Mitosis

Mitosis

During mitosis, the cells undergo duplication and division, in other words, they produce copies of themselves. This duplication process is a normal function of cells and allows for growth and the replacement of worn or damaged cells. Some cells, such as the epithelial or skin cells, reproduce frequently, whilst others, such as mature nerve cells reproduce rarely if ever. Although cellular reproduction is a normal process, sometimes the process becomes uncontrolled and in such cases tumours and cancers develop, we shall see later that certain chemicals and radiation can promote such behavior.

Traditionally mitosis is divided into two main stages; interphase and mitosis (although each of these stages consist themselves of a number of phases).

Interphase: During interphase the cell prepares for reproduction by duplicating the cellular DNA. Much of this activity is invisible to the conventional light microscope and it is only during the mitosis stage itself that the frantic cellular activity becomes apparent.

 

Mitosis:

Diagrams in the set book will help you follow the process of mitosis. There is also an excellent animation of the process at San Diego State University This really is a must visit site. You can also download a copy of this animation. Kathleen M. Fisher, Professor of Biology at San Diego has produced an excellent biology site and her description of mitosis (better than mine!) and some ideas for lessons and exercises are available at her mitosis site - I strongly recommend you pay a visit.

Mitosis consists of a series of continuous processes; prophase, metaphase, anaphase and telophase (which itself leads to cytokinesis, the actual formation of daughter cells). It is important that you are able to describe each of these phases.

Prophase

During the prophase stage, the DNA containing chromatin condense and form chromosomes. Since the DNA has already been duplicated during the interphase, each chromatid consists of two chromosomes connected by a centromere. This, and other stages of mitosis, are illustrated in set book 1.

In the latter stages of prophase, spindle fibres form. These are attached to the chromatids and reach out in opposite directions. It is at this point that the chromosomes begin to move apart and the nuclear envelope breaks up and dissolves. The centrioles separate, each radiating microtubules called asters.

Metaphase

The chromosomes move apart and at metaphase they reach the central plane of the cell. Following the frantic activity which has gone before, this seems almost like a moment of rest for the cell as the chromosomes wait patiently for the next phase of mitosis.

Anaphase

During this phase the centrioles divide to form truly separate chromosomes, these are then drawn to their individual poles, where they wait to form the daughter cells. At this point we have a single cell containing two sets of individual yet identical chromosomes.

Telophase

Following anaphase, the two daughter cells begin to form. The nucleus (or nuclei since we now have two proto-cells), previously dissolved, begins to reform as does the nuclear membrane. The cytoplasm and organelles (duplicated earlier) are distributed between the emerging cells, the chromosomes disperse (as in the interphase) and the spindle is recycled to form the cytoskeleton.

The final phase is the separation of the two daughter cells (this is called cytokinesis) as the cytoplasm separates to form the two new cells.