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

Answer

To answer the question regarding mature mRNA in eukaryotes, let's break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding mRNA Processing**:
- In eukaryotic cells, the initial RNA transcript (pre-mRNA) undergoes several modifications before it becomes mature mRNA. This process includes the removal of non-coding regions.

2. **Identifying Exons and Introns**:
- **Exons**: These are the coding sequences of the gene that remain in the mature mRNA. They are expressed and translated into proteins.
- **Introns**: These are the non-coding sequences that are present in the pre-mRNA but are removed during the processing. They do not code for proteins.

3. **The Process of Splicing**:
- Splicing is the process by which introns are removed from the pre-mRNA and exons are joined together to form the mature mRNA. This results in a continuous coding sequence that can be translated into a protein.

4. **Conclusion about Mature mRNA**:
- After splicing, the mature mRNA contains only exons and does not include introns. Therefore, in mature mRNA, exons are present while introns are absent.

5. **Evaluating the Options**:
- Option A: Exons and introns do not appear in the mature RNA. (Incorrect)
- Option B: Exons appear but introns do not appear in the mature RNA. (Correct)
- Option C: Introns appear but exons do not appear in the mature RNA. (Incorrect)
- Option D: Both exons and introns appear in the mature DNA. (Incorrect)

6. **Final Answer**:
- The correct answer is **Option B**: Exons appear but introns do not appear in the mature RNA.