With regard to mature mRNA in eukaryotes

Options

A. exons and introns do not appear in the mature RNA
B. exons and introns do not appear in the mature RNA
C. Exons appear but introns do not appear in the mature RNA
D. both exons and introns appear in fhe mature RNA

Answer

### Step-by-Step Solution:

1. **Understanding mRNA Types**:
- In eukaryotes, the initial RNA transcript synthesized from DNA is called precursor mRNA (pre-mRNA). This pre-mRNA contains both coding regions (exons) and non-coding regions (introns).

2. **Identifying Exons and Introns**:
- Exons are the segments of the RNA that code for proteins, while introns are non-coding segments that do not contribute to the final protein product.

3. **The Process of Splicing**:
- Before the pre-mRNA can be translated into a protein, it undergoes a process called splicing. During splicing, introns are removed, and exons are joined together to form the mature mRNA.

4. **Formation of Mature mRNA**:
- After splicing, the mature mRNA consists only of exons. Therefore, in mature mRNA, introns are absent.

5. **Evaluating the Options**:
- Now, let's evaluate the options given in the question:
- **Option 1**: "Exons and introns do not appear in the mature RNA" - Incorrect, as exons are present.
- **Option 2**: "Exons and introns do not appear in the mature RNA" - Incorrect for the same reason as option 1.
- **Option 3**: "Exons appear but introns do not appear in the mature RNA" - Correct, as this accurately describes the composition of mature mRNA.
- **Option 4**: "Both exons and introns appear in the mature RNA" - Incorrect, as introns are removed during splicing.

6. **Conclusion**:
- The correct answer is **Option 3**: "Exons appear but introns do not appear in the mature RNA."