What will happen to DNA if salt is added to it: (PGI Dec 2008)
## Core Concept
The addition of salt to DNA affects its structure and solubility due to the ionic interactions between the salt ions and the phosphate backbone of DNA. DNA is a negatively charged molecule due to the phosphate groups in its backbone. The presence of salt can influence the electrostatic interactions within DNA and between DNA and other molecules.
## Why the Correct Answer is Right
When salt is added to DNA, the positively charged ions (cations) from the salt interact with the negatively charged phosphate groups on the DNA backbone. This interaction helps to neutralize the negative charges on the DNA, reducing the electrostatic repulsion between the negatively charged phosphate groups. As a result, the DNA double helix becomes more stable and less likely to denature. This phenomenon is related to the concept of **Debye-Hückel theory**, which explains how ions affect the interactions between charged particles in a solution.
## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because adding salt does not directly lead to the degradation of DNA into smaller fragments. While high salt concentrations can affect DNA solubility and stability, it does not directly cause enzymatic degradation.
- **Option B:** This option is incorrect because the addition of salt does not directly result in the precipitation of DNA out of solution in a manner that would prevent PCR (Polymerase Chain Reaction) without considering other factors like alcohol precipitation.
- **Option C:** This option suggests an increase in melting temperature (Tm), which is correct but let's evaluate all options.
## Why Option D is Incorrect if Chosen
- Assuming **Option D** suggests a decrease in melting temperature or another incorrect effect, it would be incorrect because the addition of salt generally increases the melting temperature of DNA by shielding the negative charges on the phosphate backbone, thus stabilizing the double helix.
## Clinical Pearl / High-Yield Fact
A high-yield fact to remember is that the **melting temperature (Tm) of DNA** increases with the addition of salt. This is crucial in molecular biology techniques like PCR, where the optimal annealing temperature is critical for specificity and efficiency. Increasing salt concentration can help in achieving a more stable duplex, but excessive salt can also inhibit enzyme activity.
## Correct Answer: C.