The lytic cycle

The lytic or virulent bacteriophages attack and lyse their hosts during the replication cycle. This is exemplified by bacteriophage T4, whose relatively large genome contains about 200 genes. T4 attaches to the cell membrane and injects its genome into the cytoplasm. The  first  genes transcribed code  for  nucleases to  cut  up  the  bacterial genome, effectively stopping any  further host  cell activity. The remainder of the phage replication cycle relies on existing host ribosomes, RNA polymerases, and enzymes in the cell, in addition to the phage-encoded products. Phage genes are now expressed that enable the T4 DNA to be copied.

The T4 genomes are synthesized as a concatemer, with each copy linked end to end with another. T4-specific nucleases cut the concatemeric DNA into pieces, each approximately 105% of the genome. Phage head, tail, sheath, and other proteins necessary for the construction of the protein coat are now synthesized, and coalesce around the T4 chromosome. Each viral head receives the same overall complement of genes, but as the DNA molecule is slightly larger than the minimum number of genes required, the genes at each end are repeated. This terminal redundancy allows a simple but reliable method of ensuring that each phage head always receives at least one copy of each gene.

Finally, phage genes that cause the autolysis of the infected cell are transcribed, with the release of around 300 virions. Each of these has the potential to infect more cells, but if left in a population without phage resistance lytic phages will eventually wipe out all their local hosts.