Chapter 8: Microbial Genetics - Horizontal Gene Transfer

Question 1

What are plasmids?

Answer 1

• self-replicating dsDNA molecules

Question 2

What do plasmids contain?

Answer 2

• non-essential genes (not needed for survival)
  ex. genes for toxin production

Question 3

What are F plasmids? (2)

Answer 3

• fertility factors
• carry genes to make F pilus (sex pilus)

Question 4

Where are F plasmids involved in?

Answer 4

conjugation

Question 5

What are R plasmids? (2)

Answer 5

• resistance factors
• carry genes for antibiotic resistance

Question 6

What are vir plasmids? (2)

Answer 6

• virulence factors
• carry genes for toxin production

Question 7

What are three methods of horizontal gene transfer in bacteria?

Answer 7

1. Transformation
2. Transduction
3. Conjugation

Question 8

What occurs in transformation? (4)

Answer 8

• pieces of “naked” DNA are taken up by a bacterial cell
• these pieces can then be integrated into the chromosome
• recombination
• can then be passed to progeny and become a stable part of the genome

Question 9

Where can transformation occur?

Answer 9

• from dead cells, or released from plasmids

Question 10

In lecture, what occurs in transformation?

Answer 10

free DNA is incorporated into chromosomal DNA

Question 11

What is transduction?

Answer 11

• small fragments of DNA transferred between bacteria by bacteriophage

Question 12

What are bacteriophages?

Answer 12

viruses that infect bacteria

Question 13

What are the steps in transduction? (5)

Answer 13

1. Phage attaches to bacterial cell wall
2. Injects its DNA into the cell
3. Phage DNA is replicated inside bacterial cell wall
4. Phage DNA also encodes enzymes that cut the bacterium’s DNA into fragments
5. As new phages are being assembled, some accidentally receive a piece of bacteria DNA instead of phage DNA, creating a transducing particle

Question 14

In lecture, what are the most important microbes in the biosphere?

Answer 14

• DNA phages

Question 15

Are viruses living?

Answer 15

no

Question 16

Once the transducing particle is made, what can it do? (4)

Answer 16

• carry bacterial genes to another cell
• Injection mchanism is still fully functional
• But only bacterial DNA is injected
• then if the injected DNA recombines with the existing xme, it becomes a stable genetic element
  ie. will be passed to progeny

Question 17

What is conjugation? (3)

Answer 17

• bacterial mating
• common in Gram -
• mediated by genes encoded on an F factor

Question 18

How does conjugation occur? (4)

Answer 18

1. Donor cell (F+) forms an f pilus and uses it to attach to a recipient cell F-)
2. pilus retracts to bring cells together
3. the donor cell replicates the F factor as a copy is passed to the recipient
4. The recipient becomes an F+ cell