Final answer:
Structure 1 in the context of DNA replication is best described as the leading strand, synthesized continuously in the 5' to 3' direction, requiring only a single RNA primer for its elongation.
Step-by-step explanation:
The correct answer to the question 'Which statement best describes structure 1?' given the context of DNA replication is 'It is the leading strand because it is being built continuously.' This description fits the leading strand which is synthesized continuously in the 5' → 3' direction, unlike the lagging strand which is created discontinuously in segments called Okazaki fragments and requires a new RNA primer for each fragment. The leading strand only requires a single RNA primer to initiate synthesis by DNA polymerase, and the enzyme adds nucleotides in the direction of the replication fork, proceeding in a continuous fashion.