Q.What are virulent phages? Give an example.

A bacteriophage begins infection the same way regardless of which cycle follows: it attaches to its host bacterium via its tail fibres and injects its nucleic acid into the cell, leaving its protein coat outside. In the lytic cycle, this injected DNA immediately takes over the host's machinery to replicate itself and manufacture new phage heads, tails and fibres, which self-assemble into complete new phage particles; the phage enzyme lysozyme then breaks down the host cell wall, causing lysis - the cell bursts, releasing the newly made phages (the number released per cell being the burst size). Phages that always follow this destructive route are called virulent phages, with the T-even phages (such as T4) as the standard example.

In the lysogenic cycle, the injected phage DNA instead integrates into the host bacterium's own chromosome, becoming a prophage. As a prophage, it causes no harm and makes no new phage particles, but is simply copied passively along with the host chromosome at every host cell division. Under certain conditions, the prophage can excise itself from the host chromosome and switch to the lytic cycle, producing new phages and lysing the host just as a virulent phage would. Phages capable of this dual lifestyle - alternating between a dormant, integrated prophage and an active lytic phase - are called temperate phages, the bacteriophage lambda being a well-known example.

A further consequence of the lysogenic cycle is that when a prophage excises itself, it can occasionally carry away a small piece of the adjoining host bacterial DNA along with its own genome; if this phage then infects a new host, it delivers that borrowed piece of bacterial DNA into the new cell. This process is transduction, and it is exactly why temperate phages - through their capacity to integrate into and later leave a host chromosome - serve as the essential carrier underlying genetic transduction between bacteria.