Lytic and lysogenic cycles of lambda phage
When lambda phage infects a bacterium, eg E. Coli, two cycles may happen: lytic or lysogenic. Both cycles undergo a similar initial infection, when the phage binds to the host bacterium’s surface. The lytic cycle produces progeny phage particles, and is thus the more common cycle that comes after most infections. The lysogenic cycle, however, does not produce a huge number of progeny phage or break down the host cell. Instead, the λ DNA recombines with its host’s genome to produce a prophage. This is the most preferred pathway when unfavourable environmental conditions prevent intense replication of the bacterial cells. (Phages, 2013)
Initial infection occurs in the following manner: * Phage adsorbs (or attaches) to the bacterial cell (which is called the host). Viruses have different types of proteins that can attach or “dock” with a protein on the surface of the bacterial cell. * The phage penetrates the bacterium by making a hole in the cell wall with an enzyme called lysozyme (also called muramidase) or physical 'injection', degrading cell wall lipo-polysaccharides, proteins, teichoic acids, flagella, pili and the phage nucleic acid enters the bacterium. * The DNA passes through the maltose permease complex in the inner membrane and immediately circularizes using the cos sites, a 12-base G-C rich sequence. The single-strands are later joined by the host DNA-ligase. * Host DNA gyrase releases the super-coiling in the circular chromosome, causing A-T rich regions to unwind and initiate transcription. (Sanderson, 2011) * Transcription starts from the constitutive PL, PR and PR' promoters producing the 'immediate early' transcripts. Initially these express the N and cro genes, producing N, Cro and a short inactive protein. * Cro binds to OR3 preventing access to the PRM promoter preventing