not replicate independently but are copied during their host’s DNA transcription.
2) Transposons are excised, but frequently includes some flanking DNA (e.g. chromosomal DNA).
3) Transposon is reintegrated and incorporated (e.g. into plasmid or phage).
4) Transposon is transferred to another bacterium by conjugation or transduction.

Question 23

What is the clinical significance of transposons?

Answer 23

A transposon gene can confer a particular drug/antibiotic resistance (on the R plasmid), which can move to the plasmids of different bacterial genera, resulting in the rapid spread of resistant strains.