Sickle cell anaemia is caused by the subsitution of :

Options

A. Valine by glutamic acid at sixth position of alpha chain of haemoglobin
B. Valine by glutamic acid at sixth position of beta chain of haemoglobin
C. Glutamic acid by valine at sixth position of alpha chain of haemoglobin
D. Glutamic acid by valine at sixth position of beta chain of haemoglobin

Answer

**Step-by-Step Solution:**

1. **Understanding Sickle Cell Anemia**: Sickle cell anemia is a genetic disorder that affects the shape of red blood cells (RBCs). Normally, RBCs are biconcave discs, but in sickle cell anemia, they become rigid and sickle-shaped.

2. **Identifying the Cause**: The condition is caused by a mutation in the hemoglobin gene. This mutation is a type of point mutation, which means there is a change in a single base pair in the DNA sequence.

3. **Locating the Mutation**: Specifically, the mutation occurs at the 6th position of the beta chain of hemoglobin.

4. **Determining the Amino Acid Substitution**: In normal hemoglobin, the amino acid at the 6th position is glutamic acid. In sickle cell anemia, this glutamic acid is substituted by valine.

5. **Conclusion**: Therefore, sickle cell anemia is caused by the substitution of glutamic acid by valine at the 6th position of the beta chain of hemoglobin.

**Final Answer**: Glutamic acid is replaced by valine at the 6th position of the beta chain of hemoglobin (Option D).

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