Select incorrect option w.r.t. sickle cell anemia

Options

A. It is an example of point mutation
B. It occurs due to base substitution i.e. transition
C. Glutamic acid is replaced by valine at sixth position in polypeptide chain
D. Mutant haemoglobin, molecule undergoes polymerization under low oxygen tension

Answer

To solve the question of selecting the incorrect option with respect to sickle cell anemia, let's go through the statements one by one.

### Step-by-Step Solution:

1. **Identify the Nature of Sickle Cell Anemia**:
- Sickle cell anemia is a genetic disorder caused by a mutation in the hemoglobin gene. Specifically, it is known to be caused by a point mutation.

**Hint**: Remember that sickle cell anemia is a genetic disorder linked to hemoglobin.

2. **Evaluate the First Statement**:
- The first statement claims that sickle cell anemia is an example of point mutation. This is true because the disease arises from a single nucleotide change in the DNA sequence.

**Hint**: Point mutations involve a change in a single nucleotide in the DNA.

3. **Evaluate the Second Statement**:
- The second statement mentions that it occurs due to base substitution. This is also true, as sickle cell anemia results from a substitution of one base for another in the DNA sequence.

**Hint**: Base substitution is a common mechanism for point mutations.

4. **Evaluate the Third Statement**:
- The third statement states that glutamic acid is replaced by valine in the sixth position of the polypeptide chain. This is accurate; in sickle cell anemia, the sixth amino acid in the beta-globin chain of hemoglobin is indeed valine instead of glutamic acid.

**Hint**: Focus on the specific amino acid change in the hemoglobin structure.

5. **Evaluate the Fourth Statement**:
- The fourth statement indicates that a mutant hemoglobin molecule undergoes polymerization under low-oxygen tension. This statement is true as well; under low oxygen conditions, the abnormal hemoglobin can aggregate, leading to the sickling of red blood cells.

**Hint**: Consider how oxygen levels affect the behavior of hemoglobin.

6. **Conclusion**:
- After evaluating all the statements, it is clear that the second statement is incorrect regarding the specifics of the base substitution mechanism. The correct base substitution involves the change from adenine (A) to thymine (T) rather than the way it was described.

### Final Answer:
The incorrect option with respect to sickle cell anemia is the second statement regarding the base substitution.

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