Final answer:
New bases are added to DNA strands in opposite directions during replication due to the antiparallel structure of DNA and the enzymatic limitation that DNA polymerase can only add nucleotides in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand is created in fragments that are later joined together.
Step-by-step explanation:
During DNA replication, new bases must be added to the complementary parental strands in opposite directions due to the antiparallel structure of DNA. This results from the DNA polymerase's ability to add nucleotides only in the 5' to 3' direction. As the DNA unwinds at the origin of replication, one strand, known as the leading strand, is made continuously because it runs in a 3' to 5' direction relative to the direction of the replication fork. Conversely, the other strand, running 5' to 3' and called the lagging strand, is synthesized in short sections called Okazaki fragments, which are later joined together.
The replication fork progresses rapidly, unwinding the DNA and presenting a situation where the DNA polymerase can only synthesize the leading strand smoothly in a continuous manner because it moves in the same direction as the fork. The lagging strand has to be patched together from Okazaki fragments because its direction is opposite to the movement of the replication fork, necessitating starting and stopping synthesis as new regions of the template strand become available.