ABSTRACT
The viral genome carries the nucleic acid sequences responsible for the genetic code of the virus and, logically, larger genomes carry more genes, allowing these viruses to encode greater numbers of proteins, which may be structural (part of the virion) or non-structural. In infected cells the genome must be transcribed and translated into proteins, which requires the recognition of protein-encoding RNA molecules (messenger RNAs or mRNA) by cellular ribosomes. The processes of gene expression need to be appreciated in order to understand virus replication; they are well described in SECTION F of this book, and also more fully within the Instant Notes Molecular Biology volume. The virus genome, or a complementary copy of it, must also serve as a template from which new genomes can be synthesized; these ultimately assemble with viral proteins into progeny virions. Viral genomes can be of DNA or RNA in either single- or double-stranded form, although some viruses (e.g. hepatitis B virus) have strands of unequal size, so their genomes are only partially double-stranded. Genomes can be linear or circular in structure and some viruses spread their information between separate nucleic acid molecules (i.e. they can be segmented) that must all be present for successful infection of a host. Virus genomes vary enormously in size; the smallest virus infecting humans is hepatitis B virus, with a genome size of just 3300 nucleotides, whereas some herpesvirus genomes are over 500 000 nucleotides. The largest virus genome sequenced to date is that of the marine mimivirus, at over 1 181 000 nucleotides; larger than the genome of some bacteria. The gene sequences within a virus genome must ultimately be translated into virus proteins and this requires either direct recognition of the viral genome as a messenger RNA (mRNA) by cellular ribosomes, or the conversion of viral genetic information into new mRNA transcripts within the infected cell. There are to date seven known schemes by which viruses provide mRNAs for translating their genomes into proteins and these distinct strategies form the basis of the Baltimore classification of viruses mentioned in the previous SECTION and outlined in more detail below (Figure 1).
