Retrovirus structure

Retrovirus StructureOn average, retroviruses are about 90 nanometers (nm, about 0.000004 in) across, which means they are so small that they can only be seen with an electron microscope.

Retroviruses consist of a flexible outer membrane called the envelope that surrounds a protein case known as the capsid.The envelope is studded with glycoproteins, chemical receptors that enable the virus to lock on to target cells.The capsid’s hollow interior contains two identical strands of RNA.

These RNA strands make up the virus’s genetic program and store all the instructions needed to replicate the virus once it has infected a host cell.Retroviruses also contain molecules of an enzyme called reverse transcriptase.When a virus infects a cell, reverse transcriptase copies the genetic instructions in the virus’s RNA and uses it to build complementary strands of DNA.

In common with most viruses, retroviruses are highly selective about the hosts that they infect. For instance, HIV only causes disease in human cells, not the cells of other types of animals.Also like most viruses, retroviruses show no signs of life when they are isolated from living cells. Many retroviruses, including HIV, are relatively fragile.

Their ability to infect cells lessens after prolonged exposure to the environment outside a host cell.When retroviruses do infect a host cell, their mode of replication leads to frequent mutations—changes in the genetic makeup of viral offspring.These mutations enable viruses to evolve at a rapid rate.

Genetic mutation is one of many reasons why retroviral infections are difficult to treat—medicines developed to combat one retrovirus with a specific genetic makeup are not effective against mutated offspring of that retrovirus.