Lytic Phage Life Cycle

Question 1

What are the 7 steps of the lytic life cycle?

Answer 1

1. adsorption
2. infection
3. early gene expression
4. viral genome replication
5. late gene expression
6. assembly of new phages
7. lysis

Question 2

What is adsorption?

Answer 2

The phage binding to its host cell

Question 3

What happens during the adsorption step of the lytic life cycle?

Answer 3

The phage receptor protein recognizes and binds to a particular molecule on the surface of the host cell

Question 4

What host cell surface molecule will be recognized by the phage?

Answer 4

Depends on the phage. Can be LPS sugars, transport proteins, pili

Question 5

What happens during the infection step of the lytic life cycle?

Answer 5

The phage genome crosses the membrane and enters the host cell while the protein coat stays outside

Question 6

How does T4 use its tail to get its DNA inside a cell?

Answer 6

Once bound to the host, the tail fibres pull up and retract, and the phage “sits down” on the outer membrane. The tail pins tightly bind the membrane, and T4 lysozyme in the phage neck chews a hole through the PG. The tail sheath then retracts and it injects its DNA through the hole it made into the cell

Question 7

How do phages without tails get their genome into a cell?

Answer 7

Hijack membrane proteins

Question 8

What happens during the early gene expression step of the lytic life cycle?

Answer 8

Genes with promoters that look like E coli promotors will trick E coli RNA poly into transcribing them and will transcribe and translate proteins involved in initiating the replication process

Question 9

What are the 4 things expressed during early gene expression?

Answer 9

1. Proteins that repress E coli gene function
2. Enzyme that cuts up the E coli genome
3. Any enzymes needed for replication
4. Transcription factors for the late genes

Question 10

What happens during the replication step of the lytic life cycle?

Answer 10

The virus uses host proteins and a few of its own to replicate its own DNA about 100 times

Question 11

What happens during the late gene expression step of the lytic life cycle?

Answer 11

Genes with viral promotors are expressed to make the coat proteins and lysis enzymes

Question 12

What needs to have been expressed for late genes to be expressed?

Answer 12

Transcription factors and viral RNA polymerase that were expressed by the early genes

Question 13

What are the 4 things expressed by the late genes?

Answer 13

1. Coat proteins
2. Lysis proteins
3. Early gene repressors
4. Packaging proteins

Question 14

Why do the late genes express early gene repressors?

Answer 14

Shifts all energy and resources in the cell towards the late genes

Question 15

What happens during the morphogenesis step of the lytic life cycle?

Answer 15

The new phage particles get assembled

Question 16

How does T4 build its capsid?

Answer 16

Builds a scaffold and assembles the capsid on there. Motor proteins then use host ATP to shove the DNA inside. Once inside, the scaffold and motor are removed

Question 17

How does T4 build its tail?

Answer 17

It spontaneously assembles through protein interactions

Question 18

What happens during the lysis step of the lytic life cycle?

Answer 18

Phage enzymes become activated and disrupt both the cell membrane and cell wall to cause the host cell to lyse and release the new phage particles

Question 19

What are the 2 lysis enzymes used by T4? What do they do?

Answer 19

Holin: pokes holes in the membrane
Lysozyme: destroys all the PG

Question 20

What promotors do the early genes use?

Answer 20

Host promotors

Question 21

What promotors do the late genes use?

Answer 21

Viral promotors

Question 22

How is phage gene expression controlled by a regulatory cascade?

Answer 22

Early genes are expressed first, which activate expression of the late genes. The late genes then repress activation of the early genes to put all energy into expressing the late genes

Question 23

What is the burst size?

Answer 23

Average number of phage released per infected bacterial cell

Question 24

What determines the burst size of a phage?

Answer 24

Very specific to the host-virus interaction

Question 25

What is the typical burst size for a phage undergoing the lytic cycle?

Answer 25

70-100 phage

Question 26

What is a lysate?

Answer 26

Bacteriophage stock where every host cell has been lysed