The number of base substitution possible in amino acid codons is

Options

A. 261
B. 264
C. 535
D. 549

Explanation

To determine the number of base substitutions possible in amino acid codons, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Total Codons**:
- There are a total of 64 codons in the genetic code.

2. **Determine Codons that Code for Amino Acids**:
- Out of these 64 codons, 61 codons code for amino acids. The remaining 3 codons are stop codons (UAA, UAG, UGA).

3. **Understand Codon Structure**:
- Each codon consists of 3 nucleotide bases. The bases can be adenine (A), guanine (G), cytosine (C), or uracil (U).

4. **Identify Types of Base Substitution**:
- Base substitution can occur in two ways:
- **Transition**: This is when a purine is replaced by another purine (A ↔ G) or a pyrimidine is replaced by another pyrimidine (C ↔ U).
- **Transversion**: This is when a purine is replaced by a pyrimidine or vice versa (A or G ↔ C or U).

5. **Calculate Possible Substitutions**:
- For each of the 61 codons, we can substitute each of the 3 bases.
- Each base can be substituted with 3 other bases (since there are 4 bases total, and we cannot substitute with the same base).
- Therefore, for each base in a codon, there are 3 possible substitutions.

6. **Total Base Substitutions Calculation**:
- Since there are 3 bases in each codon, and each base can be substituted in 3 ways, the total number of substitutions for one codon is:
\[ 3 \text{ (bases)} \times 3 \text{ (substitutions per base)} = 9 \text{ substitutions per codon} \]
- Now, for 61 codons, the total number of base substitutions is:
\[ 61 \text{ (codons)} \times 9 \text{ (substitutions per codon)} = 549 \]

7. **Final Answer**:
- Therefore, the total number of base substitutions possible in amino acid codons is **549**.