DNA hybridisation question (AQA AS biology unit 2)

When DNA hybridisation experiments are carried out, why do biologists measure the temperature at which HALF the DNA has split into its two strands?
My biology textbook says that they measure the temperature when half the sample splits into the two strands it's made from. But it doesn't say why they record the temperature at half rather than when the strand has completely spit into two? Does anyone know why?

If anyone can advise me about this I'd be hugely appreciative!
Thank you! :smile:

Reply 1

Turkishee

DNA Hybridisation is a method which tries to compare DNA strands of different species.

Take Humans in one - you heat the DNA to break the hydrogen bonds between these base pairs.

You do the same to a different species.

The two strands of DNA are then complemented to a certain accuracy (since all living organisms evolve from a common ancestor). It's a technique to easily distinguish the DNA.

As for the temperature record - the annealed hybrid DNA will have weaker hydrogen bond, since certain segments may miss-match, it's important to note this temperature to avoid mistakes.

In essence that thermal stability provides an insight into how well bonded the two strands are.

Reply 2

stuuuuuuuuuuuu

Actually it is because of the shape of the DNA melting curve. If you look at this the graph is sigmoidal, you get a far more accurate representation of the differences between the two sequences if you measure it at 50% hybridisation. In other words small differences in % hybridisation correspond to small differences in temperature (equivalent to sequence similarity). It can be very difficult to measure exactly when the DNA is either fully separated or fully base paired and very small differences at this point of the curve correspond to large differences in temperature - as a result any measurement made here would be subject to far greater error.