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Oncornavirus Replicative Cycle

Oncornavirus Replicative Cycle -The replicative cycle of an oncornavirus takes place in the host cell
A. The oncornavirus is adsorbed on a specific receptor on the surface of the host cell and then penetrates the cell membrane.
The virus particle is uncoated and the RNA released into the cytoplasm of the cell
B. The RNA synthesizes provirus DNA in the cytoplasm with the help of primer dependent reverse lranscripta8e.
This enzyme cannot form new DNA chains de novo.
It can only extend originally existing chain (primers).The primer is cellular tRNA, a part of which has complementary bases to a sequence on virion RNA.
The tRNA primer attaches to 35s genome RNA about 135 nucleotides from its 5 terminus synthesis begins from the tRNA primer and extends to the 5' end to the 35S RNA, forming a short DNA strand

There are two possible models for further synthesis : Two 35S RNAs are used as templates The DNA is extended across the 5 terminus of the first strand and crosses the gap to the 3 terminus of the second strand.
From here it is extended across the strand to the 5 end.
In this model the DNA formed repres­ents one subunit only.
In the second model a circular template is presumed. The DNA crosses the 5' to 3' gap and copies both 35S subunits of 70S RNA
C. The RNA of the DNA-RNA hybrid is hydrolysed by the RNase H activity of reverse transcriptase, leaving only the DNA strand.
D.  The DNA strand acts as a template for the synthesis of a second DNA strand to from dsDNA. The enzyme now functions as a DNA dependent DNA polymerase, Actinomycin C inhibits this reaction, thus providing a method for preparing pure ssDNA

E. The provirus dsDNA assumes a closed circular form. This form helps integration into the host DNA because only a single recombi­national event is required.
F. The provirus DNA is integrated into the DNA of the host chromosome, and becomes a part of host genome.
It is not known whether the site of integration is random or specific in both viral and host DNA, or whether the site o[integration affects the expression of viral genes.
G. The integrated provirus DNA may remain latent or may trans­cribe RNA. In the former case the host cell is not affected by the virus.
Transcription provides RN A for the progeny viruses as well as mRNA for protein synthesis.
Host RNA polymerase is used for RNA transcription.
Possibly there are three different mRNAs for the three genes representing viral proteins in avian leukemia viruses.

The group specific antigen gene (gag) codes a large precursor polypeptide which is cleaved into the internal proteins p10, p 12, p15, and p30 (for mouse leukemia virus).
The pol gene codes reverse transcriptase and the env gene the envelope glycoproteins.
In the Rous sarcoma virus a fourth gene (src) is presumed to code a transforming protein.
In the vesicular stomatitis virus each gene transcribes a separate mRNA which translates a viral protein.
In the polio virus, on the other band, the entire genome codes a single giant polyprotein which is then cleaved into functional proteins.
Retrovirus represents an intermediate condition between the two.

Transcription probably does not begin until the host cells pass through the S-phase (DNA synthesis) and the subsequent 01 phase of the mitotic cycle.
Once it has begun.
However, viral RNA transcription Continues through successive cell divisions
H. Maturation of the virus occurs at the cell surface and the parti­cles are budded off from the plasma membrane.
Condensation of the nucleoid takes place beneath the cell membrane.

 
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