HIV AIDS virus: Rotating virus particle showing internal structure. Envelope and part of matrix shell disappear to reveal the contained core. The golden spikes that radiate out are proteins that help the virus to attach to cells. They are composed of gp120 (outer knob) and gp41 (stalk, trans-membrane section, and cytoplasmic tail). The virus is covered by a viral envelope (translucent green) derived from the host cell during budding. As the virus rotates part of the surface is cut away revealing the Matrix proteins (MA: blue cage-like structure) and the purple core (which contains the viral RNA and the enzyme reverse transcriptase). HIV is a retrovirus, so-called because it reverse transcribes its RNA into DNA. This viral DNA is then integrated into the host cell DNA.