Retrovirus infects a cell

When a retrovirus infects a cell:

• its molecules of reverse transcriptase are carried into the cell attached to the viral RNA molecules.


• The reverse transcriptase synthesizes DNA copies of the RNA.


• These enter the nucleus


• and are inserted into the DNA of the host.


• These inserts are transcribed by the host's enzymes into fresh RNA molecules which re-enter the cytosol where some are translated by host ribosomes


• while other RNA molecules become incorporated into fresh virus particles.

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References to text (Retroviruses): http://tolweb.org/treehouses/?treehouse_id=4426 http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.061.htm http://gsbs.utmb.edu/microbook/ch042.htm http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Retroviruses.html

Reproductive Cycle of a Retroviridae in a Host Cell

Flow of events during the replication of retroviruses

1. Retrovirus virions enter host cells through interaction between a virally-encoded envelope protein and a cellular receptor.
   
2. Viral RNA is transcribed into a DNA copy by the enzyme reverse transcriptase which is present in the virion.
   
3. The viral DNA copy is integrated into, and becomes a permanent part of, the host genome. This integrated DNA is referred to as a provirus.
   
4. The host cell's transcriptional and translational machinery expresses the viral genes.
   
5. The host RNA polymerase II transcribes the provirus to create new viral RNA, which is then transported out of the nucleus by other cellular processes.
   
6. A fraction of these new RNAs are spliced to allow expression of some genes, while others are left as full-length RNAs.
   
7. Viral proteins are synthesized by the host cell's translational machinery.
   
8. Virions are assembled and bud from the host cell.

Picture extracted: http://gsbs.utmb.edu/microbook/ch042.htm

Genome Structure and Virion Properties

Genome Structure
The genome of retroviridae is dimeric, unsegmented and contains a single molecule of linear. The genome is a positive-sense, single-stranded RNA. Minor species of non-genomic nucleic acid are also found in virions.


Virion Properties
Virions consist of an envelope, a nucleocapsid, and a nucleoid. Virus capsid is enveloped. Virions are spherical to pleomorphic. They are measured 80-100 nm in diameter. Surface projections are densely dispersed, small or distinctive glycoprotein spikes that cover evenly the surface. Surface projections are 8 nm long. The nucleoid is concentric, or eccentric. The core is spherical. The single-stranded, linear RNA is found to be 7-12 kb in size.

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Retroviruses

The family of Retroviruses is large and diverse that is defined by common structure, composition, and replicative properties. The members of this family replicate via a DNA intermediate using a single stranded RNA genome. They can be broadly divided into two categories – simple and complex – based on the organization of their genomes.

Retroviruses are viruses that are remarkable for their use of reverse transcription of viral RNA into DNA during replication. Members of this family include Human immunodeficiency virus (the virus that causes AIDS), feline leukemia, and several cancer-causing viruses. Being the most abundant group of subcellular parasites, they infect animals, plants and bacteria.

Structure of a retrovirus - specifically HIV

picture extracted: http://tolweb.org/treehouses/?treehouse_id=4426