With regard to mature mRNA in eukaryotes, which of the following is true?

Options

A. Exons and introns do not appear in the mature RNA
B. Exons appear but introns do not appear in the mature mRNA
C. Introns appear but exons do not appear in the mature mRNA
D. Both exons and introns appear in the mature mRNA

Answer

To answer the question regarding mature mRNA in eukaryotes, we need to understand the processing of mRNA after transcription. Here is a step-by-step breakdown of the solution:

### Step-by-Step Solution:

1. **Understanding mRNA in Eukaryotes**:
- In eukaryotic cells, the initial RNA transcript produced from DNA is called heterogeneous nuclear RNA (hnRNA). This precursor mRNA contains both exons and introns.

2. **Definition of Exons and Introns**:
- **Exons**: These are the coding regions of the gene that will be expressed as proteins.
- **Introns**: These are non-coding regions that are interspersed between exons and do not code for proteins.

3. **Splicing Process**:
- During mRNA processing, introns are removed from the hnRNA through a process called splicing. This results in the joining together of exons to form the mature mRNA.

4. **Capping and Tailing**:
- In addition to splicing, the mature mRNA undergoes capping (addition of a 5' cap) and polyadenylation (addition of a poly-A tail at the 3' end) which are essential for mRNA stability and translation.

5. **Final Composition of Mature mRNA**:
- After splicing, capping, and tailing, the final mature mRNA consists only of exons. Introns are completely absent in mature mRNA.

6. **Conclusion**:
- Therefore, the correct statement regarding mature mRNA in eukaryotes is: "Exons appear, but introns do not appear in mature mRNA."

### Final Answer:
- The correct option is: **Exons appear, but introns do not appear in mature mRNA.**

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