Retroviruses

Retroviruses fall into a unique category of viruses based on their unusual method of maintaining genetic information.  Viruses are not considered to be alive by most scientists because they don’t meet all the basic criteria for life (most notably the fact they aren’t made of cells).  Similarly to organisms, the vast majority of viruses use DNA as their primary genetic material.  Retroviruses, such as HIV, earn the ‘retro’ term because their genetic material moves in a flow which is opposite to anything else on the planet.

 According to the Central Dogma of Biology, DNA is the primary genetic material and is copied into RNA code, which in turn serves as the instructions for building proteins.  In retroviruses, however, genetic information is stored as RNA.  After the RNA is injected into the host cell during an infection, the RNA copied into DNA in a process called reverse transcription.  The newly created viral DNA, called provirus, is then integrated into the host organism’s genome and passed along during subsequent cell divisions.  In the future, this provirus can ‘wake up’ and stage a major infection on the host.

Retroviruses are fairly simple particles, with few notable components.  The outside of the retrovirus is a membrane layer, unlike most viruses.  There are only a few proteins the virus needs to do its job.  GAG proteins help locate the cell type the virus seeks to attack.  Reverse transcriptase is the enzyme responsible for copying the virus’ RNA code into DNA.  Integrase inserts the viral DNA into the host’s chromosomes.  Because the retrovirus’ genes assemble its proteins linked together instead of individually, proteases are needed to help separate the proteins after they are made.

The unique ways that retroviruses attack their hosts complicate these infections in a few ways.  Because the retrovirus inserts the provirus DNA into the host’s DNA, it can interfere with the organism’s genes. If the provirus lands in the middle of functional piece of DNA it could prevent that gene from working correctly, possibly causing cell death or cancer.  In cases where the viral DNA inserts itself between genes, the provirus never leaves, becoming part of the host’s genome.  Remarkably, 5-8% of the human genome consists of the remains of viral genes in our evolutionary past.

Retroviruses also present problems to those who try to develop cures.  Most genes change over little from one generation to the next because DNA can be stably copied many times.  Reverse transcriptase, however, has a tendency to make mutation-causing mistakes.  As a result, retroviruses change rapidly over time, foiling many possible treatments and making vaccination very difficult.