Complete information on the replication and reproductive cycle of Bacteriophage

Replication of Bacteriophage:

This process can be divided into four steps:

(1) Infection.

(2) Synthesis of phage components in the infected cells.

(3) Assembly of new phage particles.

(4) Liberation of phage particles from the host cell.

1. Infection:

Infection occurs by ‘Adsorption’ of bacteriophage by the host cells and entry of nucleic acid inside the host cell. The adsorption of bacteriophage by host cell depends on their mutual affinity.

Only a specific bacteriophage can infect a specific host cell. Some coliphage have got so much affinity that they are only absorbed at the definite place on the cell wall of the host cell. This site is known as receptor site. Adsorption is facilitated by tail fibres. It has been seen that separated tail fibres can be adsorbed on the cell wall of host cell and bacteriophage lacking in tail fibres cannot be adsorbed.

Lysozyme-Iike enzyme is secreted by host cell after the adsorption of bacteriophage. This enzyme disintegrates muramic acid—peptide complex (mucopeptide)—presents in the cell wall of the host; with this activity of the enzyme there occurs formation of one-minute hole in the cell wall of bacteria. Through this hole, nucleic acid of the bacteriophage enters inside the host cell.

According to Kellen Berger, the process of infection takes place as follows:

(a) As bacteriophage comes in contact with host cell its tail fibrils get turned aside and the tail plate comes in contact with the cell wall of the host cell.

(b) The contraction of tail sheath and a central cylindrical portion of the tail makes a small hole in the protein covering of the host cell.

(c) Through this hole all nucleic acid of the phage enters into the host cell.

After transfer of nucleic acid of bacteriophage inside the host cell, the protein covering or the shell of parasite remains attached with the host cell. This protein cover is known as ghost. The main feature of the infection is that only one parasite infects one bacterial cell. After the transfer of nucleic acid of bacteriophage there occurs a loss of infectivity of the bacteriophage.

2. Synthesis of Phage component in host cell:

The nucleic acid of bacteriophage changes the metabolism of host cell. This metabolism is directed towards the synthesis of different parts of bacteriophage. The two functions of the nucleic acid of bacteriophage are:

(i) It works as template for its replication.

(ii) It synthesizes viral specific m-RNA, which results into the synthesis of specific proteins.

Synthesis of new DNA starts by unbinding of both strands. Some basic compounds of host cell are utilized in this process. By semi-conservative replication, there is exchange of parts of the DNA of one generation with the parts of other generation. By this hybrid DNA molecules are produced.

After the synthesis of nucleic acid, the production of different proteins takes place inside the host cell. They are known as late proteins. Most of the late proteins are present in the covering of the bacteriophage. Their synthesis starts by the formation of polypeptide chains. These proteins make capsomeres of the covering of host cell. The last stage of replication is synthesis of lysozyme.

3. Assembly of new phage particle:

Nucleic acid and protein components form assembly of a new phage particle, which is called maturation. This process is controlled by the genome. First, there is condensation of DNA in the form of crystal at the tip of phage. After this protein, sub-units unite around the DNA and form head of protein. At this moment the assembly of tail starts. In this process, central core attaches with tail plate. Now there occurs formation of a sheath around the central core. In this stage, tail gets attached to the base of the head. Tail fibres attached to the tail plate arise at the end of phage development.

4. Liberation of phage particles from the host cells:

At the end of the dormant period, phage particles are liberated from the host cell. Before liberation, there occurs lysis of the host cell wall. Probably, phage DNA secretes an enzyme lysoenzyme, which helps in the lysis of host cell wall. After lysis the cell wall gets ruptured and phage particles are liberated.

In this way in a short time, there is a formation of so many bacteriophages in a bacterial cell. In one cell of the strain T₂ of E.coii. 200 bacteriophages, are produced within 24 minutes. After the first invasion of bacteriophage, bacterial cell produces only viral DNA and its protein covering. It ceases to produce protein. DNA and RNA for itself. After the synthesis of bacteriophage there occurs lysis of cell wall and liberation of bacteriophage. This cycle is known as reproductive cycle of bacteriophage.