Lytic and Lysogenic life cycle of Bacteriophages

1. Bacteriophages
Dr. Deepa M.A
Assistant Professor of Botany
Government Arts College (Autonomous)
Coimbatore – 641 018
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WVqMIngbDA
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2. https://commons.wikimedia.org/wiki/File:Phage.png
A bacteriophage is a virus which infects bacteria.

3. Life Cycle:
1. Lytic cycle – T4 and T7
2. Lysogenic cycle – Lambda and P1, M13
and X174

4. Bacteriophages: landmarks in molecular biology
• 1939 one-step growth of viruses
• 1946 Genetic recombination
• 1947 Mutation & DNA repair
• 1952 DNA found to be genetic material, restriction &
modification of DNA
• 1955 Definition of a gene
• 1958 Gene regulation, definition of episome
• 1961 Discovery of mRNA, elucidation of triplet genetic
code, definition of stop codon
• 1964 Colinearity of gene and polypeptide chain
• 1966 Pathways of macromolecular assembly
• 1974 Vectors for recombination DNA technology
Source: Principles of Virology. Flint et al, 2000.

5. • T4 is a virus which infects E.Coli, a bacteria that has been
used extensively for molecular biology research.
• The bacteriophage T4 exemplifies the life cycle of viruses.
• It exists as an inactive virion until one of its extended 'legs'
comes into contact with the surface of an E. Coli.
• Sensors on the ends of its 'legs' recognize binding sites on the
surface of the host's cell, and this triggers the bacteriophage
into action.

6. • The bacteriophage binds to the surface of the host, punctures
the cell with its injection tube, and then injects its own genetic
blueprint.
• This genetic information subverts the host cell's normal
operation and sets the cell's biosynthetic machinery to work
creating replicas of the virus.
• These newly created viruses escape from the cell and then
float about dormant until one happens to come into contact
with a new host cell.

7. T4 phage

8. In nature, the bacteriophage T4 contains about 168,800 bp of ds
DNA. This genetic blueprint contains all of the necessary
information to create new bacteriophage T4.

9. T7 - Phage
https://commons.wikimedia.org/wiki/File:Str
ucture_of_phage_T7.png

10. • A lytic virus; infects E. coli
• Life cycle ~ 30 min at 30°C
• Genome (40kbp), 55 genes, 3 classes
https://commons.wikimedia.org/wiki/File:T7
_phage_genome.png
https://commons.wikimedia.org/wiki/File:T7
_phage_genome.png

11. T7 genome programs a dynamic infection process
Class
I
Class
II
Class
III
Genome
T7 RNA expression,
host interference
Gene functions
host DNA
digestion, T7 DNA
replication
T7 particle
formation, DNA
maturation and host
lysis

13. Phage lambda
https://en.wikipedia.org/wiki/Lambda_phag
e#/media/File:Lambda_EM.jpg

14. Lambda phage is a bacterial virus that infects E. coli, and
depending on early events (and genetics), can either multiply
within cells leading to cell lysis, or the viral DNA can integrate
into the bacterial genome in a process called lysogeny.
If lambda gene transcription is activated, then lytic infection
occurs leading to phage release and infection of nearby E. coli
cells.
Alternatively, if lambda gene transcription is repressed, then the
infection is lysogenic resulting in lambda DNA integration into
the E. coli genome.
Applied Molecular Genetic methods used in the labs are usually
designed to take advantage of cell lysis.

15. https://commons.wikimedia.org/wiki/File:La
mbda_phage.jpg

16. https://upload.wikimedia.org/wikipedia/com
mons/5/51/Prophage_SVG.svg

17. Applications in microbial genetics
• Insertion vectors
• Replacement vectors
in cDNA library construction