Legend: Process by which retroviruses use a host cell to replicate.
Retroviruses contain viral RNA and several copies of reverse transcriptase (DNA polymerase). After infecting a cell, the reverse transcriptase is used to make the initial copies of viral DNA from viral RNA. Once a DNA strand has been synthesized, a complementary viral DNA strand is made. These double strand copies of viral DNA are inserted into the host-cell chromosome and host-cell RNA polymerase is used to make virus-related RNA. These RNA strands serve as templates for making new copies of the viral chromosomal RNA and serve also as mRNA. mRNA is translated into viral proteins that are used to make the virus envelope. New viral particles are assembled, bud from the plasma membrane, and are released. An example of this process is illustrated in the replication of the retrovirus, HIV (human immunodeficiency virus).
Legend: Illustration of the structure of a retrovirus.
Retroviruses are infectious particles consisting of an RNA genome packaged in a protein capsid, surrounded by a lipid envelope. This lipid envelope contains polypeptide chains including receptor binding proteins which link to the membrane receptors of the host cell, initiating the process of infection.
Retroviruses contain RNA as the hereditary material in place of the more common DNA. In addition to RNA, retrovirus particles also contain the enzyme reverse transcriptase (or RTase), which causes synthesis of a complementary DNA molecule (cDNA) using virus RNA as a template. When a retrovirus infects a cell, it injects its RNA into the cytoplasm of that cell along with the reverse transcriptase enzyme. The cDNA produced from the RNA template contains the virally derived genetic instructions and allows infection of the host cell to proceed.
The virus that causes AIDS (acquired immune deficiency syndrome) is a retrovirus. It is called HIV for human immunodeficiency virus.
Taxonomy:
Group VI: RNA Reverse Transcribing Viruses
Genome:All retrovirus genomes consist of two molecules of RNA, which are s/s, (+)sense and have 5' cap and 3' poly-(A) (equivalent to mRNA). These vary in size from ~8-11kb. Retrovirus genomes have 4 unique features:
They are the only viruses which are truly diploid.
They are the only RNA viruses whose genome is produced by cellular transcriptional machinery (without any participation by a virus-encoded polymerase).
They are the only viruses whose genome requires a specific cellular RNA (tRNA) for replication.
They are the only (+)sense RNA viruses whose genome does not serve directly as mRNA immediately after infection. These two molecules are physically linked as a dimer by hydrogen bonds (co-sediment). In addition, there is a 3rd type of nucleic acid present in all particles, a specific type of tRNA (usually trp, pro or lys) - required for replication (below).Gene order in all retroviruses is invariant:
5' - gag - pol - env - 3'
Some retroviruses have additional genes:
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