Most one‑carbon transfers are promoted by one of three cofactors: biotin, tetrahydrofolate, or s‑adenosylmethionine. s‑adenosylmethionine is generally used as a methyl group donor; the transfer potential of the methyl group in n 5 ‑methyltetrahydrofolate is insufficient for most biosynthetic reactions. however, one example of the use of n 5 ‑methyltetrahydrofolate in methyl group transfer is in methionine formation by the methionine synthase reaction; methionine is the immediate precursor of s‑adenosylmethionine. list the three methyl donors in order of methyl group transfer potential highest to lowest.

step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.