[Fzang]

Okay, so in this experiment I'm heating a short string of DNA, and around the melting point (50 C) you can see that the slope is steepest on an abs/temp graph, and thus determine the melting point.

And then there's a question in the manual I can't answer: "why is the observed melting point from heating not necessarily the same value you would obtain from cooling".

And I don't know. Is it because the structure of the DNA has been altered/damaged during heating all the way up to 70 C? If you looked at it as an "ideal molecule", heating or cooling wouldn't make a difference, would it? The measured abs and temperature datas would be the same.

[fledarmus]

You might want to research the term "hysteresis" as it applies to DNA and melting points...