In which of the following levels can gene expression not be exerted?

Options

A. transitional level
B. reverse trasncriptional level
C. transcriptional level
D. processing level

Answer

To determine the level at which gene expression cannot be exerted, we need to analyze the various levels of gene expression regulation. Here’s a step-by-step breakdown:

1. **Understanding Gene Expression Levels**: Gene expression can occur at multiple levels, including transcriptional, translational, and processing levels. Each of these levels plays a crucial role in how genes are expressed and regulated.

2. **Transcriptional Level**: This is the first step where gene expression is actively taking place. During transcription, DNA is transcribed into mRNA. This step is essential for gene expression, as it produces the mRNA that will be translated into proteins.

3. **Processing Level**: After transcription, the primary mRNA undergoes processing where introns (non-coding regions) are removed, and exons (coding regions) are joined together. This processed mRNA is what will be translated into proteins, indicating that gene expression is also exerted at this level.

4. **Translational Level**: At this level, the processed mRNA is translated into proteins. This is another crucial step in gene expression, as it results in the formation of functional proteins that carry out various roles in the cell.

5. **Reverse Transcriptional Level**: This is where the process occurs in the opposite direction, converting RNA back into DNA. This process is typically seen in retroviruses and is not a standard step in gene expression for most organisms. In this case, gene expression is effectively turned off or not exerted because the mRNA is being converted back to DNA, which does not lead to protein synthesis.

6. **Conclusion**: Based on the analysis of these levels, the reverse transcriptional level is where gene expression cannot be exerted. This is because, during reverse transcription, the normal flow of genetic information is disrupted, preventing the expression of genes into functional proteins.

### Final Answer:
Gene expression cannot be exerted at the reverse transcriptional level.