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The retrovirus (oncovirus) contains RNA. It also has a molecule called reverse transcriptase. This molecule transcribes RNA into cDNA. This cDNA is the DNA copy of viral RNA genome

RNA has Uracil instead of thymine and DNA has thymine instead of uracil. So, how can RNA be converted into DNA? Where does Uracil go and where does thymine come from?

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RNA has Uracil instead of thymine and DNA has thymine instead of uracil. So, how can RNA be converted into DNA?

I think you may want to ask a more basic question. DNA and RNA are extremely similar, with only an oxygen being the difference. This has vast consequences for organisms, and life as we know it uses DNA and RNA for very separate purposes. DNA stores the genome providing the instructions for life, and RNA in a nutshell is how that message gets converted into protein for a useful action. RNA is generally less stable than DNA, so this arrangement works out well.

DNA is regularly and constantly transcribed to RNA, so Thymine is regularly used in favor of Uracil. There is a fantastic answer here on Biology.SE already dealing with why so I suggest reading that. The how is pretty straightforward, there are different molecules used! DNA polymerases incorporate Thymine whereas RNA polymerases incorporate Uracil. The reverse transcriptase that is encoded by the retrovirus does just this: it transcribes RNA to DNA, using Thymine instead of Uracil. Both Uracil and Thymine are present in the cell and are thus available for use.

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I think it might be of use to show you the difference between uracil and thymine

They are very similar structures. The part that is involved in base pairing is actually the Nitrogen and the oxygen furthest from the sugar. See below:

So having an extra methyl group on the other side of the molecule does not interrupt basepairing. And remember, with DNA/RNA synthesis you have a template, and the new strand is based on the template. So, when the RT enzyme Medhat mentions encounters an adenine in the template RNA, it will pair it with a thymidine. And when it encounters a uracil, it is able to pair it with an adenine. The enzyme is structured specifically to only allow in thymine and not uracil, which is how it makes the distinction.

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