AIPMT PRELIMS 2010 Botany Mitosis MCQ Question

Explanation of Mitosis Stages

Mitosis is divided into several stages: prophase, metaphase, anaphase, and telophase. Each stage has specific events that characterize the process of cell division.

1. Prophase: This is the first stage of mitosis where chromatin condenses into visible chromosomes. Each chromosome is made up of two sister chromatids. During early prophase, the nucleolus starts to disappear, which is a key indicator that the cell is preparing to divide. The nuclear envelope begins to break down, and the mitotic spindle starts to form.

2. Prometaphase (Late Prophase): This is often considered a continuation of prophase. The nuclear envelope is completely broken down, and the spindle fibers attach to the kinetochores of the chromosomes. The ER continues to break down during this phase as well.

3. Metaphase: In this stage, the chromosomes are aligned at the metaphase plate (the equatorial plane of the cell). The nucleolus is no longer visible, and the spindle apparatus is fully formed.

4. Anaphase: The sister chromatids are pulled apart towards opposite poles of the cell.

5. Telophase: The chromosomes begin to de-condense back into chromatin, and the nuclear envelope re-forms around each set of chromosomes. The nucleolus also reappears during this stage.

Correct Answer

Correct Answer: B - Late Prophase

The correct answer is B) Late prophase, where both the endoplasmic reticulum and the nucleolus are no longer visible. During this stage, the breakdown of the nuclear envelope is complete, and the spindle apparatus is fully formed, allowing the cell to proceed into metaphase.

Why Other Options Are Incorrect

• Option A: Early Prophase: While the nucleolus begins to disappear during early prophase, it is not completely absent until late prophase. Therefore, this option does not fully capture the point at which both the ER and nucleolus are entirely gone.

• Option C: Early Metaphase: By early metaphase, the nucleolus and ER have already disappeared during late prophase. Hence, this option is incorrect.

• Option D: Late Metaphase: By this stage, the nucleolus and ER remain absent, but they were already gone before this phase started. Thus, this option does not represent the moment of disappearance.

Conclusion

In conclusion, the disappearance of the endoplasmic reticulum and nucleolus is a significant event that marks the transition from prophase to metaphase. The key moment for their disappearance occurs during late prophase (Option B). The early prophase (Option A) indicates the beginning of this process but does not represent their complete absence. Understanding these stages is crucial for grasping the fundamental concepts of cell division in botany and biology.