Cracking the AP Biology Exam

7

Cell Reproduction

MITOSIS: THE DANCE OF THE CHROMOSOMES

Once the chromosomes have replicated, the cell is ready to begin mitosis. Mitosis is the period when the cell divides. Mitosis consists of a sequence of four stages: prophase, metaphase, anaphase, and telophase.

STAGE 1: PROPHASE

One of the first signs of prophase is the disappearance of the nucleolus. In prophase, the chromosomes thicken, forming coils upon coils, and become visible. (During interphase, the chromosomes are not visible. Rather, the genetic material is scattered throughout the nucleus and is calledchromatin. It is only during prophase that we can properly speak about the chromosomes.)

Now the cell has plenty of room to “sort out” the chromosomes. Remember centrioles? During prophase, these cylindrical bodies found within microtubule organizing centers (MTOCs) start to move away from each other, toward opposite ends of the cell. The centrioles will spin out a system of microtubules known as the spindle fibers. These spindle fibers will attach to a structure on each chromatid called a kinetochore. The kinetochores are part of the centromere.

STAGE 2: METAPHASE

The next stage is called metaphase. The chromosomes now begin to line up along the equatorial plane, or the metaphase plate, of the cell. That’s because the spindle fibers are attached to the kinetochore of each chromatid.

STAGE 3: ANAPHASE

During anaphase, the sister chromatids of each chromosome separate at the centromere and migrate to opposite poles. The chromatids are pulled apart by the microtubules, which begin to shorten. Each half of a pair of sister chromatids now moves to opposite poles of the cell. Non-kinetochore microtubules elongate the cell.

STAGE 4: TELOPHASE

The final phase of mitosis is telophase. A nuclear membrane forms around each set of chromosomes and the nucleoli reappear.

The nuclear membrane is ready to divide. Now it’s time to split the cytoplasm in a process known as cytokinesis. Look at the figure below and you’ll notice that the cell has begun to split along a cleavage furrow (which is produced by actin microfilaments):

A cell membrane forms about each cell and they split into two distinct daughter cells. The division of the cytoplasm yields two daughter cells.

Here’s one thing to remember: Cytokinesis occurs differently in plant cells. The cell doesn’t form a cleavage furrow. Instead, a partition called a cell plate forms down the middle region.

STAGE 5: INTERPHASE

Once the daughter cells are produced, they reenter the initial phase—interphase—and the whole process starts over. The cell goes back to its original state. Once again, the chromosomes become invisible, and the genetic material is called chromatin.

BUT HOW WILL I REMEMBER ALL THAT?

For mitosis, you may already have your own mnemonic. If not, here’s a table with a mnemonic we created for you.

PURPOSE OF MITOSIS

Mitosis has two purposes:

• to produce daughter cells that are identical copies of the parent cell.
• to maintain the proper number of chromosomes from generation to generation.

For our purposes, we can say that mitosis occurs in just about every cell except sex cells. When you think of mitosis, remember: “Like begets like.” Hair cells “beget” other hair cells; skin cells “beget” other skin cells, etc.