GENERAL CHARACTERISTICS OF VIRUSES
Viruses are non-cellular or acellular infectious agents that replicate only within living
cells. Viruses are smaller than cells, ranging in size from 0.02 µm to 0.3 µm (20 nm to 300
nm). Smallpox virus, one of the largest viruses, measures about 0.2 µm (same size as some
bacteria), while Poliovirus, one of the smallest viruses, measures only 0.028 µm in
diameter. Viruses are unique in their organization and pattern of reproduction.
The virus genome consists of either RNA or DNA and never both.
Viruses lack independent metabolism. They multiply only within living cells,
using the host cell machinery.
When a virus multiplies, the genome is released from the coat. This occurs
during the infection process. Once released, the viral genome undergoes
replication and directs the synthesis of new coat proteins. New virus particles
are reconstructed from the virus genome and virus coat.