Final answer:
DNA retains its negative charge due to the ionization of phosphate groups, even at low pH levels, although extreme pH can affect its structure.
Step-by-step explanation:
DNA molecules are indeed negatively charged, and this charge is due to the phosphate groups in the DNA backbone. At low pH values, the hydrogen ions (H+) are more concentrated, and while the acidic environment can influence the structure and stability of macromolecules, the DNA will still retain its negative charge.
Each phosphate group in the DNA molecule loses a hydrogen ion, which leads to the negative charge. It's important to remember that extreme pH levels, both very acidic and alkaline, can damage macromolecules like DNA, proteins, and lipids.
However, DNA's negative charge at physiological pH is due to the ionization of the phosphates, and this characteristic remains across a range of pH levels, but is not specific to low pH only.