A retrovirus is a type of virus that stores its genetic information as RNA and converts it into DNA inside a host cell, reversing the normal flow of genetic information. This backward process, called reverse transcription, is what gives retroviruses their name and makes them fundamentally different from other viruses. HIV, the virus that causes AIDS, is the most well-known retrovirus, but the family is large and diverse, with members that cause cancer, immune deficiency, and even sequences permanently woven into human DNA.
How Retroviruses Differ From Other Viruses
Most viruses follow a straightforward rule: DNA makes RNA, and RNA makes protein. RNA viruses that don’t have a DNA stage must copy their RNA genomes directly, producing both new viral RNA and the messenger molecules needed to make proteins. Retroviruses break this pattern entirely. They carry their genome as single-stranded RNA but convert it into double-stranded DNA once inside a cell. That DNA then integrates permanently into the host cell’s own chromosomes.
This integration is what makes retroviruses so persistent. Once viral DNA is stitched into a cell’s genome, it becomes part of that cell’s genetic code. Every time the cell divides, it copies the viral DNA along with its own. The cell essentially becomes a lifelong factory for new virus, reading the inserted viral genes and producing new viral RNA and proteins. This is why infections like HIV are so difficult to cure: you’d have to eliminate every single cell harboring integrated viral DNA.
Structure of a Retrovirus
Retroviral particles are tiny, measuring 80 to 100 nanometers across. Each particle carries two copies of its RNA genome, which ranges from about 7,000 to 12,000 genetic letters in length. The RNA is wrapped in a protein shell, and the whole package is surrounded by a lipid envelope stolen from the host cell’s membrane during the budding process. Studded into that envelope are viral surface proteins that the virus uses to latch onto and enter new cells.
Packed inside each particle are three enzymes that make the retroviral life cycle possible: reverse transcriptase, integrase, and protease. These enzymes are initially produced as part of a single large protein, which the protease enzyme then cuts apart into functional pieces as the virus matures.
The Three Essential Enzymes
Reverse transcriptase is the signature enzyme of retroviruses. It reads the viral RNA and builds a double-stranded DNA copy through a complex, multi-step process involving strand transfers and template switches. The process begins almost immediately after the virus enters a cell’s cytoplasm, using a small piece of cellular transfer RNA as a starter to begin copying. The result is a complete DNA version of the viral genome.
Integrase takes over next. It trims two nucleotides from each end of the newly made viral DNA, exposing sticky ends, then splices both ends into the host cell’s chromosome. Once integrated, this viral DNA (called a provirus) is read by the cell’s own machinery just like any other gene, producing new viral RNA and proteins.
Protease acts during the final stages. New viral particles initially bud from the cell surface as immature, nonfunctional blobs. Protease cleaves the large precursor proteins inside these particles into their active forms, transforming immature particles into infectious viruses ready to infect new cells.
Retroviruses That Infect Humans
Five retroviruses are known to infect people. The two most significant are HIV-1 and HIV-2, which cause AIDS. These viruses target and destroy CD4 immune cells, the white blood cells that coordinate the body’s immune response. As CD4 counts drop, the immune system collapses, leaving the body vulnerable to infections and cancers it would normally fight off easily. HIV-1 and HIV-2 can also infect brain tissue, sometimes causing a form of cognitive decline known as subcortical dementia.
The other human retroviruses, HTLV-I and HTLV-II, work in the opposite direction. Instead of killing CD4 cells, they cause them to multiply uncontrollably. HTLV-I is linked to a rare and aggressive blood cancer called adult T-cell leukemia/lymphoma, as well as a progressive neurological condition that causes weakness and stiffness in the legs. HTLV-II causes milder immune suppression that rarely progresses. A fifth virus, HTLV-V, is associated with a less severe form of skin lymphoma.
How Antiretroviral Drugs Work
Modern treatment for HIV targets multiple stages of the viral life cycle simultaneously, which is why the standard approach uses a combination of drugs from different classes. Each class blocks a different step.
- Reverse transcriptase inhibitors come in two varieties. One type mimics the building blocks of DNA and gets incorporated into the growing DNA chain, but lacks the chemical group needed to attach the next building block, effectively terminating the chain. The other type binds directly to the reverse transcriptase enzyme and physically blocks it from working.
- Integrase inhibitors prevent the viral DNA from being inserted into the host cell’s chromosomes, stopping the virus from establishing a permanent foothold.
- Protease inhibitors block the enzyme that cuts viral precursor proteins into functional pieces, so new viral particles bud from the cell but remain immature and unable to infect other cells.
- Entry inhibitors work at the very start of the process, either blocking the virus from docking with receptors on the cell surface or preventing the viral and cell membranes from fusing together.
Combining drugs from multiple classes makes it extremely difficult for the virus to develop resistance, since it would need to mutate in several different ways simultaneously. This strategy transformed HIV from a death sentence into a manageable chronic condition for most people with access to treatment.
Ancient Retroviruses in Your DNA
Because retroviruses integrate into host DNA, infections that hit reproductive cells (eggs or sperm) millions of years ago left permanent marks. These sequences, called human endogenous retroviruses (HERVs), make up roughly 1% of the human genome. They are essentially fossil viruses, remnants of infections that occurred in our ancestors and were then passed down through every subsequent generation.
Some of these ancient viral sequences arrived over 30 million years ago, while others appeared more recently, after humans and apes diverged. Most have accumulated enough mutations that they can no longer produce functional virus. But some HERV sequences still produce proteins, and a few appear to have been co-opted for useful purposes. The ability of these sequences to move around and reshape the genome may have provided a source of rapid genetic variation that plain mutation alone could not achieve.
Retroviruses as Tools in Gene Therapy
The same ability that makes retroviruses dangerous, inserting genes into a cell’s DNA, also makes them useful in medicine. Scientists have engineered retroviral vectors by gutting the virus of its disease-causing genes and replacing them with therapeutic ones. These modified viruses can deliver corrective genes directly into a patient’s cells, where they integrate and become a permanent part of the genome.
Retroviral vectors were the first viral delivery system used in human gene therapy and remain among the most widely used. Early trials used them to treat a severe immune deficiency caused by a missing enzyme, inserting a working copy of the gene into patients’ own immune cells. They have also been used to deliver cholesterol receptor genes to liver cells in people with dangerously high inherited cholesterol, and to target brain tumors by inserting genes that make tumor cells vulnerable to antiviral drugs.
One particularly creative application involves turning the virus against itself. Researchers have explored using retroviral vectors to deliver protective genes into the very CD4 immune cells that HIV targets, or into the stem cells that produce them, potentially making new immune cells resistant to HIV infection from the start.