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In mature RBC, the nucleus is present in
A. Rabbit
B. Frog
C. Both Rabbit and Frog
D. Neither in Frog nor Rabbit

Answer
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Hint: RBCs containing nucleus are present in birds, reptiles, amphibians, etc. This lack of nucleus observed in human adults is observed as an evolutionary advantage seen in the specific taxon.

Complete Answer:
- In this specific question, we need to understand that blood is a significant component of life. It is a liquid connective tissue. It constitutes plasma fluid and the formed elements i.e. blood cells. It forms the cardiovascular system of our body where it flows to the whole body circulating oxygen and other essential elements to all the cells of the body.
- The formed elements in the blood include erythrocytes (Red blood cells), leucocytes (White blood cells), and thrombocytes (platelets). The major role of the RBCs in the blood is to circulate oxygen to the whole body and to maintain the body temperature and the pH level. Mature red blood cell doesn’t contain a nucleus.
- Moreover, they lack organelles like the endoplasmic reticulum or mitochondria. Red blood cells cannot undergo replication of DNA and mitotic division like other somatic cells. Thus, they cannot synthesize proteins on their own.
- RBCs are circulated to the body by blood flow along with the pressure. When erythrocytes are formed in the bone marrow, they are immature and have a nucleus within them. When they get fully developed, they eject their nucleus, a method called enucleation.
- Now, when the red blood cells flow into vessels, they no more contain a nucleus. The ejection of a nucleus from the cell is an advantageous evolution to be more efficient for its function i.e. delivering more oxygen and being able to move freely through the capillaries. Unlike classical mammalian erythrocytes, all non-mammalian chordates such as Amphibians have RBC carrying a nucleus in the middle of the cell.

Hence, the correct option is B.

Note: Homo sapiens have red blood, while various species have other colors of blood mainly due to the respiratory pigment. If human adults carry a nucleated RBC, then it could be a lethal sign of bone marrow cancer and anemia. In the case of the frog, the RBC is nucleated.