Teacher's Study Guide for Lesson Three Mitosis by Dr Jamie Love 2002 - 2010

Mitosis is specifically designed to distribute exact and equal amounts of genetic material so the daughter cells are genetically identical to each other (and the mother nucleus that they came from) - clones.

Mitosis increases the number of nuclei and that usually leads to an increase in cell numbers too.

Species that reproduce this way are clones and this method of reproduction is called asexual reproduction. Clones are produced without sex and without meiosis. Plants are "masters" at cloning and that is why asexual reproduction is often called vegetative reproduction or vegetative propagation.

Mitosis is a flow of events but we have divided it into four obvious stages

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase

During prophase the chromatin condenses into chromosomes.
This is the distinguishing characteristic of prophase - visible, disorganized chromosomes (a "ball of worms"). Prophase is a rather long process and is often divided into early and late prophase.

During prophase, as the chromosomes continue to condense, the centrosomes move to opposite " poles" dragging their spindles along.

Eventually the cell enters late prophase and, because the next phase will be metaphase, late prophase is often call prometaphase. During prometaphase two very important events occur. The nuclear envelope breaks down. The microtubules (fibers) attach to chromosomes.

Late in prophase two special structures grows inside of the centromere region of every chromosome - one on each chromatid. These structures are called kinetochores. ("Kinetos" is Greek for "movement".) The kinetochore attaches to the microtubules that make up the spindles from the centrosomes. Once attached, the spindles run from kinetochore (one on each chromatid) to the centrosome (one on each side), so each chromosome is held between the two centrosomes via the spindles.

A series of tugging from both kinetochores on their spindles cause the chromosomes to move back and forth until this "tug-of-war" ends in a "draw" - the chromosome is then positioned midway between the centrosomes. ALL the chromosomes behave this way.

Therefore, all the chromosomes come to line up midway between the two centrosomes. When all the chromosomes' centromeres are lying on a plane perpendicular to a line connecting the two centrosomes, the cell has entered the part of mitosis called metaphase. The plane upon which the centromeres are arranged is called the metaphase plate.

Metaphase lasts for about an hour and is the best time to see chromosomes.

At anaphase the centromere of each chromosome breaks down the middle (!) and the two sister chromatids move to opposite centrosomes. It is at anaphase that "true" nuclear division occurs.

As each kinetochore drags its chromatid along, the arms of the chromatid are swept back and they seem to form a V-shaped structure. All the chromosomes are able to coordinate the breaking of their centromeres quite well and the speed at which the chromatids move is impressive so anaphase is a sight to see! (In a living cell, of course.)

The moment the chromosome's centromere splits (during anaphase) each kinetochore acts as the center for the creation of a new centromere. Each chromatid is "puckered" inward at the kinetochore during anaphase and that is the new centromere. The trick to counting chromosomes and differentiating them from chromatids is to keep track of the centromeres. A chromosome has a single centromere and each chromosome has two kinetochores at the centromere - one on each chromatid.

By definition a chromosome has a centromere - it's a one to one correspondence. During anaphase, each chromosome becomes two separate chromosomes at that moment of chromatid separation and we now have two chromosomes from each chromosome! During anaphase and until cytokinesis occurs, the cell has twice as many chromosomes as "normal".

Once the newly created chromosomes reach their poles, anaphase is over and we enter the next and last phase of mitosis - telophase.
"Telo" is Greek for "end.

Telophase is the point in mitosis when the new nuclei are formed. A nuclear membrane forms around both bundles of chromosomes at each pole. This creates two nuclei. Also, the chromosomes begin to unwind (decondense) until eventually they are nothing more than a dim coloring of chromatin with a few nucleoli (clumps of chromatin). Each nucleus takes on the appearance of an interphase nucleus and once that is done the cell has completed telophase and mitosis.

Telophase produced the identical nuclei that we now call daughter nuclei because they will be the nuclei of the daughter cells produced from the upcoming cytokinesis.

Not only are the daughter nuclei identical to each other but they are also identical to the mother nucleus from which they came. The daughter nuclei are clones of each other and clones of their mother nucleus. Also, notice that a (mother) cell must have two centrosomes in order to orchestra mitosis, but both (daughter) cells produced (after cytokinesis) have only one centrosome each (until they enter late G2 and the whole cycle starts again).