Frameshift mutation occurs when :

Options

A. base is deleted or added
B. purine is substituted by a pyrimidine
C. purine is substituted by another purine
D. pyrimidine is substituted by another pyrimidine

Answer

### Step-by-Step Solution:

1. **Understanding Frameshift Mutation**:
- A frameshift mutation is a type of genetic mutation that occurs when there is an addition or deletion of one or more nucleotide base pairs in the DNA sequence.

2. **Causes of Frameshift Mutation**:
- Specifically, frameshift mutations are caused by the addition (insertion) or deletion of base pairs. This alteration changes the reading frame of the genetic code during translation.

3. **Impact of Frameshift Mutation**:
- When a base pair is added or deleted, the entire sequence of codons downstream of the mutation is altered. This means that the way the genetic code is read (the reading frame) is shifted, leading to potentially significant changes in the resulting protein.

4. **Consequences**:
- The altered reading frame can result in a completely different translation of the genetic code, which may produce a nonfunctional protein or a protein with an entirely different function.

5. **Conclusion**:
- Therefore, a frameshift mutation occurs specifically due to the addition or deletion of base pairs in the DNA sequence.

### Final Answer:
Frameshift mutation occurs when there is an addition or deletion of base pairs in the DNA of the gene.

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