Methylene blue can be used to stain DNA because:

• A. it forms covalent bonds with DNA bases
• B. it binds to the negatively charged phosphate groups of DNA
• C. it is hydrophobic and integrates into the DNA double helix
• D. it reacts specifically with the ribose sugar in DNA

Answer: (B)

Verified step by step guidance

1. Understand the chemical nature of DNA: DNA has a backbone made of sugar and phosphate groups. The phosphate groups carry a negative charge, which is important for interactions with stains.
2. Consider the properties of methylene blue: it is a positively charged (cationic) dye, which allows it to interact with negatively charged molecules.
3. Analyze the types of interactions stains can have with DNA: stains can bind through covalent bonds, hydrophobic interactions, or ionic interactions. Covalent bonding is strong and specific, hydrophobic interactions involve nonpolar regions, and ionic interactions involve attraction between opposite charges.
4. Evaluate why methylene blue binds to DNA: since DNA's phosphate backbone is negatively charged, methylene blue binds through ionic interactions to these phosphate groups rather than forming covalent bonds or hydrophobic interactions.
5. Conclude that the correct reason methylene blue stains DNA is because it binds to the negatively charged phosphate groups of DNA via ionic interactions.