<Tr> <Th> MeSH </Th> <Td> D000749 </Td> </Tr> <Tr> <Td_colspan="2"> (edit on Wikidata) </Td> </Tr> <P> Megaloblastic anemia (or megaloblastic anaemia) is an anemia (of macrocytic classification) that results from inhibition of DNA synthesis during red blood cell production . When DNA synthesis is impaired, the cell cycle cannot progress from the G2 growth stage to the mitosis (M) stage . This leads to continuing cell growth without division, which presents as macrocytosis . Megaloblastic anemia has a rather slow onset, especially when compared to that of other anemias . The defect in red cell DNA synthesis is most often due to hypovitaminosis, specifically a deficiency of vitamin B and / or folic acid . Vitamin B deficiency alone will not cause the syndrome in the presence of sufficient folate, as the mechanism is loss of B dependent folate recycling, followed by folate - deficiency loss of nucleic acid synthesis (specifically thymine), leading to defects in DNA synthesis . Folic acid supplementation in the absence of vitamin B prevents this type of anemia (although other vitamin B - specific pathologies may be present). Loss of micronutrients may also be a cause . Copper deficiency resulting from an excess of zinc from unusually high oral consumption of zinc - containing denture - fixation creams has been found to be a cause . </P> <P> Megaloblastic anemia not due to hypovitaminosis may be caused by antimetabolites that poison DNA production directly, such as some chemotherapeutic or antimicrobial agents (for example azathioprine or trimethoprim). </P>

Explain how vitamin b12 and folic acid deficiencies affect rbc production
find me the text answering this question