Question Video: Recalling some of the Structures Involved in Mitosis Science

The figure shows a cell going through mitosis. What is structure A? What is structure B? What stage of mitosis is shown?


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.

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