What is the minimum number of base substitutions required to change the nucleotide sequence of the HbA (normal) allele to the HbS (sickle cell) allele?

Options

A. 1
B. 2
C. 3
D. 4

Answer

To solve the question regarding the minimum number of base substitutions required to change the nucleotide sequence of the HbA (normal) allele to the HbS (sickle cell) allele, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Normal and Sickle Cell Alleles**:
- The normal hemoglobin allele (HbA) has a specific nucleotide sequence.
- The sickle cell hemoglobin allele (HbS) has a different nucleotide sequence due to a mutation.

2. **Locate the Specific Codon**:
- The mutation that causes sickle cell anemia occurs in the 6th codon of the hemoglobin gene.
- In the HbA allele, the 6th codon is CTC, which codes for the amino acid glutamic acid.

3. **Determine the Change in the Codon**:
- In the HbS allele, the 6th codon changes from CTC to CAC, which codes for the amino acid valine instead of glutamic acid.

4. **Count the Base Substitutions**:
- To change CTC (which is composed of the nucleotides C-T-C) to CAC (which is composed of C-A-C), we need to analyze the changes:
- The first nucleotide (C) remains the same.
- The second nucleotide changes from T to A (1 substitution).
- The third nucleotide (C) remains the same.
- Therefore, only one base substitution is required.

5. **Conclusion**:
- The minimum number of base substitutions required to change the nucleotide sequence of the HbA allele to the HbS allele is **1**.

### Summary:
The minimum number of base substitutions required is **1**.