DNA and RNA, like proteins, can be denatured to produce unfolded or uncoiled strands. Heating DNA to what is referred to as its “melting temperature” denatures it (the two strands of the double helix become separated). Why does a longer strand of DNA have a higher melting temperature than a shorter one?

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All right. Hello everyone. So this question says that the melting temperature of DNA is the temperature at which half of the DNA strands are in a random coil or single stranded configuration, determine whether the following statement is true or false. And state your reasoning. And the statement reads that longer strands of DNA have lower melting temperatures than shorter ones. Option A says the statement is true and option B says the statement is false with an explanation for each. All right. So first, let's just briefly clarify what the role of heat is in unwinding double stranded DNA into single strands. Recall that the double helix structure of DNA is stabilized in part by hydrogen bonds. This is because DNA being a nucleic acid is composed of several nucleotides, right? Those nucleotides contain nitrogenous bases that hydrogen bond with each other one on each strand. So for example, adenine bonds with thymine and cytosine bonds with guanine and DNA. When we're talking about RN A, it's adenine bonding with uracil, but cytosine and guanine are still there. So the hydrogen bonds between each individual strand of DNA is what maintains that double helix structure. Now, adenine and thymine has a total of two hydrogen bonds. Whereas cytosine and guanine have three hydrogen bonds making them more stable generally. So, what is the role of heat then in creating the single stranded configuration? Well, the role of heat in this context is to disrupt the hydrogen bonds that are holding the double helix together. So now let's let's think about the scenario at hand, right, how does the melting temperature change when talking about a longer DNA strip? Well, first of all, if we're talking about a longer strand, what that means is that more nucleotides are present to bind together and create that longer structure, right? And so more nucleotides means more bases and therefore more hydrogen bonds that are needed to hold that larger structure together. And so generally speaking, if more hydrogen bonds need to be disrupted, more energy would be required to break all of those hydrogen bonds or rather the higher temperature is required to break half of the hydrogen Botts present in the DNA strand. Therefore, the melting temperature must be high. So if the melting temperature is actually going to be higher in longer strands of DNA option B is correct because the statement is false. So the statement is false. And the longer the DNA strand, the more hydrogen bonding is expected, increasing DNA stability and melting temperature, more energy would be required to break half of those hydrogen bonds if there are more present. And so with that being said, thank you so very much for watching. And I hope you found this helpful.

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Key Concepts

Melting Temperature (Tm)

Hydrogen Bonds in DNA

Strand Length and Stability