- The Temperature of Melting (Tm) is defined as the temperature at which 50% of double stranded DNA is changed to single-standard DNA. Tm of DNA does not depends on
(1) Length of DNA (2) % GC content
(3) Presence of cations (4) Presence of anionsKey Factors Affecting DNA Melting Temperature (Tm)
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% GC Content
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GC base pairs form three hydrogen bonds, making them more stable than AT pairs (two bonds). Higher GC content increases Tm.
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Length of DNA
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Longer DNA molecules have more base pairs, requiring higher energy (temperature) to denature.
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Presence of Cations
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Cations (e.g., Na⁺, Mg²⁺) neutralize the negatively charged phosphate backbone, stabilizing DNA and raising Tm.
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Why Anions Do Not Influence Tm
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Anions (e.g., Cl⁻, SO₄²⁻) do not interact directly with DNA’s phosphate groups.
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Ionic strength effects on Tm are primarily due to cations, not anions.
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Anions do not stabilize or destabilize DNA structure significantly.
Summary Table
Factor Effect on Tm Explanation % GC content Increases More GC pairs → stronger hydrogen bonding and stability DNA length Increases Longer DNA → more base pairs to disrupt Presence of cations Increases Cations neutralize phosphate charges, reducing strand repulsion Presence of anions No effect Anions do not interact with DNA backbone or base pairing
Conclusion
The melting temperature (Tm) of DNA does not depend on the presence of anions, as they play no significant role in stabilizing or destabilizing the double helix.
Correct answer:
(4) Presence of anions -
