Video Transcript
The figure shows a cell going
through mitosis. What is structure A? (A) Spindle, (B) equator, (C)
centrosome, (D) centromere, or (E) chromosome. What is structure B? And what stage of mitosis is
shown?
Let’s start by reviewing how a cell
goes through mitosis or cell division. Before mitosis begins, the cell is
in interphase, during which all of the cell’s DNA is replicated so that there will
be a full set of chromosomes for each of the two cells which will result from the
division. Mitosis itself consists of four
stages; they are prophase, metaphase, anaphase, and telophase. Let’s look closer at what happens
during each of these four stages of mitosis.
During prophase, the loose strands
of DNA in the cell’s nucleus called chromatin, which were present during interphase,
coil up around proteins and condense into visible chromosomes. The number of the chromosomes
depends on the species of the organism. In humans, there would be 46
chromosomes visible in the nucleus after the chromosomes coil up and condense during
prophase. Another thing that happens during
prophase, however, is that the nuclear envelope or the membrane around the nucleus
dissolves so that the chromosomes can be moved more freely around the cell. Also, the centrosomes, which are
the organelles that form the mitotic spindle, begin to develop spindle fibers for
later use.
By metaphase, the centrosomes have
already migrated to opposite ends of the cell. And during this stage, the spindle
fibers attach to the centromeres of the chromosomes, which allows the mitotic
spindle by shortening or lengthening its fibers to move the chromosomes around the
cell. During metaphase, the chromosomes
are pulled by these fibers to line up in a single-file row along the middle of the
cell or the equator of the cell.
Then in anaphase, the sister
chromatids separate from each other and are pulled apart by the spindle fibers
towards opposite ends of the cell. Finally, in telophase, the cell
begins to physically divide in two. Nuclear envelopes reform for each
side of the dividing cell and the highly condensed visible chromosomes start to
decoil or decondense back into loose strands of DNA or chromatin.
So with that information about
mitosis in mind, let’s look back at the questions. We are asked, what is structure
A? A is pointing to a centrosome. So, the correct answer is (C),
centrosome.
What is structure B? B is pointing to the fibers
emanating from the centrosome or the mitotic spindle. So, the correct answer is the
spindle.
And finally, what stage of mitosis
is shown? Well, we can see that the
centrosomes are at opposite ends of the cell and that the mitotic spindle is formed
with the spindle fibers attached to the centromeres of the chromosomes. And perhaps most importantly, we
can see that the chromosomes are lined up at the equator or midline of the cell. This happens during metaphase. So, the diagram is showing a cell
in the stage of metaphase.
