What does it mean to say the two strands of a DNA double helix are antiparallel?

A. The two helices are parallel but don't bond together because they repel each other.

B. If you read them from left to right, one strand runs 5' to 3', and the other runs 3' to 5'.

C. They will be transcribed into completely different strands of RNA, making two different polypeptides.

D. The leading strand is built using Okazaki fragments instead of being continuously elongated.

B). If you read them from left to right, one strand runs 5' to 3', and the other runs 3' to 5'.

Explanation: Being antiparallel means that one of the two strands runs in 3'->5' direction while the other runs in a 5'->3' direction. The antiparallel nature of the two DNA strands make DNA synthesis to be semidiscontinous. Because DNA synthesis always proceed in the 5'->3' direction, the leading strand is synthesized continuously while the lagging strand is synthesized discontinuously in short pieces called Okazaki fragments. The continuous or leading strand is the one in which 5'->3' synthesis proceeds in the same direction as replication fork movement while the discontinuous or lagging strand is the one in which 5'->3' synthesis proceeds in the direction opposite to the direction of fork movement.