A cistron is:

Options

A. Part of a RNA polymerase
B. Type of a RNA polymerase
C. DNA segment coding for a polypeptide
D. Both (a) and (c)

Explanation

To answer the question "A cistron is:", we will analyze the options provided and identify the correct definition of a cistron step by step.

### Step 1: Understanding the term "cistron"
A cistron is a term used in genetics to refer to a segment of DNA that encodes for a specific polypeptide or protein. It is essentially a functional unit of heredity.

### Step 2: Analyzing the options
1. **Part of RNA polymerase**: This option is incorrect because a cistron is not a part of RNA polymerase; rather, it is a segment of DNA.
2. **Type of RNA polymerase**: This option is also incorrect as a cistron is not a type of RNA polymerase.
3. **DNA segment which is coded for polypeptide**: This option is correct. A cistron is indeed a segment of DNA that contains the information necessary to produce a polypeptide.
4. **Both end C**: This option does not provide a clear definition and is not relevant to the concept of a cistron.

### Step 3: Conclusion
Based on the analysis of the options, the correct answer is that a cistron is a **DNA segment which is coded for polypeptide**.

### Final Answer
A cistron is a DNA segment which is coded for polypeptide.

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