Why are sterile nuclease-free tubes useful in DNA isolation?

Correct answer: To reduce DNA degradation and contamination

Sterile, nuclease-free tubes are used to keep isolated DNA safe and reliable. Nucleases are enzymes that can cut or digest DNA, so their absence reduces unwanted DNA breakdown. Sterility also prevents outside microorganisms, DNA, proteins, or other material from entering the sample. This is especially important when the extracted DNA will be tested, amplified, or used in recombinant-DNA work.

Therefore, option A is correct because it combines the two main benefits: less nuclease-mediated degradation and less contamination. Options B, C, and D describe unrelated processes. A tube does not make DNA into protein, fill cells with starch, or convert ethanol into water. Proper tubes alone are not enough; clean handling and suitable storage are also needed.