In eukaryotes, after transcription of mRNA, some of its nucleotides are removed before it is translated into a polypeptide. The nucleotides which are removed from mRNA are called :

Options

A. Exons
B. Upstream sequences
C. Unusual bases
D. Introns

Answer

To solve the question, we need to understand the process of mRNA processing in eukaryotes. Here’s a step-by-step breakdown:

### Step 1: Understand mRNA Transcription
- In eukaryotic cells, the process of transcription produces a precursor mRNA (pre-mRNA) from DNA. This pre-mRNA contains both coding and non-coding sequences.

**Hint:** Remember that mRNA is synthesized from DNA during transcription.

### Step 2: Identify the Components of Pre-mRNA
- The pre-mRNA consists of coding sequences known as exons and non-coding sequences known as introns. Exons are the parts that will be expressed as proteins, while introns are intervening sequences that do not code for proteins.

**Hint:** Focus on the definitions of exons and introns.

### Step 3: mRNA Processing
- Before the pre-mRNA can be translated into a polypeptide, it undergoes processing. This includes the removal of introns and the splicing together of exons to form mature mRNA.

**Hint:** Think about what happens to the pre-mRNA before it becomes mature mRNA.

### Step 4: Identify the Removed Nucleotides
- The nucleotides that are removed during this processing step are called introns. These sequences are not included in the final mature mRNA that is translated into a protein.

**Hint:** Consider which sequences are left in the mature mRNA after processing.

### Conclusion
- The nucleotides that are removed from mRNA during processing in eukaryotes are called **introns**.

**Final Answer:** The nucleotides which are removed from mRNA are called **introns**.