The document summarizes the lytic cycle or vegetative cycle of bacteriophages. It involves 5 key steps - 1) attachment of the phage to the host bacterial cell, 2) penetration of viral DNA into the cell, 3) synthesis of new viral components using the host cell's machinery, 4) assembly of new viral particles, and 5) lysis of the host cell and release of progeny viruses to infect new hosts. The cycle results in the destruction of the host bacterium and production of new phages to perpetuate the viral life cycle.

1. SRI PARAMAKALYANI COLLEGE
REACCREDITED WITH B GRADE WITH A CGPA OF 2.71 IN THE SECOND CYCLE OF NAAC
AFFILIATED TO MANOMANIUM SUNDARANAR UNIVERSITY, TIRUNELVELI.
ALWARKURICHI 627 412, TAMIL NADU, INDIA
POST GRADUATE & RESEARCH CENTRE
DEPARTMENT OF MICROBIOLOGY
(Government Aided)
ACADEMIC YEAR 2021-2022
ll SEM CORE: VIROLOGY(ZMBM23)
UNIT-2 LYTIC CYCLE
SUBMITTED TO, SUBMITTED BY: JANAKI SUJITHA.K
GUIDE: Dr.C.MARIAPPAN,Ph.D REG NO: 20211232516111
ASSISTANT PROFESSOR, I MSC MICROBIOLOGY
SRI PARAMAKALYANI COLLEGE, SRI PARAMAKALYANI COLLEGE,
ALWARKURICHI. ALWARKURICHI.

2. Lytic cycle / Vegetative cell cycle
Introduction:
 Bacterial viruses exhibiting a lytic cycle only are known as virulent bacteriophages or lytic
phage.
 Virulent bacteriophage cause the death and destruction of the host bacterium.
 Lytic or vegetative life cycle results in the lysis (rupture) of the host cell and the release of
numerous viral progeny.

3.  It involves in the reproduction of viruses using a host cell to manufacture more viruses; the
viruses then burst out of the cell.
 The normal process of viral reproduction involving penetration of the cell membrane, nucleic
acid synthesis and lysis of the host cell.
Steps of lytic cycle :
1. Attachment
2. Penetration
3. Synthesis
4. Assembly
5. Release

4. The stages of the bacteriophage lytic cycle :

5. 1. Attachment
 A virus requires a host cell for multiplication.
 In a first step, a virus attaches itself onto the host cell.
 Proteins in the “tail” of the phage bind to a specific receptor on the surface of the
bacterial cell by Adsorption.
 Adsorption is a specific biochemical reaction; hence bacteriophages can infect only
certain host bacterial cells.
 When the numbers of phages adsorbed is less, it may only affect the permeability of
the membrane.

6.  Lysis of the host cell occurs only if an adequate number of phages invade the bacterium.
 Infection usually does not occur in the absence of adsorption.
 However, infection of the bacterium directly by naked phage nucleic acid can occur, and this
process is known as transfection.
Attachment

7. 2. Penetration
 Adsorption is followed by penetration of phage nucleic acid into the bacterial cell.
 After the virus attaches to its host, it introduces its genetic material into the cell by
injection.(Tail sheath contracts, attaching the internal tail tube to host plasma membrane
where DNA is released)
 Penetration through the cell wall is facilitated by an enzyme lysozyme present in the tail core
that produces a hole on the bacterial cell wall.
 The phage DNA then passes through the central tail core into the host cytoplasm, while the
empty protein capsid remains attached to the cell surface.

8.  More than one phage can adsorb to a host cell and inject its DNA. If more bacteriophages are
adsorbed, they make so many holes in the cell wall that it causes lysis of the cell, a
phenomenon called lysis from without.
 After entry, the phage DNA is rearranged to a circular form, and this process is known as
circulation of phage DNA.
Penetration

9. 3. Synthesis
 Once the phage genome enters the cytoplasm, it subverts the host nucleic acid and
protein synthesis apparatus and initiates the synthesis of viral protein and DNA.
 Synthesis of phage components begins immediately after penetration of the phage
nucleic acid. Early proteins are the first products to be synthesized.
 These early proteins include the enzymes, such as nucleases, synthetic enzymes, and
RNA polymerase. These early proteins are essential for the building of complex
molecules peculiar to the phage.

10.  Subsequently, late proteins are synthesized, which include
(a) protein subunits of phage
(b) structural proteins necessary for the virion self-assembly
(c) enzymes involved in maturation
(d) proteins used in the release of bacteriophages from the cell
During this period, the synthesis of bacterial proteins, DNA, and RNA stops. It is noteworthy that
phage DNA, head proteins, and tail proteins are synthesized separately in the bacterial cell.
Replication

11. 4. Assembly
 The protein subunits of the phage head and tail aggregate spontaneously to form the
compact capsid.
 The DNA is condensed into a compact polyhedral shape and packaged into the head.
 The assembly of the phage component into the mature infective phage particles is
known as maturation.
Assembly and maturation

12. 5. Release
 New mature progeny phages are released by lysis of the bacterial cells.
 During the replication of phages, the bacterial cell wall is weakened by lysozyme.
 Finally, the cell wall bursts as a result of osmotic pressure. This is known as lysis from
within.
Release

13.  The interval between the entry of phage nucleic acid into the bacterial cell and the
appearance of the first infectious intracellular phage particle is known as the eclipse phase.
 The average yield of daughter phages from the infected bacterial cell is known as the burst
size.
 The period during which the number of infectious phages released rises is known as the rise
period.
Lytic cycle is enumerated by a plaque assay. A plaque is a clear area, which
results from the lysis of bacteria. Each plaque arises from a single infectious phage.

14. References:
 Subhash Chandra Parija - Textbook of Microbiology and
Immunology (2012).
 https://bio.libretexts.org
 https://www.technologynetworks.com

15. Skills gained by the seminar
 Confidence level
 Time management
 Better communication skills
 Searching ability