How does vitamin B12 and folic acid supplementation in megaloblastic anemia leads to the improvement of anemia?

Deficiency of folic acid or vitamin B12 leads to functional folic acid deficiency which affects cells that are dividing rapidly because they have a large requirement for thymidine for DNA synthesis. Clinically, this affects the bone marrow, leading to megaloblastic anemia. So, when patients with megaloblastic anemia are supplemented with folic acid and vitamin B12, anemia improves due to increased DNA synthesis in the bone marrow. Ref: Harper's Illustrated Biochemistry, 26th Edition, Page 494; Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Pages 269, 276-7, 280