Lysogenic Phage Life Cycle

Question 1

Can all phages do the lytic cycle?

Answer 1

Yes

Question 2

Can all phages do the lysogenic cycle?

Answer 2

No

Question 3

What is lysogeny?

Answer 3

The process of a temperate phage integrating into the host genome

Question 4

Does the phage get replicated when it’s integrated into the host genome?

Answer 4

No new phage get produced, but the viral DNA replicates every time the host divides

Question 5

What is a prophage?

Answer 5

The viral DNA inside the host genome

Question 6

What is a lysogen?

Answer 6

Bacterial cell with a prophage integrated into its genome

Question 7

How does having a prophage alter the behaviour of Vibrio cholerae?

Answer 7

Only cells that have an integrated prophage cause cholera. The toxin gene is actually a viral gene and not a bacterial gene

Question 8

Why would the lysogenic life cycle be advantageous to a virus?

Answer 8

Allows it to spread throughout the bacterial population without killing their hosts

Question 9

What are the 9 steps of the lysogenic life cycle?

Answer 9

1. Adsorption
2. Infection
3. Viral DNA circularizes
4. Decision
5. Expression of integrase and early gene repressor
6. integration
7. Host replicates
8. Induction
9. Lytic cycle

Question 10

Why does the λ phage genome circularize once its in the cell?

Answer 10

COS sites - complementary sticky ends

Question 11

What happens during the decision step of the lysogenic life cycle?

Answer 11

The phage “decides” to undergo the lytic or lysogenic cycle

Question 12

What would cause a temperate phage to undergo the lytic cycle?

Answer 12

If the host cell was unhealthy, stressed, or starving

Question 13

What would cause a temperate phage to undergo the lysogenic cycle?

Answer 13

If the host cell is healthy and not stressed

Question 14

How does a phage “decide” to undergo the lytic or lysogenic cycle?

Answer 14

It can sense the cell’s environment and detect chemicals that indicate stress

Question 15

Will plating bacteria on complete media favour the lytic or lysogenic cycle?

Answer 15

Lysogenic

Question 16

What happens during the lysogenic gene expression step of the lysogenic life cycle?

Answer 16

The phage expresses an early gene repressor protein to block the lytic cycle and expresses integrase

Question 17

What happens during the integration step of the lysogenic life cycle?

Answer 17

The viral DNA is integrated into the host genome through site specific recombination

Question 18

What does integrase do?

Answer 18

Catalyzes site specific recombination between attP and attB sites to integrate the phage DNA into the genome

Question 19

Does the bacterial genome have an attB or an attP site?

Answer 19

attB

Question 20

Does the phage genome have an attB or an attP site?

Answer 20

attP

Question 21

Why doesn’t the bacterial chromosome break when the phage becomes integrated?

Answer 21

The phage DNA is also circular. If it was linear it would break the chromosome

Question 22

What happens during the host cell replication step of the lysogenic life cycle?

Answer 22

The bacterial cells replicate as per usual, and it also replicates the phage DNA and it gets passed onto the daughter cells

Question 23

What happens during the induction step of the lysogenic life cycle?

Answer 23

The prophage cuts itself out of the genome and undergoes the lytic cycle

Question 24

How does the prophage know when to cut itself out of the genome?

Answer 24

λ recognizes complexes of RecA and single stranded DNA that results from DNA damage, which causes the early gene repressor protein to be degraded

Question 25

Why does the phage need both integrase and excissionase to cut itself out of the bacterial genome?

Answer 25

Integrase catalyzes the recombination, but can’t recognize the hybrid attP/B sites. Excissionase recognizes those and tells integrase where to cut

Question 26

What is a virulent phage?

Answer 26

A phage that can only undergo the lytic cycle

Question 27

What is a temperate phage?

Answer 27

A phage that can undergo either the lytic or lysogenic life cycle