A retrovirus is a single-stranded RNA virus that stores its nucleic acid in the form of mRNA genome ( 5 'cap and 3'poly A tail). In most viruses, DNA is transcribed into RNA, and then RNA is translated into protein. However, in retroviruses their RNA is reverse-transcribed into DNA, which is integrated into the host cell's genome (when it becomes a provirus) and then undergoes the usual transcription and translational process to express the genes carried by the virus, i.e., RNA DNA RNA Polypeptide