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Internist infectious disease

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Retroviruses

Alexey Portnov, medical expert
Last reviewed: 19.11.2021

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Structure

Viruses belonging to this family have a number of the following features, peculiar only to them.

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Structure of retroviruses

The genome is a single-stranded unfragmented positive RNA, consisting of 9,000-9,700 nucleotides, but in the form of two identical molecules that are bound at their 5 'ends. Therefore, their genome is diploid. Retroviruses are the only family of viruses with a diploid genome.

The virion contains a reverse transcriptase, i.e., an RNA-dependent DNA polymerase, or a revertase. For this reason, the family received the name (English retro - back, back). This enzyme, called a polymerase complex, consists of several domains and has 3 kinds of activity: reverse transcriptase, RNase H, and DNA-dependent DNA polymerase.

Due to the presence of reverse transcriptase, the RNA gene of the virus in the cell is converted into a DNA-gene and in this form is integrated into the chromosome of the host cell, as a result of which it either dies (HIV) or turns into a tumor (oncovirus).

Due to the fact that the function of reverse transcriptase is not controlled, the enzyme allows many errors. This entails a high frequency of mutations in the genes encoding the structural proteins of the virus, i.e., its constant variability, which creates difficulties in creating effective vaccines.

By the structure of the nucleocapsid and its location in the virion, retroviruses are divided into 5 forms: A, B, C, D, E. In type A viruses, the spherical nucleocapsid occupies the most part of the virion. In a type B virus, a nucleocapsid of circular shape is located eccentrically. In a type C virus, the globular nucleocapsid is located in the center of the virion. The type D virus has a cylindrical nucleocapsid (shell type) with a central location in the virion. Viruses of type E are similar in morphological features to viruses of type C, but for a number of other properties they differ from them.

All retroviruses have common structural genes: gag, pol, env, but antigenic links between the genera of the viruses are either absent, or extremely weak.

The family Retroviridae includes three subfamilies.

Spumavirinae - "foaming" viruses; such a name is given because during the multiplication in the cell culture there is an intense symplastic formation, which gives the culture a "foamed" appearance. The links of these viruses with any pathological processes are not established.
Oncovirinae - oncogenic viruses, i.e. Viruses responsible for the transformation of a normal cell into a tumor cell.
Lentivirinae - viruses - pathogens of slow infections. This virus belongs to this subfamily , which causes AIDS.

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