Describe the anti-parallel orientation of double-stranded DNA and the significance of 5' to 3' directionality for synthesis.

Describe the anti-parallel orientation of double-stranded DNA and the significance of 5' to 3' directionality for synthesis.

• A. The two strands run in the same direction; DNA is synthesized 3' to 5' along the template.
• B. The two strands run in opposite directions; DNA polymerases synthesize in the 5' to 3' direction, reading templates 3' to 5'.
• C. The two strands are parallel but opposite; replication proceeds 5' to 3' on both strands.
• D. The strands alternate directions in a helix; synthesis occurs 3' to 5' on the growing strand.

Answer: (B)

Antiparallel arrangement means the two DNA strands run in opposite directions: one strand has its 5' end opposite the other's 3' end. DNA polymerases can only add new nucleotides to a free 3' end, so synthesis occurs in the 5' to 3' direction. To copy the DNA, enzymes read the template strand from 3' to 5' and build the new strand in 5' to 3'. This setup explains how both strands are copied simultaneously: the leading strand is made continuously toward the fork, while the lagging strand is made in short 5' to 3' pieces that are later joined, all still extending from a 3' end. In short, the two strands run opposite ways, and synthesis proceeds in the 5' to 3' direction by reading templates 3' to 5'.

Antiparallel arrangement means the two DNA strands run in opposite directions: one strand has its 5' end opposite the other's 3' end. DNA polymerases can only add new nucleotides to a free 3' end, so synthesis occurs in the 5' to 3' direction. To copy the DNA, enzymes read the template strand from 3' to 5' and build the new strand in 5' to 3'. This setup explains how both strands are copied simultaneously: the leading strand is made continuously toward the fork, while the lagging strand is made in short 5' to 3' pieces that are later joined, all still extending from a 3' end. In short, the two strands run opposite ways, and synthesis proceeds in the 5' to 3' direction by reading templates 3' to 5'.