The concept of a transcriptionally regulated system was first given by:

Options

A. Avery and McLeod
B. Hershey and Chase
C. Francois Jacob and Jacque Monod
D. Watson and Crick

Explanation

To solve the question regarding who first proposed the concept of a transcriptionally regulated system, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the origin of the concept of a transcriptionally regulated system. This involves identifying the scientists who contributed to this concept.

2. **Define Transcriptionally Regulated System**: A transcriptionally regulated system refers to how genes are expressed and regulated at the transcription level. This includes elements such as structural genes, promoters, regulators, and terminators.

3. **Identify Key Components**: Recognize that the system includes:
- **Structural Genes**: The genes that are being expressed.
- **Promoter**: The region where transcription begins.
- **Regulator**: Proteins that control the expression of the genes.
- **Terminator**: The sequence that signals the end of transcription.
- **Operon**: A functional unit of DNA containing a cluster of genes under the control of a single promoter.

4. **Recall Historical Contributions**: The question provides multiple choices of scientists who may have proposed this concept:
- Every and McLeod
- Hershey's and Chase
- Francis Crick and Jacob Monod
- Fortson and Crick

5. **Select the Correct Answer**: Based on the information provided in the video transcript, the concept of a transcriptionally regulated system was first given by Jacob and Monod. Therefore, the correct answer is option number 3: Jacob and Monod.

### Final Answer:
The concept of a transcriptionally regulated system was first given by Jacob and Monod.

---