Now, what exactly is a retrovirus? Well, a retrovirus is a virus that contains RNA molecules as well as a special enzyme that catalyzes the formation of viral DNA from that viral RNA. So that's why we have reverse transcriptase because it synthesizes our nucleotides in the opposite direction to what is normally seen in nature. Normally we synthesize DNA to RNA, but in this case it goes from RNA to DNA in the opposite process. So HIV is a retrovirus, which means it has an enzyme called reverse transcriptase that can form viral DNA from the viral RNA. So these blue molecules, once inside the cytoplasm attached to the red RNA molecules and they begin to synthesize the viral DNA.