A student recently wrote in with a question: "I had a quick question as to whether DNA is able to bind to RNA. In the explanation for Question 4 in the Biology Bank 1 Qbank it states that Uracil would not be found in a DNA probe (which is similar to a primer) then in the explanation for Question 25 it says " A DNA strand can certainly hybridize with an RNA polymer. In fact, this process must be able to occur for transcription to take place". So if RNA (which would contain uracil) can bind to DNA then why does Q4 say you can't have uracil?"
While you are correct that cells will use RNA primers when copying DNA, a DNA probe actually uses DNA primers instead of RNA. Thus, since DNA doesn't include uracil, there wouldn't be uracil in the DNA primer. In other words, the Q4 explanation says you can't have uracil because uracil wouldn't be found in a molecule of DNA.
DNA and RNA do hybridize during transcription; indeed, it is through complementary base pairing that pre-mRNA is synthesized.
I hope this helps and good luck!
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