AIPPG 2003 Dental MCQ Answer 03

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The Correct Answer is D

The cell cycle is divided into two main stages: interphase and the mitotic phase. When a cell first arises it is in interphase. Interphase is the working phase of the cell cycle, the time when the cell is going about its usual business. During interphase, your brain cells are computing, your muscle cells are contracting, and the cells lining your small intestine are digesting. During interphase, the chromosomes inside your cell nuclei are not visible under a light microscope. That is because the chromosomes are very thinly spread out, so you can’t see them.

A cell normally stays in interphase unless it receives specific signals from its environment. Once such a “go ahead” signal is received and processed, the cell then moves on to continue the cell cycle.

To continue the cell cycle and begin the process of cellular reproduction, a cell must first copy (or replicate) the chromosomes within its nucleus. This is accomplished during a sub-part of interphase known as the S phase, or synthesis phase. During the S phase, every chromosome in the nucleus is copied. When a chromosome is duplicated, the two copies are called sister chromatids and they stay attached to one another at a location called the centromere.

Imagine yourself standing in a room. You are a chromosome and the room is the nucleus. Now imagine that I copy you, producing an identical twin. You and your twin stay joined at the hip, attached to one another like conjoined twins (sometimes called Siamese twins). You and your twin would make up a pair of sister chromatids with the centromere joining you at the hip. You look around and see that, within the room of the nucleus, you are one of 46 sets of identical conjoined twins, each joined at the hip. These are the 46 sets of sister chromatids found in the nucleus at the end of the S phase.

Let’s pause here for a moment so I can catch my breath and you can process the information you’ve learned so far about the cell cycle:
Most of the time a cell is in interphase, going about it usual work. DNA is copied during interphase before cell division.

Alright then. Let’s now focus our attention on the second stage of the cell cycle: mitosis.

When a cell is reproducing itself, the most important task is to provide the correct amount of DNA to each new cell. This can be complex, because each cell has a lot of DNA, and it all has to be separated without tangles or mistakes—a daunting task that requires a fairly elaborate process known as mitosis. The purpose of mitosis is to duplicate the nucleus and the chromosomes it contains.

The cell does not normally spend much of the cell cycle in mitosis but many biology instructors do. Understanding mitosis is essential to understanding cell reproduction. But part of the reason I think we study mitosis is because the process is so elegant and beautiful. When you watch a cell undergoing mitosis under the microscope, it is very much like watching a highly coordinated dance troupe. In a human cell, the 46 dance partners (that is, the duplicated chromosomes) appear and assemble on then stage, the pairs move to the middle of the stage and then, suddenly, all of the dance pairs separate and rapidly move to opposite sides of the stage where each clump of dancers disappear. Can you picture that in your mind?

Now, let’s put this analogy into the language of biology.

Mitosis is a continuous process that is divided into four stages called prophase, metaphase, anaphase, and telophase. I usually remember these as PMAT, which sounds like the name of a standardized test, like pre-med aptitude test. PMAT: prophase, metaphase, anaphase, telophase.

Let’s talk about what happens during each of these stages.

Prophase is the stage when the dance partners first make their appearance; that is, when the duplicated chromosomes first become visible. In many respects this is the phase with the most going on. The nuclear envelope disappears. The chromosomes condense and become visible. From each end of the cell a network of microtubules (or very little tubes) forms. This network of fibers is called a spindle. Each chromosome attaches its centromere to a fiber in the spindle. The duplicated chromosomes then start to move to the center of the cell. Imagine 46 pairs of dance partners appearing randomly on the stage and then moving their way toward the center.

During metaphase, all of the duplicated chromosomes arrive and line up at the center of the cell. Imagine the 46 dance partners all arranged two by two in a line that runs down the center of the stage.

Anaphase begins when the sister chromatids are separated from one another. That is, each conjoined twin is ripped apart from its partner, and the two twins move in opposite directions away from one another. This movement is achieved by motor proteins on each centromere that drag the chromatids along the tracks laid down by the spindle. Imagine the two lines of dance partners suddenly moving away from each other, each line backing up, away from the other line. Each chromosome is now single, separated from its duplicated partner.

During the final stage of mitosis, telophase, the single chromosomes arrive at opposite ends of the cell. On the stage, the two sets of 46 individual dancers arrive at the opposite ends of the stage, begin to mingle, and disappear. The chromosomes unwind, become invisible once again, and two new nuclear envelopes form, one around each set of 46 chromosomes.

We have now achieved the goal of mitosis: we’ve created two nuclei, each with an identical set of chromosomes.

Once mitosis is over, we still have to divvy up the rest of the cell, such as the cytoplasm and the organelles. This process, known as cytokinesis, actually starts as telophase is ending. The result is two new daughter cells, each with its own nucleus and its own share of the cytoplasm.

So there you have it. We’ve completed one cell cycle, starting in interphase, duplicating the chromosomes during the S phase, separating them during mitosis, and then forming new cells during cytokinesis.