A change in medium composition in a bacterial culture, results into growth of a specific section of that population. This section survived because:

Options

A. These bacteria can convert themselves according to changing medium
B. These bacteria already had the gene that provides them selective advantage in changing medium
C. These bacteria survived by chance
D. Exact reason can't be predicted

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question states that a change in the medium composition of a bacterial culture leads to the growth of a specific section of that population. We need to determine why this particular section survived.

2. **Analyzing the Scenario:**
When the medium changes, certain bacteria in the culture are able to survive while others are not. This suggests that the surviving bacteria possess some advantageous traits that allow them to thrive in the new conditions.

3. **Identifying the Key Factor:**
The key factor for survival in a new medium often relates to genetic traits. In this case, the surviving bacteria likely have genes that confer resistance to the specific conditions of the new medium (e.g., resistance to antibiotics).

4. **Considering Genetic Variation:**
Bacteria can possess plasmids, which are circular pieces of DNA that can carry genes providing advantages such as antibiotic resistance. If a bacterium has a plasmid with a gene that allows it to survive in the new medium, it will thrive while others without that gene will not.

5. **Evaluating the Options:**
- **Option 1:** "These bacteria can convert themselves according to changing medium." This is incorrect because bacteria do not change instantly; they rely on existing genetic traits.
- **Option 2:** "These bacteria already had the gene that provides them selective advantage in changing medium." This is correct as it explains that the surviving bacteria possessed pre-existing advantageous genes.
- **Option 3:** "These bacteria survived by chance." This option is misleading as survival is based on genetic traits, not random chance.
- **Option 4:** "Exact reason cannot be predicted." This is incorrect because we can predict that the survival is due to specific genetic advantages.

6. **Conclusion:**
The correct answer is that the section of bacteria that survived had pre-existing genes that provided them with a selective advantage in the changing medium.

**Final Answer:**
These bacteria already had the gene that provides them selective advantage in changing medium.

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