When a gene expresses itself, it "switches on" to produce a protein. The gene does so by first directing the synthesis of an intermediary molecule called messenger ribonucleic acid (mRNA). To transfer a gene's information from DNA to mRNA, base pairing is used. However, there is one change. RNA uses a new rule: A matches only with U, and C only with G. So the base sequence of an mRNA molecule resembles that of the DNA molecule from which it was copied, except the base U appears anywhere the base T would appear in DNA.

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