The document discusses the life cycle of viruses, specifically bacteriophages. It describes two main cycles - the lytic cycle and lysogenic cycle. The lytic cycle involves the viral DNA replicating separately from the host cell and ultimately causing the cell to burst. The lysogenic cycle involves the viral DNA integrating into the host cell's genome where it can remain dormant for generations before entering the lytic cycle. The key difference between the cycles is that the lytic cycle results in host cell death while the lysogenic cycle allows the host cell to survive and replicate normally.

1. LIFE CYCLE OF VIRUS
Presented by :Urooj Umer
Presented to : Ma’am Maria
CMS no :404570

2. CONTENTS
 Virus
 Bacteriophage
 Forms of bacteriophage
 Structure of bacteriophage
 Life cycle of virus
 Lytic cycle
Steps in replication of bacteriophage
 Lysogenic cycle
 Difference between lytic and lysogenic cycle.
 References

3. VIRUS
 LITERAL MEANING
The word virus is derived from LATIN word
called VENOME meaning POISON .
 DEFINITION
Viruses are non cellular infectious entities
which contain either DNA or RNA normally
encased in proteinaceous coat and
reproduce only in living cells .
Microscopic view of virus
virus

4. BACTERIOPHAGE
DISCOVERY
Bacteriophage was
discovered independently by
FRIEDRICK TWORT in
1915 and FELIX
D.HERELLE in 1917 .
 It is a basically a type of
virus Felix D. Herelle Fredrick Twort

5. CONTI….
 LITERAL MEANING
The term the Greek word which
means BACTERIA EATER .
 DEFINITION
A bacteriophage also known
informally as a phage is
a virus that infects and
replicates within bacteria and
archaea.

6. STRUCTURE OF BACTERIOPHAGE
Earlier researches on bacteriophages were mainly on
phages that infect Escherichia. coli , of these the best
are T phages (T2 and T4 are mainly used in phage
studies). Basically electron microscopic studies
reveal that it resembles tadpole.
 HEAD
 The head is an elongated pyramidal , hexagonal
, prism shaped structure to which tail is attached
 Head is 1150 Å-long and 850 Å-wide .
 With in the head double stranded DNA molecule
is present .

7. CONT….
 TAIL
 Bacteriophage T4 belongs to the Myoviridae
family.
 Myoviridae phages have the most
complex tail structures, generally
consisting of a baseplate with tail fibers
and a long, non-contractible tube
surrounded by a contractile sheath.
 SHEATH
 The sheath consists of 138 copies of the
tail sheath protein, which surrounds the
central non-contractile tail tube.

8. CONT…
1. One side of sheath is collar
2. The other side is end plate ( base plate).
 TAIL FIBRES
 Bacteriophage T4 has two sets of tail
fibers:
1. Long tail fibers that are the initial
receptor binding proteins an
2 . Short tail fibers that bind subsequently
and trigger the infection
 used for attachment to the host cell

9. Bacteriophage

10. STRUCTURAL FORMS OF
BACTERIOPHAGE
Bacteriophages
Bacteria eater
Icosahedral
Having 20 faces
Polio virus ,
adenovirus
Enveloped
Having outer
wrapping
HIV VIRUS
Helical
Rod shaped
Tobacco mosaic
virus

11. Icosahedral virus
Enveloped virus
Helical virus

12. CYCLES INVOLVED IN REPLICATION
OF BACTERIOPAHGE
There are two cycles
involved in replication of
bacteriophage. These are as
follows:
• Lytic cycle
• Lysogenic cycle

13. GENERAL STEPS INVOLVED
IN BOTH LYTIC AND
LYSOGENIC CYCLES

14. LYCTIC CYCLE
• In the lytic cycle, the viral DNA exists as a
separate free floating molecule within
the bacterial cell, and replicates
separately from the host bacterial
• The lytic cycle is named for the process of
lysis, which occurs when a virus has
infected a cell, replicated new virus
particles, and bursts through the cell
membrane.

15. CONT…
• About 25 minutes after the initial infection approx
200 new bacteriophages are formed and then
bacterial cell bursts , called lysis.
• The phage at this stage is called virulent phage.
(A bacteriophage that causes the destruction
of the host bacterium by lysis )
• It is also called master – slave relationship

16. STEP WISE VIEW
OF LYTIC CYCLE

17. CYCLIC VIEW OF LYTIC CYCLE

18. GRAPHICAL VIEW OF LYTIC CYCLE

19. LYSOGENIC CYCLE
• Lysogeny is characterized by integration
of the bacteriophage nucleic acid into the
host bacterium's genome or
formation of a circular replicon in the
bacterial cytoplasm.
• In this condition the bacterium
continues to live and reproduce normally.
• The genetic material of the
bacteriophage, called a prophage.

20. CONT...
• Sometimes the viral DNA gets detached from the host’s
chromosome and lytic cycle stats again . This is called
induction .
• Induction involves either a spontaneous or
environmentally induced excision of prophage from the
bacterial chromosome
• The phage that causes Lysogeny is called temperate
phage .
• This is also called host guest relationship.

21. STEPWISE VIEW OF
LYSOGENIC CYCLE

22. CYCLIC VIEW OF
LYSOGENIC CYCLE

23. CYCLES

24. LYTIC CYCLE LYSOGENIC CYCLE
Viral DNA interact into host DNA. Viral DNA does not integrate into host
DNA.
Host DNA is hydrolyzed . Host DNA is not hydrolyzed.
Productivity of viral DNA is high . Productivity of viral DNA is low.
Virus is virulent . Virus is not virulent.
Occurs within a short period of time Takes time.
Does not have prophage. Has prophage state.
Host cell is lysed . Host cell is not lysed.
DIFFERENCE BETWEEN
LYTIC AND LYSOGENIC CYCLE

25. REFERENCES
• https://en.wikipedia.org/wiki/Lysogenic_cycle
• https://pediaa.com/difference-between-lytic-cycle-and-
lysogenic-cycle/
• ocratic.org/questions/what-are-the-differences-between-a-
lytic-infection-and-a-lysogenic-infection
• https://www.researchgate.net/figure/Phage-life-cycles-and-
types-of-phage-mediated-transduction-A-Temperate-phage-
life_fig1_318170069
• https://www.slideshare.net/suganyakunju/bacteriophages-
71259201