Coloured transmission electron micrograph (TEM) of T2 bacteriophage viruses (orange) attacking an Escherichia coli bacterium. Each phage consists of a large DNA- containing head and a tail composed of a central sheath with several fibres. The fibres attach to the host cell surface, and the phage DNA is injected into the cell through the sheath. It instructs the host to build copies of the phage (progeny, in cell). Destroying the host’s DNA releases nucleotide building blocks, from which phage DNA is synthesised. It is then packaged in the progeny, which are released when the host bursts. Magnification: x64,000.