Heterochromatin is stained by which dye?

• A. Hematoxylin
• B. Eosin
• C. Methylene blue
• D. Sudan Black

Answer: (A)

Heterochromatin is densely packed DNA, which makes it highly basophilic. Basic (cationic) stains like hematoxylin bind strongly to the negatively charged phosphate groups in DNA, so heterochromatin takes up hematoxylin and appears dark blue/purple in routine stains. This is the same staining principle used in hematoxylin-eosin preparations, where hematoxylin colors nuclei and other DNA- or RNA-rich structures, while eosin stains cytoplasmic proteins pink. The other dyes target different components: eosin dyes proteins, Sudan Black stains lipids, and methylene blue can stain nucleic acids in some contexts but is not the standard dye for highlighting heterochromatin. So hematoxylin best reveals heterochromatin due to its affinity for DNA.