With regard to mature mRNA in eukaryotes

Options

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

Explanation

### Step-by-Step Solution:

1. **Understanding mRNA in Eukaryotes**:
- In eukaryotic cells, the initial RNA transcript produced during transcription contains both coding and non-coding sequences. The coding sequences are called exons, while the non-coding sequences are called introns.

2. **Role of Introns and Exons**:
- Exons are the segments of RNA that code for proteins, while introns do not code for proteins and are considered non-coding regions.

3. **Processing of Pre-mRNA**:
- The initial RNA transcript (pre-mRNA) undergoes a processing step before it becomes mature mRNA. This processing includes the removal of introns and the joining together of exons.

4. **Splicing**:
- The process of removing introns and joining exons together is known as splicing. This results in the formation of a continuous coding sequence in the mature mRNA.

5. **Final Composition of Mature mRNA**:
- After splicing, the mature mRNA consists only of exons, as introns are removed during the processing phase. Therefore, mature mRNA does not contain any introns.

6. **Conclusion**:
- Based on the understanding of the mRNA processing in eukaryotes, the correct statement regarding mature mRNA is that it contains exons but does not contain introns.

### Answer:
The correct option is **B**: Exons appear but introns do not appear in mature RNA.

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