Methyl-tetrahydrofolate is the active form of folic acid.  It is used for one-carbon transfers throughout the body. This form of folic acid is responsible for assuring an adequate supply of thymidylate. Thymidylate is one of four nucleotide bases needed to construct DNA molecules.  DNA synthesis will stop during cell division if there is not enough thymidylate.  As you can see, folic acid plays an important role in DNA synthesis.