Question

Write a brief write up on Replication strategies of retro viruses,how the retro viruses have evolved and how they do transcription and translation.

Note: Mention complete details of the journal and papers in the reference section(additionally has to be added in the above write up) that were used to write above write up.

Answer 1

Retroviruses are enveloped ribonucleic acid viruses containing a single strand of RNA in their genome. HIV that causes AIDS, is a retrovirus. Reverse transcription and integration into the host genome are hallmarks of retroviral replication. An essential part of the retrovirus life cycle is converting the RNA genomic material back to DNA. This involves an RNA-dependent DNA polymerase called reverse transcriptase that uses an RNA as a template to make DNA.

The viral RNA genome contains a packaging signal for selective encapsidation into viral particles and a binding site for a tRNA primer for initiation of reverse transcription

Like other RNA-dependent polymerases, reverse transcriptase is very error prone because it contains no proofreading activity. The replication scheme is as follows:

1. Viral RNA in the host cell base pairs with a specific tRNA molecule to provide a primer for DNA replication.

2. Reverse transcriptase makes DNA towards the 5' end of the viral RNA.

3. RNase H partially degrades RNA from the RNA-DNA duplex, removing the 5' terminus.

4. The 3' terminus of the viral RNA base pairs with the overhanging DNA strand, forming a circle-like structure.

5. Reverse transcriptase makes a DNA copy of the entire genome, using the 3' end of the DNA strand as a primer.

6. The RNA strand of the duplex is removed and a DNA strand complementary to the first DNA strand is synthesized to form a duplex.

7. The duplex integrates into the host chromosome.

Once inside the genome, the integrated viral DNA can persist in a noninfectious state for many years, with most of its own genes turned off. Environmental stresses, still undetermined, can trigger excision of the integrated viral genome and return the virus to an infectious state.

Transcription and translation in retro viruses

retroviruses function differently, as their RNA is reverse-transcribed into DNA, which is integrated into the host cell's genome (when it becomes a provirus), and then undergoes the usual transcription and translational processes to express the genes carried by the virus.The information contained in a retroviral gene is thus used to generate the corresponding protein via the sequence: RNA → DNA → RNA → polypeptide.

Evolution

Studies of retroviruses led to the first demonstrated synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes. It has been speculated that the RNA to DNA transcription processes used by retroviruses may have first caused DNA to be used as genetic material. In this model, the RNA world hypothesis, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create DNA from the RNA templates.