How nucleotides are added in DNA Replication

The replication of DNA begins at sequence of nucleotides called the origin of replication
Helicase unwinds the double-stranded DNA helix and single-strand binding proteins react with the single-stranded regions of the DNA and stabilize it
DNA Polymerase III is the major enzyme involved in DNA replication.DNA polymerase III can only add a nucleotide to the '3' end of pre-existing chain of nucleotides and it cant initiate a nucleotide chain
Therefore, an RNA polymerase, called a primase, constructs a RNA primer, a sequence about 10 nucleotides, complementary to the parent DNA
DNA polymerase III can then add deoxyribonucleotides to synthesize the new complementary strand of DNA
Because the two parent strands of DNA are antiparallel, they are oriented in opposite directions and must therefore be elongated by different mechanism
The leading strand elongates toward the replication fork by adding nucleotides continuously to its growing '3'end
IN contrast, the lagging strand, which elongates away from the replication fork , is synthesized, discontinuously as a series of a short segments, called Okazaki fragments
When the DNA Polymerase III reaches the RNA primer on the lagging strand, it is replaced by the DNA polymerase I, which removes the RNA and replace it with DNA. DNA ligase then attaches and forms phosphodiester bonds
The DNA is further unwound, new primers are made, and DNA polymerase III jumps ahead to begin synthesizing another Okazaki fragment
For simplicity, DNA polymerase III has been depicted as separate units, one acting on the leading strand and the other acting on the lagging strand
The current view of DNA polymerase III is that the two subunit functions together with the DNA on the lagging strand folding to allow the dimeric DNA polymerase molucule to replicate both strand of the parental DNA duplex simultaneously
Proteins other than DNA polymerase III are not shown

from : http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf