What chemical force must be overcome in order to separate the two DNA strands during replication?

Hydrogen bonds

Explanation:

DNA strands are held together by pair of complementary base pairs with Purines; Adenine and Guanine pairing with Pyrimidine: cytosine and Thymine;therefore Adenine pairs with Thymine,and Cytosine with Guanine.Theses bases are held together by the hydrogen bonds.

Two hydrogen bonds are in A-T, and three between C-G.

The process of formation of  another DNA molecules from the parent template is called replication. Under this process the  parent  double DNA strands unwinds,and fresh nucleotide line on the parent strands serving as the template.

Thus new double DNA stands are formed with one back bone of alternate sugar  and phosphate containing the new DNA strands,while the other strands conserving the parents strands contains the old back bone of sugars alternated with phosphate.This is semi conservative replication because, one of the double stands is new and the other is from the original parent strand.

Other form of replication is called conservative replication in which an entirely new double strands is formed,different from the parent DNA.

It begins with  the the unwinding of the DNA strands by the activity of enzyme Helicase which breaks down the hydrogen bonds between bases.This is followed by  attachment of primer to the old strands, and the catalysis by Enzyme DNA polymerase which ensure that new nucleotide are attach in 3'-5' direction to form a new strands.

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