In eukaryotes, first the DNA of the specific gene unwinds. Then enzymes, known as RNA polymerases, use the DNA sequence, the pairing rules (U-A, G-C, C-G, A-T), and available RNA nucleotides to efficiently copy the DNA sequence into RNA. Thus, the DNA nucleotide adenine matches the RNA nucleotide uracil; thymine matches adenine; cytosine matches guanine; and guanine matches cytosine.

An RNA sequence can go to DNA, too. A process known as reverse transcription (see above) can convert an RNA sequence into DNA by using an enzyme known as reverse transcriptase. First observed in retroviruses like HIV, reverse transcriptase has also been identified as playing a role in the copying of DNA segments from one site to another in the genome.