Which of the following best describes the events that occur during prophase, the first phase of mitosis?

• A. Chromosomes align at the metaphase plate, and spindle fibers attach to their centromeres.
• B. Chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and the mitotic spindle begins to form.
• C. Two new nuclei form around the separated chromosomes, and the cell begins to divide.
• D. Sister chromatids are pulled apart toward opposite poles of the cell.

Answer: (B)

Verified step by step guidance

1. Understand the context: The question is asking about the events that occur during prophase, which is the first phase of mitosis. Mitosis is the process of cell division that ensures equal distribution of genetic material to two daughter cells.
2. Recall the key events of prophase: During prophase, chromatin (the uncondensed form of DNA) condenses into visible chromosomes. Each chromosome consists of two sister chromatids joined at a centromere.
3. Identify the structural changes: The nuclear envelope, which encloses the nucleus, breaks down during prophase. This allows the chromosomes to interact with the mitotic spindle.
4. Understand the role of the mitotic spindle: The mitotic spindle, a structure made of microtubules, begins to form during prophase. It is responsible for organizing and separating the chromosomes during later stages of mitosis.
5. Eliminate incorrect options: Compare the provided options to the events of prophase. For example, chromosome alignment at the metaphase plate occurs during metaphase, not prophase. Similarly, the formation of two new nuclei happens during telophase, and the separation of sister chromatids occurs during anaphase.