Bacteriophages are viruses that infect and replicate within bacteria, featuring a structure that includes a head, tail, base plate, and tail fibers for attachment. They can undergo two life cycles: the lytic cycle, which involves rapid replication and lysis of the host cell, and the lysogenic cycle, where phage DNA integrates into the bacterial genome and replicates with it, causing no immediate harm. Under stress, the lysogenic cycle can shift to the lytic cycle.

1. BACTERIOPHAGE
LYTIC AND LYSOGENIC
CYCLE
Understanding the viral life
cycles in bacteria
By:
YAZHINI RAJ P.M.

2. BACTERIOPHAGE
DEFINITION:
A bacteriophage, or phage, is a type
of virus that specifically infects and
replicates within bacteria.

3. STRUCTURE
Head (Capsid): Contains the phage's
genetic material (DNA or RNA).
Tail: A sheath-like structure
attached to the head. Acts as a
conduct for injecting genetic
material into the host bacterium.
Base Plate: Located at the end of
the tail. Facilitates attachment to
the bacterial surface.
Tail Fibers: Extend from the base
plate.Help in recognizing and
binding to specific receptors on the

4. LYTIC CYCLE
• Definition:
A process where the phage
replicates and lyses the host cell.
• Steps:
• Attachment
• Penetration
• Biosynthesis
• Maturation
• Lysis

5. STEPS:
• Attachment: Phage attaches to specific
receptors on the bacterial surface.
• Penetration: Phage injects its DNA into
the host cell.
• Biosynthesis: Phage DNA replicates and
phage proteins are synthesized.
• Assembly: New phage particles are
assembled.
• Release: Host cell bursts, releasing new
phages.

7. LYSOGENIC CYCLE:
• Definition: A process where the phage DNA
integrates into the host genome and replicates
with it without killing the host.
• Steps:
• Attachment
• Penetration
• Integration
• Prophage Replication
• Induction (trigger to enter lytic cycle)

8. STEPS:
• Attachment: Phage attaches to the bacterial
cell.
• Penetration: Phage injects its DNA into the
host.
• Integration: Phage DNA integrates into the
bacterial chromosome, becoming a prophage.
• Prophage Replication: Prophage replicates
along with the bacterial DNA.
• Induction: Under stress conditions, prophage
can excise from the genome and enter the lytic
cycle.

10. D/B LYTIC AND LYSOGENIC
• Lytic Cycle: Quick
reproduction, host
cell death.
• Lysogenic Cycle:
Dormant phase, no
immediate harm to
host.