Re: how are bonds between nucleotides broken?

The double helix unravels with the help of enzymes called helicases. The 
action of these enzymes creates the single stranded templates needed for 
semi-conservative (template-directed) synthesis of the new DNA strands. 
These enzymes are molecular motors that move along the DNA (driven by 
hydrolysis of ATP) and unwind the double helix so that the DNA polymerases 
can have access to the template strands.

Following are three sites that give good overviews of DNA replication and 
the role helicases play in theis proecess.

http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/eukarychrom/eukaryo2.htm 

http://www.hms.harvard.edu/news/releases/1099helicase.html 

http://faculty-staff.ou.edu/S/Barbara.Safiejko-Mroczk-
1/3113/Lectures/Lecture%2013/13.dna.replicat.html