Higher yields of desired gene product (protein) are obtained .
A. By maintaining the cell culture at lag phase .
B. By dramining out the used medium from time to time .
C. By adding the drained medium along with fresh medium at period intervals and regular stirring .
D. By adding fresh medium and throughly aerating it .

Options

A. A,B,C and D
B. B,C and D only
C. B and D only
D. A,C and D

Answer

To determine how to obtain higher yields of the desired gene product (protein), let's analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding Cell Culture Phases**:
- In cell culture, there are different growth phases: lag phase, log phase, stationary phase, and death phase. The lag phase is characterized by slow growth, which is not ideal for maximizing protein yield. Therefore, maintaining the cell culture at lag phase (Option A) is not beneficial for higher yields.

2. **Draining Used Medium**:
- Draining out the used medium from time to time (Option B) is a good practice. As cells grow, they consume nutrients and produce waste products. Removing the used medium helps to eliminate waste and replenish nutrients, which is essential for maintaining healthy cell growth.

3. **Adding Drained Medium**:
- The suggestion to add the drained medium along with fresh medium at periodic intervals (Option C) is not advisable. The drained medium contains waste products and depleted nutrients, which would not contribute positively to cell growth or protein production.

4. **Adding Fresh Medium and Aeration**:
- Adding fresh medium and thoroughly aerating it (Option D) is crucial for increasing yields. Fresh medium provides new nutrients, and aeration ensures that cells receive adequate oxygen, which is vital for their metabolism and growth. This option is the most effective for enhancing protein production.

### Conclusion:
Based on the analysis:
- **Correct Option**: D (Adding fresh medium and thoroughly aerating it) is the best choice for obtaining higher yields of the desired gene product.