Mitosis

The following pictures were taken with one of our classroom microscopes. They depict the main steps in the process of cell division. All pictures are at 1000x magnification (before being magnified by your screen). The pictures are of the cells of onion root tips. The tips are used because they grow fast and the cells are quite large. The cells were stained to reveal the nucleic acids (chromosomes) which are usually quite difficult to see.

Each picture is accompanied by a paragraph. Read the paragraphs carefully and see if you can list the events that occur at each phase in the life of the cell.

For most of a cell's life it is not dividing. We call this non-dividing time interphase [INT-ur-FAYZ]. During interphase, a cell is doing all of the things it was designed to do. The DNA that makes up the chromosomes is being read to create messenger RNA which is in turn used as a template to create proteins. During interphase the cell must also duplicate its chromosomes and prepare for the time when the cell will divide to form two daughter cells.

The process of mitosis actually begins with a stage referred to as prophase [PRO-FAYZ]. During prophase the chromosomes coil and become visible as pairs of sausage-shaped things withing the nucleus. When removed from the cell at this point, they often form a shape like the letter "X". The cell's centrioles move to opposite sides of the cell. Tiny fibers attach to the centrioles and to the centromere of each chromosome forming the spindle shaped network of fibers that will pull the chromosomes to opposite ends of the cell. The nuclear membrane also disappears at this time. (Can you list the events that happen during prophase? Look back through this paragraph and try to.)

After prophase the cell enters metaphase [MEHT-uh-FAYZ]. During metaphase the chromosomes line up at the middle of the cell. At this point each chromosome consists of two exact copies of the cell's original chromosome. The copies, called chromatids, are held together at the centromere. The centromere in the middle of each chromatid is attached to a spindle fiber which is also attached to one of the centrioles. Metaphase ends when the centromeres are ready to separate and allow the chromatids to be drawn to opposite ends of the cell.

After metaphase the cell enters anaphase [AN-uh-FAYZ]. During anaphase the pairs of identical chromatids separate at their centromeres and are drawn to opposite ends of the cell by the spindle fibers. This permits each daughter cell to have a copy of each of the cell's original chromosomes.

When a cell finishes anaphase it enters telophase [TEHL-uh-FAYZ]. During telphase the spindle fibers break down and a new nuclear membrane forms around the chromosomes in each half of the cell.

After the successful division of the nucleus and during the later part of telophase the cell begins to separate into two new cells. In this photograph and the one above, you can see the formation of a new cell wall between the daughter cells. The process of dividing the cell in half is called cytokinesis [SYT-o-kuh-NEE-sus]. Note that cytokinesis actually begins during telophase.