Draw and label the lytic cycle for the T4 phage, Label and include: attachment, penetration, biosyn- thesis, maturation, release, viral capsid, viral genome, assembled virions.

Answer:

Bacteriophage are viruses which kill bacterias. T4 bacteriophage infects E. coli and multiply via lytic cycle. Structure of T4 bacteriophage consist of a capsid, collar , tail.

Viral Capsid / head is contains viral genome ( double stranded DNA ) encapsulated by many proteins subunits called capsomeres. Collar coonects head and tail. Tail contains base plate and fibres which help in attachment of phage to bacterial cells and penetrate viral genome into bacteria.

Viruses reproduce by two methods : via lytic cycle and lysogenic cycle.

Lytic cycle is a type of reproductive process in which bacteriophage infects bacterial cells and use bacterial cells machinery for multiplying viral genome and assemble them with viral capsid and release assembled virions after maturation via lysis of bacterial cells. It involves following steps :

Attachment, penetration, biosynthesis, assembly, maturation and release.

Attachment : T4 bacteriophages are attached to bacterial cell surface with the help of tail fibres. Bacteriphages haves specific binding sites for receptors of bacterial cells and thus adsorbed on bacterial cell wall.

Penetration : After adsorption on bacterial cell, phage tail sheath contracts , and tail inject like a needle and release enzymes which forms a hole in bacterial cell surface. Phage release its viral genome inside bacterial cell through tail base plate . Only viral genome penetrates inside bacterial cell. Other parts of phage - viral capsid, collar , tail firbre remains outside the cell.

Biosynthesis : During this step, viral genome uses host machinery and start replicating its viral genome. After replication, transcription and translation occurs to synthesise viral genome encoded proteins and enzymes to from different parts of phage (capsomeres, tail fibres, tail sheath, base plate). Bacterial chromosome is degraded.

Assembly and maturation : During maturation different parts of phage are assembled to form new virions. Capsomeres assembles to form viral capsid and viral genome is inserted into it. Collar and tail sheath, base plate and tail fibres assembles to form new assembled virions. These assembled virions are mature phage's and are ready for release.

Release : Enzymes synthesised by viral genome like lysozymes or holin, disintegrates cell wall of bacteria and hence all the assembled virions are released out of cell. And bacterial cell dies.

In diagram cell shown is a E. coli bacterial cell and virus infecting is T4 bacteriopage.

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