Bacteriophage: Definition, Structure, and life cycle. Replication of Bacteriophage, Lytic cycle and Lysogenic cycle

Table of Contents

  • Bacteriophage introduction
  • Discovery
  • Structure
  • Life Cycle of Bacteriophage
  • Lytic Cycle
  • Lysogenic Cycle


Bacteriophage:

Virus that infect bacteria is called bacteriophage are phage. Bacteriophage are obligate intercellular parasites that multiply inside bacteria by making use of the host biosynthetic machinery.


Bacteriophage: Definition, Structure, and life cycle. Replication of Bacteriophage,  Lytic cycle and Lysogenic cycle

Discovery:

Bacteriophage are discovered independently by, Twort in England in 1915,in later in 1917, by D' Herelle in Oaris. They are widely distributed in nature. They have been isolated with great frequency from the faeces of man and animals, sewage, water and soil.

Structure:

The basic structural features of bacteriophage are is follows.

Size:

Most phages range in size from 24-200 nm in length. 

Head or Capsid:

All phages contain a head structure which can vary in size and shape. Some are isosahedral (20 sides) other are filamentous. The head or Capsid is composed of many copies of one are more different proteins. Nucleic acid (DNA or RNA) are present inside the protein head. The Capsid is a protective cover for Nucleic acids.

Tail:

Most of the phages have tails attached to the phages head. The tail is a hollow tube through which the nucleic acid passes during infection. The tail size can vary in different phages while some phages do not even have a tail structure.

In some complex phages like T4 have a base plate and one are more tail fibers attached to it. The base plate and tail fibers are involved in the attachment of the phage to bacterial cell. The phages which do not have base plate and tail fibers the use other structure for attachment of the phage to bacterium.


Bacteriophage: Definition, Structure, and life cycle. Replication of Bacteriophage,  Lytic cycle and Lysogenic cycle


Life cycle of Bacteriophage (Replication of Bacteriophage)

Bacteriophage are classified into two broad group according to their life cycle, lytic and lysogenic cycle. The main difference between these two life cycle is that a lytic phage kills (lyses) its host bacterium very soon after the initial infection, resulting the release of progeny phages.

In lysogenic cycle, phage have a choice between two life styles. After infection either it can follow a lytic life cycle ( kills the host cell and produces many progeny) or alternatively it can enter the lysogenic pathway, during which its DNA become integrated ito the chromosome of the host and persists in the host cell in a latent state for a substantial period of time.

 

a.Lytic cycle

In this cycle the phage is regarded as virulent or master and the bacterial cell (host)is regarded as slave. In lytic cycle the phage first attaches itself by its tail to the cell wall of bacterium at a point called receptor site. The phage contains an enzyme called lysozyme, which digest the cell wall of bacterium. Thus an opening is formed in the bacterial cell wall. The phage contracts and injects its DNA inside the host while the protein coat and the tail remain outside.

Inside the bacterial cell the phage DNA takes over the biosynthetic machinery of the host to synthesize its own DNA and protein molecule. The phage multiplies and increases in number. The daughter phages exert pressure on the cell wall of bacterium. Thus the bacterial cell ruptures (lysis occurs) and release the daughter phages, which are now ready to attack new bacteria and start their cycleagain.This type of life cycle in which the bacterium cell bursts is called lytic cycle of the phage.


Bacteriophage: Definition, Structure, and life cycle. Replication of Bacteriophage,  Lytic cycle and Lysogenic cycle

b.Lysogenic cycle

In this cycle, the phage does not kill or destroy the bacterium(host).Both thephage and bacterium live and multiply in a peaceful coexistence.In this case the phage becomes a harmless guest and the bacterium acts as a host. Sometimes when the phage DNA enters the bacterial cell, instead of taking over the control of biosynthetic machinery of the host it becomes associated and mixed up with the bacterial chromosome in a friendly atmosphere. In this condition the bacterium continues to live and reproduces normally.The phage DNA passes to each daughter cell of bacterium in all successive generations.

Thus the number of phages increases without any harm or damage to the bacterium cell. Therefore this relation is called guest- host relation. This type of cycle is called Lysogenic cycle. Sometimes, however, the phage-DNA is separated from the bacterial chromosomes and becomes re-activated to become virulent and hence destroy te bacterial cell and starts the lytic cycle again.

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