Final answer:
RNA molecules are synthesized by adding new nucleotides to the 3' end of the growing strand, following a 5' to 3' direction, during a process catalyzed by RNA polymerase.
Step-by-step explanation:
When RNA is synthesized, new nucleotides are added to the 3' end of the growing RNA strand. The enzyme responsible for this process is RNA polymerase, which synthesizes RNA using the template strand of DNA. As RNA polymerase moves along the DNA, it unwinds the DNA ahead of itself and adds RNA nucleotides to the 3' end of the RNA molecule. This process follows a 5' to 3' directionality, which means that RNA polymerase attaches each new nucleotide to the 3'-hydroxyl (3'-OH) group at the end of the RNA strand. The DNA strand used as the template for RNA synthesis is the antisense strand, and the RNA synthesized is complementary to this DNA template strand.