Multiplication or Life Cycle of Phages

Phages multiply through one of two very different life cycles. In the lytic (virulent) cycle, the host cell is ultimately destroyed to release new virus particles, through five defined steps: (i) Adsorption - the phage's tail fibres recognise and bind specific receptor sites on the bacterial cell surface (a step called landing), and the tail fibres then bend to anchor the base plate firmly to the cell (pinning); (ii) Penetration - the phage's own lysozyme enzyme digests a small opening in the host cell wall, the tail sheath contracts using ATP, and the DNA alone is injected into the cell without using the host's own metabolic energy (this DNA-only injection step is called Transfection); the emptied protein shell left outside is called a 'ghost'; (iii) Synthesis - the phage DNA takes over the host's biosynthetic machinery, shutting down and degrading the host's own DNA and protein synthesis while directing the cell to make phage proteins and replicate phage DNA instead; (iv) Assembly and Maturation - the newly made phage DNA and protein coats, synthesised separately, come together (maturation) to form complete phage particles - around 300 new phages within about 20 minutes of infection; (v) Release - the phage particles accumulate inside the host and are finally released when the host cell wall lyses (bursts). In the lysogenic (avirulent) cycle, no new virus particles are made at all: the injected linear phage DNA circularises and integrates into the bacterial chromosome by recombination, becoming a 'prophage'. Two repressor proteins made from phage ge …