Megaloblastic anemia is a nuclear-cytoplasmic asynchrony: DNA synthesis is blocked while RNA and protein synthesis continue, so the erythroid precursor grows without dividing. The defect is in thymidylate synthesis, not in hemoglobin production.
The blocked step
Thymidylate synthase converts deoxyuridylate (dUMP) to thymidylate (dTMP) using a methyl group donated by 5,10-methylene-THF. Without that methyl donation, dTMP cannot be made, and dTMP is the only de novo source of the thymine nucleotide needed for DNA. RNA uses uracil instead of thymine, so RNA synthesis is unaffected — which is exactly why the cytoplasm keeps growing while the nucleus stalls.
Tissues affected by the thymidylate block
- Bone marrow — highest proliferation rate, so megaloblastic changes appear first and produce macrocytic anemia.
- Gastrointestinal epithelium — rapid turnover, so glossitis, anorexia, and malabsorption accompany the anemia.
- Slowly dividing tissues such as skeletal muscle and mature neurons — not directly affected by the thymidylate block itself.
This is the Chapter 2 principle applied to a different vulnerable step. In energy metabolism, high ATP demand made nervous tissue and heart fail first. Here, high proliferation rate makes bone marrow and gut epithelium fail first. The tissue pattern follows the pathway that is blocked, not a fixed list of symptoms.