Hello Rina,

I'm not sure I fully understood your question: do you mean why a longer, double-stranded DNA have a higher Tm if compared with a shorter one?

If so, it depends on the number of hydrogen bonds, which is higher if the fragment is longer.

If your question relates to the increased stability of DNA/RNA hybrids, you can check on ResearchGate the following question: 'DNA RNA hybrid is more stable than dsDNA , why?' Olivier Switzeny gave a detailed answer there.

Beacuse long extended strand has higher hydrogen bonding, so more energy need to break it. If those extended strand have more GC pair than Tm will even increased... so its mainly the H bond play a role.

This depend on density of GC pairs.