What are retroviruses? 16th April 2024 – Tags: AIDS, genetics, genome, HIV, Infection, retroviruses, rna, Virus

Today we will look at viruses that have RNA in their structure, namely retroviruses. As we delve into the topic of immunology, we come across such creations as viruses. Viruses themselves are small microbes that infect human cells. Once infected, they use these cells as a replication site. In addition, viruses have many points of differentiation based on which we can assign them to smaller groups. One division of viruses is based on the type of genetic material found in the virus.

What are retroviruses?

Retroviruses are a genus of viruses belonging to the Retroviridae family. They use RNA as their genetic material. In addition, their name derives from a special enzyme that is essential for their life cycles – reverse transcriptase. The key steps in the life of retroviruses that distinguish them from other viruses are reverse transcription and genome integration.

Retroviruses that cause disease in humans:

• HIV virus – the virus that causes AIDS disease
• T-lymphotropic virus type 1 and type 2 – a sexually transmitted virus that causes T-cell leukemia in adults

Structure of retrovirus:

A retrovirus also differs in its structure from other types of viruses. Retroviruses are built from two concentric outer circles of lipid bilayer, which inside contains a complex of envelope proteins. The capsid proteins are hexagonal in shape, and the copies of the RNA genome are in the form of loop bound nucloproteins.

The viral envelope is mainly composed of lipids and glycoproteins encoded by viral genes. The envelope mainly serves a protective function against the extracellular environment. In addition, it helps the retrovirus to fuse with the host cell membrane.

Inside the retrovirus is genetic material in the form of RNA. It consists of an RNA dimer with a 5′ cap and a poly-A tail at 3′ in length. At the end are contained non-coding genes that aid in replication.

The replication cycle of a retrovirus:

Most commonly, this cycle is exemplified by the replication of the HIV virus. Below we will divide this process into stages:

1. Specific glycoproteins are present on the surface of the viral envelope. These structures allow binding to specific receptors present on the surface of specific leukocytes
2. Fusion of the virus and the host cell membrane occurs. Removal of the capsid proteins occurs and the viral protein and RNA are released
3. Reverse transcriptase catalyzes the reaction to synthesize a strand of DNA complementary to the viral RNA
4. Reverse transcriptase catalyzes the reaction to synthesize the second DNA strand, complementary to the first
5. The resulting double-stranded DNA is incorporated into the cell’s DNA as a provirus. DNA integration occurs
6. The provirus genes undergo transcription into RNA molecules, which will act as genomes in the next generation of viruses and serve as mRNAs for translation of viral proteins
7. Among the virus proteins, we can distinguish capsid proteins, reverse transcriptase (an enzyme formed in the cytosol) and envelope glycoproteins (formed in the ER)
8. The vesicles transport the glycoproteins to the cell membrane
9. The capsids are then assembled around the viral genome and reverse transcriptase molecules
10. At the very end of the cycle, the new virions detach from the host cell

Sources:

1. Standford.edu
2. Gentaur
3. Genome
4. Department of Health