Lecture 15 Phage Lambda

Question 1

Lytic cycle

Answer 1

Late genes code for coat proteins and lytic enzymes to complete viral replicates and release them from the cell (lysis)

Question 2

Temperate phages on Petri dishes

Answer 2

Bacteriophage lambda is a temperate phage it can choose between lysogeny or virulence (lytic pathway)

usually it chooses lysogeny which produces dark turbid plaques on a Petri dish

If it chooses to be lytic it will create a clear plaque - literally a hole in the bacterial later (“lawn”) on Petri dish

Question 3

Lysogeny

Answer 3

Integration of phage DNA into host chromosome creating hybrid called prophage. Requires enzyme integrate encoded by an early gene. Lambda has evolved an attachment region that matches host bacterium (E. Coli) DNA which helps to find recombination site.
Integrate lines up regions, catalysed dsDNA break and joins in phage DNA forming heteroduplex.

Question 4

How was lysogeny discovered?

Answer 4

Study of mutants

Usually lysogeny preferred so hybrid plaques more common

Some mutants always have clear plaques (no lysogeny only lytic)

Mutants found to have mutations in one of 3 genes encoding proteins named cl cll and clll (clear 1,2 & 3)

A mutation in 1 of the 3 clear genes causes a phage to always choose lytic cycle

so those genes must promote lysogeny or prohibit lytic cycle in wild type

Question 5

Phage Lambda has all it’s essential genes arranged in 2 operons

Answer 5

Leftwards operon codes lysogeny

Rightwards operon