Viruses are the smallest, simplest living organism. They can be seen by the electron microscope.
Characteristics of viruses:
(1) The simple viruses use the enzymes of the host cell for both their protein synthesis and gene replication the more complex ones contain up to 200 genes and are capable of synthesizing, through their host many structural proteins and enzymes themselves.
(2) Viruses are either virulent, destroying the cell in which they occur or temperate, becoming integrated into their host genomes and remaining stable there for long periods of time.
(3) Viruses may consist of nucleic acid, capsids, envelopes, and tail-fibers.
(4) The protein costs largest encloses the nucleic acid is called a capsid. it may be of different shapes. the capsid is made up of protein subunits called capsomeres.
(5) The simplest viruses consist of a single molecule of a nucleic acid (DNA or RNA) surrounded by a capsid, which is made up of different protein molecules.