A bacteriophage is a virus that infects bacteria. Lambda phage is a temperate bacteriophage that has two life cycle choices: lytic and lysogenic. During lysogeny, the lambda repressor binds to the operator region (OR) on the phage DNA and represses transcription of lytic genes, allowing the phage genome to remain dormant as a prophage integrated into the bacterial chromosome.

3. A bacteriophage is a virus that
infects and replicates within
bacteria.
Eg: Lambda phage, T4, T7
Introduction

13. • The mRNA transcript produced from the PR promoter does not go beyond PR' at this stage.
For the rest of the cycle, lambda phage has two choices: Lytic & Lysogenic

14. Mechanism of Molecular switching
• During the lysogeny phase CI is switched
ON and cro is OFF.
• The operator OR is constituted of three
binding sites ORI, ORII and ORIII
• During the lysogenic state, the lambda
repressor at OR is bound at ORI and ORII,
sites adjacent to each other.
• At these sites, the repressor represses
the right ward transcription from the
promoter PR.
• The expression of cro and other lytic
genes is therefore turned OFF.

15. Important Definitions – 2 marks
• virulent phage – A bacteriophage that can only follow the lytic cycle.
• prophage – A phage genome covalently integrated as a linear part of the bacterial chromosome.
• lysogeny – The ability of a phage to survive in a bacterium as a stable prophage component of the bacterial genome
• temperate phage – A bacteriophage that can follow the lytic or lysogenic pathway.
• integration – Insertion of a viral or another DNA sequence into a host genome as a region covalently linked on either side to
the host sequences.
• excision – Release of phage from the host chromosome as an autonomous DNA molecule.
• induction of phage – A phage’s entry into the lytic (infective) cycle as a result of destruction of the lysogenic repressor, which
leads to excision of free phage DNA from the bacterial chromosome.
• plasmid – Circular, extrachromosomal DNA. It is autonomous and can replicate itself.
• episome – A plasmid able to integrate into bacterial DNA.

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