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Unwinding of the Strand in Replication

 

Unwinding of the Strand in Replication
 The DNA double helix has to unwind in order to separate the two strands for replication. The T2 bacteriophage DNA molecule consists of about 200,000 nucleotide pairs and is about 70 microns in length. Since there are 10 base pairs per turn of the double helix, the molecule would have to undergo 20,000 rotations in unwinding during replication.

It therefore follows that replication must go on simultaneously with unwinding. It cannot wait for the complete unwinding of the DNA molecule. The unwinding of the DNA double helix is brought about by DNA unwinding proteins (DNA binding proteins). The proteins bind preferentially to single strands of DNA and promote unwinding  (denaturation) of double helical DNA.

The region of the double helix where the proteins bind forms a Y-shaped replication fork. About 200 molecules of the unwinding protein may be found at each fork. The binding of proteins to DNA is cooperative. An each molecule of the unwinding protein binds to DNA it enhances the binding of the next molecule. 1his results in further unwinding of the double helix.

The DNA unwinding protein is a tetramer with each of the four subunits having a molecular weight of 18,500 to 22,000. The protein may bind to DNA as a monomer or a dimer. Each protein subunit has a binding site of 8.10 nuc1eotides. Thus the 200 molecules of the protein at the replication fork separate about 200 x 10 = 2,000 base pairs.

DNA polymerases II and III will not propagate synthesis when the template DNA strand becomes longer than 50 nucleotides. This results in a decrease in the rate of synthesis. In E. coli the addition of DNA unwinding protein overcomes this inhibition in the case of polymerase II but not polymerase III.

Inhibition can be overcome in the case of polymerase In by the addition of protein factors I and II. As elongation continues, the DNA unwinding proteins are released. They can again participate in unwinding the parental strands.
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