What is the basis for the difference in how the leading and lagging strands of DNA molecules are synthesized? a. The origins of replication occur only at the 5′ end. b. Helicases and single-strand binding proteins work at the 5′ end. c. DNA polymerase can join new nucleotides only to the 3′ end of a pre-existing strand, and the strands are antiparallel. d. DNA ligase works only in the 3′→5′ direction.

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1. The correct answer is c. DNA polymerase can join new nucleotides only to the 3′ end of a pre-existing strand, and the strands are antiparallel.
2. DNA polymerase adds nucleotides in a 5′ to 3′ direction. This means it adds nucleotides to the 3′ end of the growing strand.
3. Since DNA strands are antiparallel, one strand (the leading strand) can be synthesized continuously in the direction of the replication fork.
4. The other strand (the lagging strand) must be synthesized in short segments, called Okazaki fragments, which are later joined together by DNA ligase.
5. This difference in synthesis is due to the antiparallel nature of DNA strands and the specific functionality of DNA polymerase.