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Retrovirus Reproduction

Retroviridae

For medical students2 min readUpdated 2026-10-10

Retroviruses are a group of RNA viruses that utilize reverse transcription to copy their genome. A hallmark of their life cycle is the formation of a provirus, which integrates into the host cell's DNA, after which new viral particles are synthesized using host cellular machinery.

GenomeTwo copies of plus-sense single-stranded RNA (diploid genome)
Key EnzymeVirion-associated reverse transcriptase
HIV TargetsCD4+ T-helper cells, macrophages, dendritic cells, and microglial cells
Intracellular FormProvirus integrated into the host chromosome
DurationFrom several hours (rapid) to several days (chronic/latent)

Attachment and Entry

The first stage of retrovirus reproduction (e.g., HIV) is adsorption. Specific glycoproteins, such as gp120, are located on the virion surface. They bind to CD4 receptors on the plasma membrane of target cells. Successful viral attachment also requires coreceptors on the host cell surface.

Following attachment, penetration and uncoating occur. The viral envelope fuses with the host cell's plasma membrane, releasing the viral nucleocapsid—containing the genomic RNA and essential enzymes, including reverse transcriptase—into the host cytoplasm.

Reverse Transcription and Integration

The release of viral RNA into the cytoplasm triggers reverse transcription. This unique mechanism allows the virus to convert its genetic information from RNA into DNA.

The process occurs in two steps:

  1. Virion reverse transcriptase uses the genomic RNA as a template to synthesize a complementary minus-sense DNA strand, yielding linear cDNA.
  2. A plus-sense DNA strand is synthesized using the minus-strand DNA template. The result is a double-stranded circularized cDNA.

The completed circular DNA migrates from the cytoplasm into the cell nucleus, where it integrates into the host chromosome. The integrated viral genome represents recombinant DNA and is termed a provirus.

Synthesis of Viral Components

The integrated provirus serves as a permanent template for producing new viruses. At this stage, transcription is driven by host cellular machinery—specifically, host DNA-dependent RNA polymerase.

Two types of RNA are synthesized from the provirus:

Following synthesis, these proteins undergo processing in preparation for the assembly of new viral particles.

Virion Release

Assembled viral components form the nucleocapsid, after which the virus exits the cell. Retroviruses leave the host cell via budding.

During budding, the viral core becomes wrapped by a modified patch of the host plasma membrane, forming the envelope (supercapsid).

The matrix protein (M protein) plays a crucial role during budding. This hydrophobic protein lines the inner surface of the envelope (beneath the modified membrane). It acts as a mediator between the nucleocapsid and the outer lipoprotein shell, maintaining the structural integrity of the mature virion.

Frequently asked questions

Which receptors and coreceptors are required for HIV adsorption?

HIV adsorption and entry require interaction with a primary receptor and chemokine coreceptors.

  • Primary receptor (CD4) — surface viral glycoprotein gp120 binds here initially.
  • Coreceptors (CCR5 and CXCR4) — chemokine receptors whose secondary binding is a critical step for membrane fusion and viral entry.

Viral coreceptor tropism can shift: early in infection, the virus binds predominantly to CCR5 on macrophages, whereas later stages often show a shift to CXCR4 on T cells.

What enzymes are contained within the retrovirus nucleocapsid?

The nucleocapsid (core) of retroviruses contains a complex of enzymes required for viral replication, encoded by the pol gene:

  • Reverse transcriptase (p66/p51) — RNA-dependent DNA polymerase that synthesizes DNA from viral RNA.
  • Integrase (p31–32) — mediates the insertion of viral DNA into the host chromosome.
  • Protease (p10) — cleaves precursor polyproteins into functionally active mature proteins.
  • RNase H (p15) — part of the virion enzymatic complex.
Which viral glycoproteins are embedded in the HIV envelope?

The HIV envelope contains viral glycoproteins forming approximately 72 mushroom-shaped spikes, encoded by the env gene and consisting of two subunits:

  • Transmembrane glycoprotein (gp41) — anchored in the lipid bilayer, forms the base of the spike, and mediates fusion between the viral envelope and the host cell membrane.
  • Surface glycoprotein (gp120) — non-covalently linked to the transmembrane protein, exposed on the exterior (forming the "cap"), and responsible for initial binding to target cell receptors.
What are the main structural genes contained in the retrovirus genome?

The retroviral genome (using HIV as an example) contains three primary structural genes arranged from the 5' to 3' end:

  • gag (group-specific antigen) — encodes internal virion structural proteins, including matrix, capsid, and nucleocapsid proteins.
  • pol (polymerase) — encodes replication enzymes, including reverse transcriptase, integrase, protease, and RNase H.
  • env (envelope) — encodes surface and transmembrane envelope glycoproteins: gp120 and gp41.

In HIV, these genes initially encode large precursor polyproteins that undergo proteolytic processing.

What is the function of reverse transcriptase?

This enzyme synthesizes DNA using viral RNA as a template. It first generates a minus-sense DNA strand and then builds a complementary plus-sense strand on it, resulting in double-stranded circular DNA.

What is a provirus?

A provirus is double-stranded viral DNA that has integrated into a host cell chromosome. In this form, the virus can persist within the cell and serve as a template for synthesizing new viral RNAs.

Which cells does HIV infect?

HIV primarily infects cells bearing the CD4 receptor on their membrane, including CD4+ T-helper cells, macrophages, dendritic cells, and microglial cells.

What role does the M protein play in retrovirus structure?

The matrix (M) protein is located on the inner side of the viral envelope. It acts as a connecting link between the inner core (nucleocapsid) and the outer envelope, ensuring virion stability during budding.

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