At physiological pH DNA is
**Core Concept**
At physiological pH (around 7.4), DNA is a negatively charged molecule due to the presence of phosphate groups in its backbone. This charge is crucial for DNA's interactions with other molecules and its overall structure.
**Why the Correct Answer is Right**
DNA's negative charge arises from the deprotonation of phosphate groups at physiological pH. The phosphate groups have a pKa of around 1.5-2.5, which means they are mostly deprotonated at pH 7.4. This results in a negatively charged DNA molecule that interacts with positively charged molecules, such as histones, to form chromatin.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but for the sake of explanation, let's assume it's a distractor. In that case, it would likely be incorrect because DNA's charge is not dependent on its sequence or the presence of specific bases.
* **Option B:** This option is also not provided, but it might be incorrect if it suggests that DNA is neutral or uncharged at physiological pH. This is not true, as the phosphate groups contribute a significant negative charge.
* **Option C:** This option is not provided, but it might be incorrect if it implies that DNA's charge is dependent on its conformation or tertiary structure. While DNA's structure can affect its interactions with other molecules, its overall charge is determined by the phosphate groups.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that DNA's negative charge plays a crucial role in its interactions with other molecules, such as histones, and its overall structure within the cell nucleus.
**Correct Answer: D. Negatively charged.**