Lytic cycle PDF

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lytic cycle...

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The multiplication process of a virulent phage is called lytic cycle because the host bacterial cell is lysed at the end. This process is divided into following stages: (a) Adsorption (Fig. 6.57): It is the attachment of virus particle to the specific host bacterial cell. The attachment is at the specific receptor sites present on the wall of host cell. T-even phages (T2, T4 etc., attach with the help of their tail fibres at the host receptor sites. (b) Penetration (Fig. 6.57): The next phase is the injection of nucleic acid of the virion into the host cell. The nucleopeptidal host cell wall is hydrolyzed by lysozyme present at the tip of the tail making a hole, through which the viral nucleic acid is injected into the host cell via tail tube. This happens when tail fibres, after attachment, bent, bringing the base plate in contact with the bacterial cell wall. The tail sheath contracts and the central tube (needle) is pushed through the hole in the wall. All this process is an active process and takes place on the expense of ATP. The protein coats remaining outside, attached to the host cell wall, are called “Ghosts”.

Fig. 6.56. Conversion of lysogenic state of virulent state in phage infected bacteria. (c) The eclipse stage: This stage shows the following viral DNA activity inside the host cell, (i) immunity against further infection by phages of the same type by the production of specific enzyme called “repressors” (ii) suppression of all cellular activity of the host (iii) synthesis of new enzymes by phage DNA utilizing amino acid pool of the host cell. These are called Early Proteins

(iv) These enzymes are utilized to seal the hole in the cell wall, to destruct the DNA of the host (v) the fresh DNA molecules then synthesize a new type of proteins called Late Proteins identified as viral coat proteins and viral lysozymes. The coat proteins form monomers which are then assembled into capsid and other viral components. (d) Maturation: It is the assembly of various components into mature or complete virions. Head and tails are first assembled separately and then the two are attached to form hundreds of new phage particles. The time period between the injection of viral nucleic acid and the first appearance of new phage progeny is the eclipse period, which is about 12 minutes in T2 phages. The total time taken from the introduction of nucleic acid to the rupture of host cell wall is called Latent period. It is about 18 minutes for T2 phages. (e) Lysis and release of new virions: The cell wall bursts at the end of latent period and the virions are liberated. This phenomenon is called lysis. The number of virions produced per host cell is specific and is termed as the burst size. Generally it is 200 – 300. The main stages of lytic cycle have been shown in Fig. 6.57.

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