Bacteriophage lambda I

Question 1

What are bacteriophages ?

What are their main functions ?

Answer 1

Bacteriophages = acterial viruses
Minimal functions :
- Protection of nucleic acid
- Delivery of nucleic acid
- Conversion of infected cell to produce phage
- Release of phage

Question 2

What are the main 2 phage structure types ?

Answer 2

Icosahedral head w/ a tail (e.g. lambda)

Filamentous

Question 3

Describe the life cycle of a phage undergoing lytic growth.

Answer 3

Infection : phage attaches to bacterium, DNA injected into cell
Early dev : enzymes for synthesis are made, replication begins, replication begins
Late dev : genomes. heads + tails are made, DNA packaged into heads, tails attached
Lysis : cell broken to release progeny phages

Question 4

Describe the life cycle of a phage undergoing lysogenic growth.

Answer 4

Infection
Phage DNA is integrated into bacterial genome, bacteria live happily ever after
Lysogenic bacterium is immune to further infection
Induction : Phage DNA is released and enters lytic cycle (e.g. after exposure to UV light)

Question 5

What is a prophage ?

Answer 5

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

Question 6

What is a temperate phage ?

Answer 6

Temperate refers to the ability of some bacteriophages (notably coliphage λ) to display a lysogenic life cycle.

Question 7

What is a virulent phage ?

Answer 7

A bacteriophage that causes the destruction of the host bacterium by lysis.

Question 8

What is induction ?

Answer 8

Upon detection of host cell damage, such as UV light or certain chemicals, the prophage is excised from the bacterial chromosome in a process called prophage induction. After induction, viral replication begins via the lytic cycle.

Question 9

What is phage immunity ?

Answer 9

The incoming phage can inject its DNA into the cell, but the DNA is immediately repressed and no transcription of genes or translation of phage proteins initiates. Therefore, lambda lysogens are immune to infection by other lambda phage particles.

Question 10

What favors lysogeny ?

Answer 10

• high MOI (Multiplicity of Infection)

- low levels if nutreitns

Question 11

Plques can appears clear or turbid on an agar plate.

What does this mean ?

Answer 11

Clear –> lysis –> lytic cycle

Turbid –> cells present –> lysogeny

Question 12

How long is the bacteriophage lamdba genome ?

How many genes does it contain ?

Answer 12

• Linear dsDNA, 48,502 bp
• 12 bp cohesive ends
• 46 genes identified

Question 13

Descrivbe the cascade controlling lytic development.

Answer 13

Early :
- phage genes are transcribed by host RNA polymerase
- regulator gene(s) : RNA Pol, sigma factor or antitermination factor
Middle :
- early product causes transcirption of middle genes
- regulator gene(s) : sigma factor or antitermination factor
- structural genes : replication enzymes etc.
Late :
- middle product causes transcription of late genes
- structural genes : phage components

Question 14

What are the key point of the lytic cascade ?

Answer 14

• The early genes are transcribed by host RNA polymerase
• These include regulators required for delayed early expression
• Delayed early expression includes regulators for transcription of the late genes
• Expression of groups of genes occurs in an ordered manner

Question 15

Which genes are expressed during the lytic life cycle ?

In what order ?

Answer 15

Early : host RNA Pol transcribes N and cro from Pl and Pr
Delayed early : pN permits transcription from same promoters to continue past N and cro
Late : Transcription at Pr (between Q and S) and pQ permits it to continue through all genes

Question 16

What are the key point of gene expression during the lytic life cycle of lambda ?

Answer 16

• N is an anti-termination factor that allows RNA Pol to continue past a terminator
• Q is the product of a delayed early gene and allows anti-termination to transcribe the late genes
• lambda DNA circularizes after infection; the late
  genes form a single transcription unit

Question 17

How does lambda DNA replication occur ?

Answer 17

Via a rolling circle mechanism :

• initiation occurs on one strand, with a nick at the origin
• elongation of growing strand dosplaces old strand
• after one revolution the displaced strand reaches unit length
• continued elongation generates a displaced strand of multiple unit lengths

Question 18

During the lytic cascade, what genes are expressed at the :

• immediately early stage ?
• delayed early stage ?
• late stage ?

Answer 18

Immediately early : 
- cro = -ve regulator
- N - antiterminator
Delayed early : 
- CII, CIII regulators
- 7 recombination genes
- 2 replication genes
- Q = antiterminator
Late : 
- 10 head genes
- 11 tail genes
- 2 lysis genes