At the replication origin DNA helicase attaches to a strand of DNA and begins to break apart hydrogen bonds in order to unravel a section of the double helix. The section of DNA that is unwound is called the replication bubble and the “Y” shaped sections are called the replication forks. In order to stop the unwound section from binding back together, single strand binding proteins react with the single strand portions on the DNA causing them to stay separated. Although the leading strand is replicated continuously in the 5' to 3' direction, the DNA polymerase still needs to know where to start adding the complimentary nucleotides. So a short strand of RNA called a primer, binds at the 3' end of the pre-existing chain of nucleotides to serve as a starting point. Elongation begins when DNA polymerase enters the replication bubble and proceeds to add nucleotides one at a time to create a complimentary stand to the original template strand of DNA. Once the complimentary strand is built, the single strand binding proteins release from the DNA and leave. Polymerase I comes and proof reads the base pairing and ligase follows to fix the first starting point where the primer was, by catalyzing the formation of phosphate bonds between nucleotides. The lagging strand is replicated non-continuously, because it's 5' to 3' direction is the opposite direction to the movement of the replication fork. Therefor, the complimentary strand of DNA is synthesized in short segments known as Okazaki fragments. An RNA primer acts as the beginning point of the elongation for each new strand of DNA. The enzyme known as primase is needed to make the primer. When the primer is in place, polymerase III can start building the complimentary strand in fragments by adding new nucleotides. Polymerase I then comes and dismantles the RNA primer and fills those spaces with the proper nucleotides and performs its exonuclease activity by proof reading and editing the base pairing...
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