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Retroviruses

Retroviridae

For medical students3 min readUpdated 2026-10-10

Retroviruses are a family of spherical, enveloped RNA viruses characterized by the presence of the enzyme reverse transcriptase. This enzyme allows them to synthesize DNA from a viral RNA template and integrate their genetic material into the host cell genome as a provirus.

MorphologySpherical virions ranging from 80 to 130 nm in size
Genome TypeDiploid: two identical copies of single-stranded positive-sense RNA (+ssRNA)
Key EnzymeReverse transcriptase (RNA-dependent DNA polymerase)
Cellular TargetCD4+ T lymphocytes (for HTLV group viruses)

General Characteristics and Virion Structure

The family Retroviridae comprises a unique group of viruses containing approximately 150 different species. Members of this family are spherical particles with a diameter varying between 80 and 130 nanometers. The virion possesses a complex, multi-layered organization. Externally, it is covered by a supercapsid, which is an outer lipid envelope. Beneath this envelope lies the nucleocapsid core, characterized by an icosahedral type of symmetry.

The most defining feature of all family members is the presence of a specific enzyme—reverse transcriptase, also known as RNA-dependent DNA polymerase. This enzyme is physically closely associated with the viral genetic material. The viral genome itself has a unique structure: it is diploid and consists of a complex of two absolutely identical subunits of single-stranded positive-sense RNA (+RNA).

Organization of the Genetic Apparatus

The organization of the retroviral genome is strictly ordered. The genome contains structural envelope proteins, enzymes, and specific regulatory elements. In addition, approximately 30 different oncogenes (oncoantigens) are known to exist within the genome.

Structural genes are arranged in a specific sequence (from the 5'-end to the 3'-end):

At the flanks, structural genes are bounded by Long Terminal Repeats (LTRs). These regulatory sequences perform a critical function: they control viral transcription and integration. Structurally, LTR elements include binding sites for cellular polymerases and tRNA (serving as a primer), a transactivator gene that acts as a transcription enhancer, and repeated sequences at the ends necessary for successful integration of the viral DNA into the host cell chromosome.

Life Cycle and Replication

Retroviruses have strict requirements for cultivation conditions: they do not replicate in embryonated chicken eggs. For successful in vitro propagation, sensitive cell cultures or susceptible animals are used.

The initial stages of the replicative cycle begin with the penetration of the virion into the target cell via endocytosis. Once inside, the virus undergoes uncoating: the nucleocapsid is released from the vacuole, leading to the activation of reverse transcriptase.

Subsequent reverse transcription proceeds in three obligatory steps with the direct participation of reverse transcriptase:

  1. A minus-strand DNA is synthesized using the original viral RNA as a template. Cellular tRNA is used as a primer for this process.
  2. Enzymatic cleavage (degradation) of the original template viral RNA occurs.
  3. A plus-strand DNA is synthesized using the newly synthesized first DNA strand as a template.

The outcome of these reactions is the formation of a linear double-stranded DNA molecule. Due to the presence of LTR regulatory structures, this molecule circularizes. In the final replication step, the synthesized circular viral DNA integrates into the host cell genome, forming a provirus. Subsequent transcription of the integrated provirus genes is carried out by the host cellular enzyme, RNA polymerase II.

Classification and Clinical Significance

The family Retroviridae is characterized by wide diversity and high pathogenicity. Approximately 150 species of these viruses are capable of inducing tumors in animals. In modern taxonomy, the family is divided into two major subfamilies:

Among the vast number of retroviruses, four primary species are recognized as clinically significant for humans. This group includes the human immunodeficiency viruses (HIV-1 and HIV-2, which are classic members of the family) and the human T-lymphotropic viruses (HTLV-1 and HTLV-2).

The HTLV group (Human T-lymphotropic virus) belongs to the genus Deltaretrovirus. Their primary target in the human body is CD4+ T lymphocytes. Oncogenicity has been reliably proven for HTLV-1 and HTLV-2.

Frequently asked questions

What enzymes, besides reverse transcriptase, are part of the retroviral virion?

In addition to reverse transcriptase, retroviral virions contain several other enzymes required for replication encoded by the pol gene:

  • Integrase — enzyme for inserting viral DNA into the host cell genome.
  • Protease — enzyme for processing viral polyproteins.
  • RNase H — enzyme that degrades the RNA template during viral genome replication.
What receptors and coreceptors does HIV use to enter the target cell?

To enter the target cell, HIV uses the primary receptor CD4 and chemokine coreceptors CCR5 and CXCR4. The process includes:

  • Primary receptor — CD4, which initially binds the viral surface glycoprotein gp120.
  • Coreceptors — chemokine receptors CCR5 and/or CXCR4, secondary binding to which is critical for viral entry.

Early in infection, the virus typically exhibits tropism for macrophage CCR5 receptors, whereas later it often shifts to CXCR4 on T cells.

What specific diseases does HTLV-1 cause in humans?

HTLV-1 belongs to human T-lymphotropic viruses and has proven oncogenicity, being the etiologic agent of adult T-cell leukemia/lymphoma (ATLL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Its primary cellular targets are CD4+ T lymphocytes.

Through what mechanism do new retrovirus virions exit the infected cell?

New retrovirus virions exit the infected cell via budding. During release, the viral particle acquires a patch of the host cell plasma membrane, which forms the outer lipid bilayer envelope (supercapsid) of the virion. In HIV, this process is facilitated by accessory proteins such as Vpu, which aids in the efficient release of new virions.

What surface and transmembrane glycoproteins does the env gene encode in HIV?

The env gene in human immunodeficiency virus encodes outer envelope structural proteins (glycoprotein spikes). Depending on the viral type, this gene encodes:

Virus TypeSurface GlycoproteinTransmembrane Glycoprotein
HIV-1gp120gp41
HIV-2gp140 / gp105gp36

In HIV-1, surface glycoprotein gp120 projects outward and mediates initial receptor binding, while transmembrane glycoprotein gp41 anchors in the lipid bilayer and drives membrane fusion.

What is the main feature of retroviruses?

They are reverse-transcribing viruses. The presence of the enzyme reverse transcriptase allows them to synthesize DNA from a viral RNA template for subsequent integration into the host genome.

In what form does the viral genome integrate into the host cell chromosome?

Integration occurs in the form of a provirus. Following reverse transcription, the double-stranded viral DNA circularizes and integrates into the host genome.

What type of genome is characteristic of the family Retroviridae?

The genome is diploid. It is represented by a complex of two identical single-stranded positive-sense RNA (+RNA) subunits.

Which cells are targeted by HTLV?

The primary targets of human T-lymphotropic viruses (HTLV-1 and HTLV-2) are CD4+ T lymphocytes.

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