LYTIC CYCLE virology.pptx

PG AND RESEARCH DEPARTMENT OF MICROBIOLOGY - SRI PARAMAKALYANI COLLEGE - ALWARKURICHI

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 RE G 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. T he 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

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