While replicating, the entire DNA molecule to keep the whole molecule stabilized does not open in one go because it would be highly expens energetically. Actually, unwinding creates tension in the molecule as uncoiled parts.

Actually, unwinding creates tension in the molecule as uncoiled parts starts forming supercoils due to the interaction of exposed nucleotides.

Instead, the helicase enzyme acts on the double stran at ori site (origin of replication) and a small stretch is unzipped. Immediately, it is held and stabilized by single-strand binding proteins.

Slowly with the help of enzymes, exposed strands are copied as a point of unwinding moves and ahead in both directions.

It gives the appearance of a Y-shaped structure which is called the replication fork.

The two functions that the monomer units of NTPs play are 

(i) They pair up with exposed nucleotides of the template strand and make phosphodiester linkages and release a pyrophosphate.

(ii) Hydrolysis of this pyrophosphate by enzyme pyrophosphatase releases energy that will facilitate making hydrogen bonds between free nucleotides and bases of the template strand.