At physiological pH DNA is ?
## Core Concept
DNA (Deoxyribonucleic acid) has a specific structure and charge at physiological pH due to the presence of phosphate groups and nitrogenous bases. The phosphate groups in the DNA backbone are ionized at physiological pH.
## Why the Correct Answer is Right
At physiological pH (around 7.4), the phosphate groups in the DNA backbone are deprotonated, giving them a negative charge. This is because the pKa of the phosphate groups is around 6.5-7.0, which means at pH 7.4, they are mostly in the form of PO4^3- (phosphodiester linkage). The nitrogenous bases have pKa values that are either too high or too low to be ionized at physiological pH, except for the imino group of cytosine and the amino groups which do not significantly contribute to the overall charge. The overall negative charge on DNA is crucial for its interaction with positively charged histone proteins in chromatin.
## Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because at physiological pH, DNA does not have a significant positive charge; rather, it has a net negative charge due to the phosphate groups.
**Option B:** This option suggests a zwitterionic form which, while partially correct in that DNA has both positive and negative charges, does not accurately represent the net charge of DNA at physiological pH.
**Option D:** This option is incorrect because DNA is not neutral at physiological pH; it carries a significant negative charge.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the negative charge of DNA is crucial for its packaging into nucleosomes and higher-order structures within the cell nucleus, and for its interaction with proteins such as histones.
## Correct Answer Line
**Correct Answer: C. **