Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Organization of Genomes
Life Cycle of Retroviruses

The retroviral genome is represented by a single-stranded (+)RNA, with a size of up to 10 kb. The (+)RNA is capped at the 5’ end and contains a poly(A) tail at the 3’ end, along with direct terminal repeats (Fig. 11.22).

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Fig. 11.22. Retroviral (+)RNA

The viral RNA encodes 3 types of Proteins: structural Proteins of the virion core; enzymatic proteins (Reverse Transcriptase, integrase, protease); and proteins comprising the outer lipoprotein envelope of the virion.

The viral particle contains two (+)RNA molecules, meaning Retroviruses are diploid.

A special role in The life cycle is played by reverse transcriptase. This enzyme uses single-stranded RNA as a template to synthesize double-stranded DNA. Reverse transcriptase requires a primer to function, The Role of which is played by a cellular tRNA molecule located within the virion. Reverse transcriptase also exhibits RNase H activity, enabling it to degrade the RNA strand from an RNA-DNA duplex.

The formation of linear DNA mediated by reverse transcriptase on the retroviral (+)RNA template occurs in the Cytoplasm. As a result of reverse METABOLISM/31.html">Transcription, the resulting DNA is somewhat longer than the retroviral (+)RNA. Following synthesis, the DNA enters The Nucleus, where it is converted into a circular form. Subsequently, the retroviral DNA is integrated into the host chromosome. During Cell Division, the provirus replicates alongside the chromosome and is passed on to daughter Cells. The retrovirus genome integrated into the cellular chromosome is transcribed to produce RNA, a process carried out by cellular RNA polymerase II. Some of the primary transcripts undergo capping, splicing, and polyadenylation. These processed RNAs serve as templates for Protein Synthesis. Another portion of the transcripts undergoes the aforementioned modifications with the exception of splicing; these molecules are packaged into the virion (and may also serve as templates for the synthesis of certain proteins) (Fig. 11.23).

Fig. 11.23. Retroviral life cycle