Fatima, temperature is not necessarily inducing hydrolysis of DNA, which is in general a quite stable molecule... otherwise you would also get no PCR product! Short cycles of high temperature can be sustained quite safely by oligonucleotides as well as longer DNA fragments.

The other factor to consider is that you use primers in quite large excess (you often see a low band of unused primers in your agarose gel when you run your PCR products), so you can also tolerate a certain amount of degradation.

Acidic or alkaline conditions are much more dangerous for nucleic acids due to the nucleophile reaction that can occur on the phosphate bonds...

Melting temperature refers to the temperature at which DNA strands dissociate from each other and become single stranded (or more correctly when 50% of a DNA duplex is single stranded and 50% double stranded). During your denaturation cycle,  the DNA strands are 'melted' from each other, and the primer is also dissociated from the template.

 "Metling" means separation of strands as Tom describes. It simply keeps away the primer from the single stranded DNA till the time the annealing temperature is provided for the primers to bind . 'Melting' has not be taken here in literal sense.

I know... but I mean how come Primers for example Tm=65*C are safe from the 95*C .. what does protect primers in the denaturation stage?

Fatima, temperature is not necessarily inducing hydrolysis of DNA, which is in general a quite stable molecule... otherwise you would also get no PCR product! Short cycles of high temperature can be sustained quite safely by oligonucleotides as well as longer DNA fragments.

The other factor to consider is that you use primers in quite large excess (you often see a low band of unused primers in your agarose gel when you run your PCR products), so you can also tolerate a certain amount of degradation.

Acidic or alkaline conditions are much more dangerous for nucleic acids due to the nucleophile reaction that can occur on the phosphate bonds...

Thank you Dr. Pietro exactly that is answer I was looking for

thank you all for your help :)

Hi, just you have to refer back to molecular biology courses to understand very well the phenomenon, as said before temperature wasn't the only condition that can affect or regulate PCR phenomena, refer back to definition of strengency, one time you understand well that thing you'll not ask anymore about PCR, regards

Also, the denaturation, affects hydrogen bonds, while the primers and each strand of DNA consisted of nucleotides connected with phospho-diesteric bond, which is much stronger.