ABSTRACT
An intact virus, called a virion, consists of a small nucleic acid genome within a protein coat called a capsid. The capsid consists of regular arrays of a few viral proteins. Depending on the virus, the nucleic acid may be single- or double-stranded RNA, single- or double-stranded linear DNA, or single- or double-stranded circular DNA. Many types of virus exist, each with a limited host range in certain plants, animals or bacteria. Bacterial viruses are called bacteriophages. When permissive animal cells are infected with a DNA virus, the virus enters the lytic cycle, multiplies and lyses the cell to release the viral progeny. If a nonpermissive animal cell is infected with the virus, however, the virus may not enter a lytic cycle but instead may become integrated into the host genome or may become a plasmid and replicate at a low level without killing the cell.
