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Bacteriophages are bacteria infecting viruses.

Bacteriophages are bacteria infecting viruses. They are also called ‘phage’ or simply bacterial virus as any group of viruses that infect bacteria are referred to as Bacteriophage. A bacteriophage is a virus that parasitizes bacteria and reproduces inside it. They are of different shapes and show genetic variations. They may contain DNA or RNA as genetic material and may have gene count ranging from four to several thousand. The name bacteriophage describes an entity’s bactericidal ability and it translates to ‘’bacteria eater’’ in English. Not only do bacteriophages infect the bacteria but also archaea- the single-celled prokaryotic organisms.

A typical bacteriophage is composed of a polyhedral head, a short collar, and a helical tail.

After the phage infects the host cell and inserts its genetic material into the host cell, it follows either of the two life cycles, they are-

If they uptake the lytic cycle, bacteriophages infect the host cell and kill it to release progeny viruses. Steps involved in this cycle are as follows

This is the first step of infection by phage in which the bacteriophage attaches itself to the surface of the host cell or bacteria. For attachment to take place, the tips of the tail fibers attach to specific receptor sites on the surface of the bacterial cell.

In the next step, the tail sheath of the phage contracts after adsorption has taken place. The base plate and the tail fibers attach firmly to the bacterial cell surface. The phage lysozymes weaken a part of the host cell wall and the hollow core is pushed downwards through it. The phage DNA is then injected inside the bacterial cell.

The components of new virus particles are produced after the genetic material of the phage is released into the host cell. The sub-units of phage then appear which includes the head, tail, and late protein. Early proteins and specific enzymes carry out the synthesis. Components of phage are also present in the nucleus and the cytoplasm.

When the cells mature, the head and tail protein of phage DNA which is surrounded by a protein coat assemble. Ultimately, virion forms by the addition of tail structures.

The lysis of the bacterial cell takes place releasing the progeny phages. During the replication, phage enzymes weaken the cell wall of bacteria.

It is another pathway of viral reproduction in a host cell. In this phase the integration of phage nucleic acid into the host cell genome or the formation of a circular replicon in the cytoplasm of the host cell takes place. The host bacterial cell continues to live and reproduce normally in this phase. The genetic material of the phage also called prophage is transmitted to daughter cells at each subsequent cell division. The lysogenic cycle is different from the lytic cycle in the respect that the lysogenic cycle does not lyse the host cell straight away. The prophage may be converted into the lytic phase either naturally or artificially by physical or chemical agents. The bacteria carrying prophage viruses without being lysed are known as “lysogenic bacteria”. In the event of multiplication of lysogenic bacteria, the prophage might be lost due to excision.  

Bacteriophages infect and can only replicate in host bacterial cells. After the phage infects the host cell and inserts its genetic material into the host cell, it follows either of the two life cycles- Lytic Cycle in which it infects the host bacterial cell, lyses it and releases progeny viruses and Lysogenic Cycle in which the phages integrate with the host cell chromosomes to give rise to the prophage. The prophage eventually brings about the lysis of the bacterial cell naturally or artificially by physical or chemical agents.

It is a phase of replication of phage characterized by the integration of phage nucleic acid into the host cell genome, or the formation of a circular replicon in the cytoplasm of the host cell. The host bacterial cell continues to live and reproduce normally in this phase. The genetic material of the phage also called prophage is transmitted to daughter cells at each subsequent cell division. The prophage may be converted into the lytic phase either naturally or artificially by physical or chemical agents.

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