Viral Genomic Organization
Viral genome includes a single type of nucleic acid. Viral DNA or RNA may be single-stranded (e.g. ortho- and paramyxoviruses) or double-stranded (reoviruses), segmented (orthomyxoviruses, etc.) or non-segmented (togaviruses, picornaviruses and many others), positive or negative, circular or linear.
Protein-encoding strand of nucleic acid is called positive, or plus-strand. This strand serves as a direct template for subsequent transcription and translation.
RNA(+) containing viruses (picornaviruses, togaviruses) use positive-sense nucleic acid as a messenger RNA for protein synthesis. This RNA is considered to possess the infectivity, being able to induce the infectious process directly after virus penetration.
Negative or minus strand means the nucleic acid chain complementary the positive one. In that case viral protein formation is impossible without preliminary synthesis of positive chain on the negative strand template. For this purpose (–) RNA viruses contain RNA polymerase. It catalyzes the complementary RNA(+) synthesis within infected cells. The latter serves as mRNA.
The smallest DNA-containing organisms are hepadnaviruses with genome size of 3.2 kbp, the largest poxviruses have the genome of 375 kbp.
Retroviruses carry reverse transcriptase, an enzyme that performs single strand DNA copy of viral RNA template. Then a second complementary strand of DNA is polymerized. This DNA molecule is capable of further integration with DNA of the infected animal or human cell, and retrovirus comes into provirus state. Integrated DNA serves as a template for transcription and translation of retroviral proteins.
Poxviruses are the most intricately constructed viruses. Virions carry different enzymes in their own transcriptional system for nucleic acid and protein synthesis.
Nucleic acid structure of human pathogenic viruses of the main importance is presented in the Table below.