Viral replication. Coloured scanning electron micrograph (SEM) of polyoma virus particles (red) replicating in a host cell's nucleus (green). Two nucleoli are also seen (brown). Viruses are only able to replicate within another cell, as they are so small they lack the machinery needed to reproduce their DNA (deoxyribonucleic acid) and produce their own proteins. The virus' DNA enters the host cell's nucleus, where it is replicated and transcribed into RNA (ribonucleic acid), which in turn is translated into the virus' proteins. The DNA and proteins then assemble into virus particles, leave the nucleus and then leave the cell by destroying the cell wall (lysis).