Lagging strand

From Palaeos.org

DNA replication by DNA polymerase requires a short RNA primer sequence and works only in one direction, 5' to 3'. Since the two strands of DNA are antiparallel, this creates a problem. As a DNA helicase separates and unwinds the DNA strands, on one strand, the leading strand, the separated strand DNA strand is exposed 3' to 5'. Synthesis of a new antiparallel strand can then proceed from a new 5' end, and follow the helicase without interruption. However, on the other strand, the lagging strand, DNA synthesis must proceed in the reverse direction. This is accomplished by a secondary feature of the helicase. Every 500 or 1000 nucleotides, the helicase stimulates a primase to nick the lagging strand and insert a few RNA nucleotides as a primer. The DNA polymerase then proceeds to replicate that strand -- still 5' to 3' but now in the opposite direction because it is working on the opposite strand of the original DNA. Replication of the lagging strand continues until the polymerase reaches the last point at which the lagging strand was replicated. A DNA ligase then removes the primer from this previous fragment (an Ozaki fragment), and zips the two Ozaki fragments together.

Credits

ATW?