Cell Division
Overview
There are three portions of the cell cycle - interphase, mitosis, and cytokinesis. During interphase, the cell is not dividing but the genetic material is duplicating. During mitosis, the chromosomes divide. Cytokinesis is the division of the cytoplasm, which occurs during in the later stages of mitosis.

Introduction

When a cell divides into two cells, the genetic material needs to be duplicated so that each of the resulting daughter cells contains the same genetic material as the original mother cell.

When does cell division occur in our bodies? Cell division occurs after a male and female sex cell have united (fertilization) and the resulting zygote divides into two cells. These two cells divide into four cells, and the process continues. Also, cell division continues throughout the life of an organism since cells must reproduce to replace worn-out and dead cells. Sometimes cell division becomes uncontrolled and the cell divides again and again resulting in a mass of cells that push out their neighboring cells. The result? Cancer. For more information about cancer, see the September, 1996 issue of Scientific American. This issue is available online for a period of time and its contents can be accessed by clicking here.

The stages of the cell cycle are as follows: interphase and then mitosis, which consists of the following stages: prophase, metaphase, anaphase, and telophase. As mentioned earlier, cytokinesis (division of the cytoplasm) accompanies mitosis).

You can watch a Quicktime movie of the entire process by clicking here. (Requires Quicktime plug-in.)

In order to understand the cell cycle and mitosis in particular, you must understand that this is a cell in an organism and that the chromosomes in that cell were inherited from that organism's parents. In the paragraphs to follow, the cell from this hypothetical organism contains 2 pairs of chromosomes.

In the following series of pictures, you will see a cell in which there are 4 chromosomes. In other words, there are two pairs of chromosomes. Of each pair, one chromosome was inherited from the organism's mother and the other member of the pair was inherited from the organism's father. Those chromosomes that were inherited from the organism's mother are pink and those from the father are blue in the diagrams below. In humans, there would be 23 pairs of chromosomes. Obviously, for the sake of space, only 2 pairs of chromosomes are shown in this cell.

Interphase
During interphase, the genetic material is duplicating. The genetic material consists of DNA and proteins (called histones) associated with it. Collectively, the DNA and the histones is called chromatin. During interphase, the chromatin is visible in the nucleus as a dark mass and individual chromosomes are not visible. It is not until later (during prophase of mitosis) that the DNA condenses and becomes visible as chromosomes.

In the pictures below, you would not really be able to see the genetic material but for the sake of making the material more understandable, the genetic material is shown first as simple strings. As you can see during from the pictures, the genetic material has duplicated during interphase.

Early interphase
Late interphase
The Stages of Mitosis
Listed below is an outline of the events that occur during each stage of mitosis. Keep in mind as you read through this outline that the purpose of mitosis is to produce two daughter cells that are genetically identical to the mother cell. Therefore, the cell must duplicate the genetic material and then give one copy to one daughter cell and the other copy to the other daughter cell.

 

Prophase
During prophase, DNA condenses and the chromosomes become visible. The fact that the DNA has duplicated becomes evident during this stage of mitosis since each chromosome can be seen to consist of two parts. These two parts are genetically identical to each other. Each half is called a chromatid. In other words, a duplicated chromosome consists of a chromatid and its sister chromatid. The two chromatids are attached to each other and the location where the two are attached is called a centromere.

This chromosome is a duplicated chromosome and therefore consists of two halves. Each half is called a chromatid. In this diagram, one chromatid has been colored as a light pink and the other chromatid has been colored as a darker pink.

How do the two chromatids of a chromosome compare genetically? In other words, are they similar, identical, or quite different from each other?

The circle between the two chromatids represents the point of attachment between the two chromatids. The point of attachment is called the centromere.

Just outside the nucleus of animal cells are two pairs of cylindrical structures called centrioles. (For the sake of simplicity, the centrioles are not shown in the diagrams. (These structures are not usually visible under the light microscope. They are usually represented simply as dots in diagrams.) (Plant cells do not have centrioles.) During prophase they migrate to opposite poles of the cell. Their purpose is to produce spindle fibers. The purpose of the spindle fibers is to pull the two chromatids of a chromosome apart from each other and pull them toward opposite poles of the cell. (This pulling apart will occur in a later stage of mitosis.) Collectively the spindle fibers are called the spindle apparatus, or spindle.

During prophase, the nuclear membrane disintegrates. In other words, the nucleus disappears.

The chromosomes also begin to migrate toward the cell equator during prophase. When they have reached the cell equator, the cell has reached the next stage - metaphase.

 

Metaphase
This stage is characterized by the chromosomes lying on the cell equator.

 

Anaphase
During anaphase, the following events occur:

  1. The spindle fibers pull on the chromatids so that one chromatid is pulled toward one pole of the cell and the other chromatid is pulled to the opposite pole.
  2. Anaphase begins once the two chromatids begin to separate and ends when they reach the poles.
  3. The chromatids are now individual chromosomes. (The term, chromatid, applies to the two parts of a duplicated chromosome.)
  4. Cytokinesis, splitting of the cytoplasm, usually begins during anaphase. It will continue through the next stage, telophase. In animal cells, the cell constricts around the middle resulting in the formation of a cleavage furrow. In plant cells, a cell plate begins to form which will separate the two daughter cells from each other.
 

Telophase

  1. This phase begins when the chromosomes have reached the poles.
  2. During this stage the chromosomes decondense (that is, the DNA stretches back out).
  3. A nuclear membrane develops around the set of chromosomes at one pole and around the set at the opposite pole.
  4. Cytokinesis is completed. Two daughter cells are formed, which are genetically identical to each other. These two cells will now enter interphase and then mitosis.