A new DNA strand elongates only in the 5' to 3' direction because _____.
a. DNA polymerase can add nucleotides only to the free 3' end
b. the polarity of the DNA molecule prevents addition of nucleotides at the 3' end
c. Okazaki fragments prevent elongation in the 3' to 5' direction.
d. DNA polymerase begins adding nucleotides at the 5' end of the template
e. replication must progress toward the replication fork

Polymerase DNA always works in one direction, synthesizing the new 5' to 3' oriented chains and adding nucleotides to the 3' end of a new synthesis chain by forming phosphodiester bonds between the phosphate of a nucleotide and the sugar of the anterior nucleotide.

Because DNA polymerase only acts in a 5' to 3' direction, replication along a chain, the leading chain, occurs continuously. The synthesis of the opposite chain, the delayed chain, occurs discontinuously because the DNA polymerase must wait for the replication fork to open. Over the delayed chain, short segments of DNA called Okazaki fragments (named after Reiji and Tsuneko Okazaki, the scientists who discovered these fragments) are synthesized as polymerase DNA works out of the replication fork. Ligase DNA catalyzes the covalent bonds between Okazaki fragments in the delayed chain to ensure there are no gaps in the phosphodiester skeleton. Finally, the first ones are removed and these gaps are filled by the DNA polymerase.