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Retroviruses
Last reviewed: 04.07.2025
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[ Structure of retroviruses
The genome is represented by single-stranded non-fragmented positive RNA, consisting of 9000-9700 nucleotides, but in the form of two identical molecules that are linked by their 5'-ends. Therefore, their genome is diploid. Retroviruses are the only family of viruses with a diploid genome.
The virion contains reverse transcriptase, i.e. RNA-dependent DNA polymerase, or revertase. The family received its name based on this feature (English retro - back). This enzyme, called the polymerase complex, consists of several domains and has 3 types of activity: reverse transcriptase, RNase H and DNA-dependent DNA polymerase.
Due to the presence of reverse transcriptase, the RNA genome of the virus in the cell is converted into a DNA genome 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 (oncoviruses).
Since the function of reverse transcriptase is not controlled, the enzyme makes many mistakes. This leads to 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.
Retroviruses are divided into 5 forms according to the structure of the nucleocapsid and its location in the virion: A, B, C, D, E. In type A viruses, the spherical nucleocapsid occupies most of the virion. In type B viruses, the nucleocapsid is round and eccentrically located. In type C viruses, the spherical nucleocapsid is located in the center of the virion. In type D viruses, the nucleocapsid is cylindrical (like a projectile) and is located centrally in the virion. Type E viruses are similar in morphological features to type C viruses, but differ from them in a number of other properties.
All retroviruses have common structural genes: gag, pol, env, but antigenic links between genera of viruses are either absent or extremely weak.
The Retroviridae family includes three subfamilies.
- Spumavirinae - "foaming" viruses; this name is given because during reproduction in cell culture, intensive symplast formation occurs, which gives the culture a "foamed" appearance. No connection between these viruses and any pathological processes has been established.
- Oncovirinae are oncogenic viruses, i.e. viruses responsible for the transformation of a normal cell into a tumor cell.
- Lentivirinae - viruses that cause slow infections. This subfamily includes the virus that causes AIDS.