Cistron can be defined as :

Options

A. The portion of DNA that codes for a polypeptide
B. The sequence of mRNA that codes for a protein
C. It is a mature mRNA
D. The structural gene found prokaryotes.

Answer

### Step-by-Step Solution:

1. **Definition of Cistron**: A cistron is a segment of DNA or RNA that encodes a specific polypeptide. This means that it contains the necessary information to produce a particular protein during the process of protein synthesis.

2. **Understanding Polycistronic and Monocistronic**:
- **Polycistronic**: In some operons, like the lac operon, a single mRNA molecule can code for multiple polypeptides. This is referred to as being polycistronic.
- **Monocistronic**: In contrast, some genes are monocistronic, meaning that a single mRNA molecule codes for only one polypeptide.

3. **Analyzing the Options**:
- **Option 1**: "The portion of DNA that codes for polypeptide." This option aligns with the definition of a cistron, as it directly refers to the segment of DNA responsible for coding a specific polypeptide.
- **Option 2**: "The sequence of mRNA that codes for a protein." This is incorrect because a cistron refers to the DNA or RNA segment, not the mRNA itself.
- **Option 3**: "A mature mRNA." This is also incorrect; a cistron is not a mature mRNA but rather the coding region within the DNA or RNA.
- **Option 4**: "The structural gene found in prokaryotes." This is misleading as cistrons can be found in both prokaryotes and eukaryotes and are not strictly defined as structural genes.

4. **Conclusion**: The correct definition of a cistron is the first option: "The portion of DNA that codes for polypeptide."