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Replication

Replication is used to make many copies of DNA so that the number of cells or parts of an organism can increase and replicate to offspring.

First, the DNA strands must be slightly separated so that high speed separation can take place. Then complementary replication of one strand is performed. The two strands are separated and are called the leading strand and the lagging strand. As the strands are separated, this puts twisting forces onto the DNA, so the DNA must be untwisted. The replicated DNA must be repaired. In eucaryotes, the telomeres must be replicated.

Initiation

Some initiator proteins bind to a replication origin. They break a few hydrogen bonds between the nucleotide base pairs of the DNA strands. In eucaryotes, there are many replication origins, while in bacteria, there is only one. It is thought that the reason is that the eucaryote replication is much slower than for bacteria because of the histones.

Separation

A helicase protein uses ATP hydrolysis to move along the DNA and break the hydrogen bonds of the DNA strands. The separation of the strands is called the replication fork. Two helicases move away from the replication origin, so there are two replication forks, and the whole entity is called a replication bubble. There are also single strand binding proteins. (SSB's) that bind to the DNA and each other to straighten the DNA, and keep the DNA strand from binding to itself.

Complementary Replication

A DNA primase is bound to the helicase, and is called a primasome. The DNA primase starts replication by loading an RNA primer. onto the DNA strand. A DNA polymerase binds triphospates to the DNA strand that it is bound to in a complementary manner. The DNA polymerase can periodically fall off of the DNA strand, so a protein that is called a sliding clamp is attached to the DNA polymerase. A clamp loader loads the clamp onto the DNA strand behind the DNA polymerase. The DNA polymerase only builds the new DNA chain in the 5' to 3' direction.

Leading Strand

On the leading strand, the DNA polymerase can copy the whole strand.

Lagging Strand

On the lagging strand, the chain must also be built in the 5' to 3' direction. This means that the DNA must be built in a backstitching manner. The DNA primase periodically loads a primer onto the lagging strand. Then a DNA polymerase will bind, and translate until it runs into another primer. These segments of DNA between the primers are called Okazaki fragments.

Untwisting

There is a protein called topoimerase that puts nicks into one strand of the DNA, and this allows the DNA to twist around the other rung of the DNA. This removes the torsional forces caused by the helicase separation.

DNA Repair

On the DNA, there are many types of repair that can be performed to ensure high integrity of replication. For one type of repair, a DNA ligase repairs the nicks between the Okazaki fragments.

Telomere Replication

In eucaryotes, the telomeres are replicated by a protein called a telomerase.

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