II.

Structure

HIV belongs to the retrovirus family of viruses, whose members share a unique method of replicating themselves when they infect living cells. Retroviruses store their genetic information in molecules of ribonucleic acid (RNA). However, unlike other RNA viruses, retroviruses use RNA as a template (master pattern) for forming deoxyribonucleic acid (DNA), the genetic material that puts viral replication instructions into effect. This process, called reverse transcription, is the exact opposite of the normal flow of genetic information in living things, in which DNA serves as the template for RNA formation (see Genetics).

HIV consists 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 onto target cells. Inside the capsid reside two identical strands of RNA. These RNA strands make up the virus’s genetic program and store all the instructions needed to replicate HIV once it has infected a host cell. HIV also contains molecules of an enzyme called reverse transcriptase. When HIV infects a cell, reverse transcriptase copies the genetic instructions in the virus’s RNA and uses the instructions to build complementary strands of DNA.