HIV

HIV is a roughly spherical retrovirus approximately 120 nanometers in diameter. (About 25 million would fit on the head of a pin.) Retroviruses use RNA as their genetic material, carrying with them a protein called reverse transcriptase (RT) to copy their RNA into DNA. Another protein, called integrase, then integrates the viral DNA into the cell's genome, and the cell begins to make the proteins needed to produce a functioning virus. But the RT does sloppy work, so the DNA copies of HIV's genome are always a little different than the RNA originals. This is one of the root causes of HIV's diversity.

HIV has nine genes, three of which (gag, pol, and env) encode for polyproteins that get broken into smaller proteins. The Gag proteins make up the physical infrastructure of the virus; for example, Gag-p24 and Gag-p6 proteins protect the virus's RNA/protein core. The Pol proteins provide enzymes needed by the virus to reproduce, while the Env proteins stud the virus's surface and are used to enter a host cell. The remaining six genes encode for proteins that enhance expression of the virus's genes and inhibit the expression of the cell's genes.

Adapted from an image provided by Lou Henderson, hendersl@ncifcrf.gov.