What are Retro Viruses?

Answer 1

Retroviruses are RNA viruses. Once they infect a living cell, they synthesize a complementary DNA strand (cDNA) of this RNA molecule using the enzyme REVERSE TRANSCRIPTASE (basically RNA dependent DNA polymerase). The enzyme reverse transcriptase is coded by the viral genome itself and not the host's. The infecting virus carries an already synthesized enzyme. The RNA molecule in this RNA-DNA hybrid is then degraded. The DNA molecule then uses the host polymerases (DNA dependent DNA polymerase) and synthesize a daughter DNA strand to form a dsDNA molecule which then intergrates into the host genome.
Central dogma follows DNA to RNA. Since these viruses go the other way around they are called as RETROVIRUSES. The well known HIV is an example for retrovirus.