Video Transcript
Which of the following correctly
describes the events that occur in anaphase? (A) Two nuclear membranes begin to
reform around the chromosomes. (B) Spindle fibers shrink and pull
each chromosome to opposite sides of the cell. (C) Pairs of chromosomes line up
along the equator or middle of the cell, and spindle fibers attach to each
chromosome. Or (D) the chromosomes condense,
spindle fibers form, and the nuclear membrane begins to break down.
Mitosis is the process of cellular
division to grow and replace body tissues. In order to answer this question,
let’s review the steps of mitosis.
Our question asks us about
anaphase, which is the third out of the four main steps: prophase, metaphase,
anaphase, and telophase. Let’s review the events that occur
in each of these stages so that we can answer this question.
Before mitosis begins, the
chromosomes within the cell’s nucleus are loosely packed in a form called
chromatin. In prophase, this chromatin begins
to condense and compact so you can see the individual chromosomes. The membrane that surrounds the
nucleus starts to break down, and structures called spindle fibers begin to form
outside the nucleus. If we look at our options in this
question, we can see that option (D) describes the events that happen in prophase,
so this answer is incorrect.
In metaphase, these condensed
chromosomes line up in the middle of the cell, which is sometimes called the
equator. Then the spindle fibers that were
formed in prophase attach to each chromosome. These events are all described in
option (C) of our question, so this is incorrect.
The third phase is anaphase. Here, the spindle fibers will begin
to retract or shrink, pulling the chromosomes to opposite sides of the cell. This makes sure that the
chromosomes carrying genetic material are divided equally between both ends of the
dividing cell. Option (B) correctly describes
these events. This could be the answer we are
looking for.
Let’s take a quick look at the last
stage of mitosis. The last stage of mitosis is called
telophase. Here, two nuclear membranes begin
to form around the chromosomes at opposite poles of the parent cell. This results in the formation of
two daughter nuclei, and the cell itself begins to split into two. This completes mitosis, and a
single cell is divided into two identical daughter cells. As we can see in our answer
choices, option (A) describes telophase, so this answer is incorrect.
So now we have our answer. The option that correctly describes
anaphase is (B), spindle fibers shrink and pull each chromosome to opposite sides of
the cell.
