What is the directionality of DNA strands and what does 5' to 3' indicate?

• A. Strands are antiparallel.
• B. Synthesis occurs 3' to 5'.
• C. Strands are parallel; synthesis is 5' to 3'.
• D. Strands are antiparallel; synthesis occurs 5' to 3' with nucleotides added to the 3' end.

Answer: (D)

DNA strands run in opposite directions, so they’re antiparallel. Each strand has a 5' end (with a phosphate) and a 3' end (with a hydroxyl). The key point is that DNA polymerase can only add new nucleotides to a free 3' hydroxyl group, so synthesis proceeds in the 5' to 3' direction. As a result, the growing strand extends from its 3' end, and the two strands are oriented in opposite directions relative to one another. This also explains why the leading strand is made continuously toward the fork while the lagging strand is synthesized in short 5' to 3' segments (Okazaki fragments) away from the fork. The phrase 5' to 3' indicates both the direction in which nucleotides are added and the orientation of the strand ends.