In which step of cell division does the nucleus disappear and spindle fibers are formed?

A. Metaphase
B. Prophase
C. Anaphase
D. Telophase

B. Prophase

The correct answer is B. Prophase.

In order to understand why this is the correct answer, let's break down the process of cell division, known as the cell cycle.

The cell cycle consists of several stages: interphase, prophase, metaphase, anaphase, and telophase.

During interphase, the cell prepares for division by growing, replicating its genetic material, and performing normal cell functions.

Once interphase is complete, the cell enters prophase, which is the first stage of actual division. In prophase, several key events occur:

1. The nucleus starts to break down: The nuclear envelope, a membrane that surrounds and protects the nucleus, begins to disintegrate, causing the nucleus to disappear. This allows other cellular structures and organelles to access the genetic material within the nucleus.

2. Chromatin condenses into chromosomes: The chromatin, a combination of DNA and proteins, becomes tightly coiled and condensed into distinct structures called chromosomes. This condensation makes it easier for the genetic material to be separated evenly during division.

3. Spindle fibers are formed: Spindle fibers, which are made of microtubules, start to form and extend from opposite poles of the cell towards the center. These fibers play a crucial role in separating the replicated chromosomes during later stages of cell division.

So, in summary, the nucleus disappears and spindle fibers are formed during the prophase stage of cell division. Therefore, the correct answer to the question is B. Prophase.