Question

Explain in full detail why retrovirus infections are always productive.

Answer 1

viruses can exist as nonliving virions outside the host and transform into pathogens once the enter a suitable host.Viruses unlike bacteria can reproduce only inside the host cells.Once the viruses enter the host cell they may remain latent without causing any disease or may immediately enter the phase of reproduction and kill the host cell

Most of the viruses have nuclear material in form of RNA and cause cytocidal infections however DNA viruses are known to produce persistent infections.Cytocidal infections are further classified into productive and abortive infections

The infection is said to be productive as the virus successfully completes replication cycle and produces progeny virions.

eg. retroviruses and herpes viruses.

The viron of retro viruses is single stranded ,linear , and non segmented with a positive polarity. Unique feature of this family of viruses is the reverse transcription of the virion RNA into linear double-stranded DNA and the subsequent integration of this DNA into the genome of the cell. Until the discovery of these viruses it was believed that the transfer was unidirectional that is from DNA to RNA but with the discovery of these viruses the central dogma had to be revised . since it involved the copying of RNA into DNA by a virus-coded polymerase, reversing the flow of genetic information,the terms "Retroviruses" and Reverse transcriptase have been coined.

Endogenous retroviruses (ERV) consist of the genetic material of extinct, the genomic constitution of which is similar to that of extant retroviruses. Human ERVs have been integrated within human DNA over the course of evolution.and are passed from one generation to the next nearly 5 percent of the human genome. HERVs are suspected of having influenced the evolution of certain elements of the human genome. They also have been implicated in certain human diseases, including multiple sclerosis

The most important and unique step in retroviral growth cycle are reverse transcription and integration.Reverse transcription generates a progenitor proviral DNA copy from which the entire viral progeny of the cell is derived by polymerase-II-mediated transcription though it is much error prone but once it integrates with the host genome it makes the infection permanent as integrated proviruses are only rarely lost from the cellular genome and remains forever.

Retroviruses also have developed unique mechanisms of pathogenicity involving the transfer or transcriptional activation of specific cellular genes based on genetic recombination between virus and cell and between viral genomes.They also show a series of effects on cell replication survival from pronounced growth stimulation to cell death.