Why are Okazaki fragments necessary?

• A. Because DNA polymerase can only synthesize new DNA in 3' to 5' direction.
• B. Because replication occurs in multiple origins.
• C. Because DNA polymerase can only synthesize new DNA in 5' to 3' direction, requiring lagging strand to be made in short segments.
• D. Because RNA primers must be replaced.

Answer: (C)

DNA polymerase can only add nucleotides to a growing strand in the 5' to 3' direction. Because the two strands of DNA are antiparallel, copying one strand toward the fork is straightforward and continuous (the leading strand), but copying the other strand would require building in the opposite direction. To keep synthesis in the 5' to 3' direction on the lagging strand, the cell makes it in short pieces called Okazaki fragments, each starting with a primer. Later, these fragments are connected into one continuous strand after the RNA primers are removed and replaced. So Okazaki fragments exist because of the directional constraint of DNA synthesis combined with the opposite orientation of the two strands.