Video Transcript
Which of the following correctly describes the events that occur in prophase? (A) Chromatin condenses into chromosomes, spindle fibers form, and the nuclear membrane begins to break down. (B) Pairs of chromosomes line up along the equator, middle, of the cell, and spindle fibers attach to each chromosome. (C) Spindle fibers shrink and pull each chromosome to opposite sides of the cell. Or (D) two nuclear membranes begin to reform around the chromosomes.
You might already know that mitosis is a process of cellular division and it has four distinct steps. These four steps occur in the following order: prophase, metaphase, anaphase, and telophase. Let’s review the events that happen in each of these stages so we can answer our question. In prophase, the first step of mitosis, the chromatin in the nucleus of the cell begins to condense, so the chromosomes become visible. Spindle fibers are also formed in this stage, which will be used later. The nuclear membrane also begins to break down during prophase. If we look at our answer choices, we can see that option (A) lists out all these events. Since we are looking for the option that best describes prophase, this could be our answer.
Let’s look through the rest of the options before we come to a conclusion. Answer choice (B) says pairs of chromosomes line up along the equator, or the middle, of the cell and spindle fibers attach to each chromosome. This describes metaphase, which is the second stage of mitosis, which you can see here. You can remember what occurs in metaphase because it starts with the letter M, and so does the word middle, which is where the chromosomes line up in the cell. So answer choice (B) is incorrect.
Answer choice (C) states that the spindle fibers shrink, pulling the chromosomes to opposite sides of the cell. This describes what happens in the third phase of mitosis, anaphase. You can remember what happens in anaphase because it starts with the letter A, just like the word apart. And anaphase is the stage where the chromosomes are being pulled apart. So answer (C) is incorrect.
Finally, answer choice (D) says that two nuclear membranes begin to reform around the chromosomes. This describes what happens in telophase, which is the fourth and final stage of mitosis. This results in the formation of two daughter nuclei, and the original cell begins to split into two. This completes mitosis: a single parent cell is divided into two identical daughter cells. So answer (D) is incorrect. So the answer that correctly describes prophase is option (A). Chromatin condenses into chromosomes, spindle fibers form, and the nuclear membrane begins to break down.
