Define a cistron. Giving examples differentiate between monocistronic and polycistronic unit.

Explanation

### Step-by-Step Text Solution

**Step 1: Define a Cistron**
A cistron is defined as a segment of DNA that encodes for a single polypeptide chain. This includes not only the coding sequence for the polypeptide itself but also the adjacent regulatory regions that control the expression of that polypeptide. Additionally, a cistron may encompass sequences that code for tRNA or rRNA, and it can also have regulatory functions for other cistrons.

**Step 2: Differentiate Between Monocistronic and Polycistronic Units**
- **Monocistronic Unit**: A monocistronic transcription unit contains all the necessary regulatory and coding sequences for the synthesis of a single polypeptide. In this case, the term "mono" indicates that it codes for one polypeptide. An example of a monocistronic unit is found in eukaryotic cells, such as the gene encoding insulin.

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