Mitosis and Cell Division

Errors in Cell Division

Errors in mitosis can result in changes in chromosome number or the loss of cell cycle control, which can lead to cancer.

The daughter cells formed at the end of the cell cycle should have the same number of chromosomes as the parent cell. For example, in humans, each parent cell starts with 46 chromosomes. Each chromosome replicates during interphase so that during anaphase, when the chromosome copies separate and move toward opposite poles, each of the new nuclei will have the correct number (46). However, sometimes this separation does not occur as planned. Failures during mitosis or cytokinesis potentially cause the formation of cells with an abnormal chromosome number. Cells may have more or fewer chromosomes than the usual for that organism. Cells with abnormal chromosome numbers are often unstable and are usually removed from the population by apoptosis. When cells are damaged during prometaphase and metaphase, they will eventually leave mitosis. Cells leaving mitosis at this point do so because of a degradation of various cyclins. When failures occur during cytokinesis, the resulting daughter cells will have the incorrect number of chromosomes. One would have too many, and the other would have too few. Having abnormal numbers of chromosomes at a certain point during the cell cycle usually results in the failure of mitosis or cytokinesis.

Anytime an organism has an abnormal number of copies of a particular chromosome, it is known as aneuploidy. The term polyploid means having more than two paired sets of chromosomes. This is especially common in plants and can be the result of errors in mitosis. Polyploid plants can also be formed purposefully to combine advantageous traits in crop plants. It is possible for certain organisms to end up with triploid (3n) or even tetraploid (4n) chromosomes. Organisms with these types of chromosome numbers tend to have larger cells. An example of a polyploid organism is a triploid banana. These fruits tend to be larger in size than other bananas and lack seeds. Seedless watermelons are also a triploid fruit.

Polyploidy in Plants

Wild bananas are diploid. Diploid bananas contain seeds and are smaller than the ones found in supermarkets today. Domestic bananas, however, are triploid (3n), which makes them polyploid-more than two pairs of chromosomes.
Cancer is a disease that is caused by any mutation that leads to uncontrolled cell growth. One of the most common mutations that can lead to cancer prevents the cell from stopping at the checkpoint between the G1 and S phases. There are many ways for the cell-cycle control system to fail. Cancer cells ignore signals that inhibit cell division. Cancer can also occur as the result of a mistake during mitosis. Here, the signaling mechanisms that turn off mitosis (such as those that send a cell into the resting G0 phase) malfunction, allowing cells to divide out of control. Some cells simply do not stop at the established checkpoints. For example, they may pass from G1 to S or from G2 to M thereby avoiding the usual inspection. Others stop at random points in the cell cycle or interact in unusual ways with cyclins. In a cancerous cell, cell division never stops, so damaged chromosomes and organelles accumulate, replicate, and are passed on to the daughter cells. Often, these cells enter apoptosis because the damage is too severe. All types of cancer cells will continue dividing as long as they continue to receive nutrients. Uncontrolled growth is considered benign if it doesn't affect other tissues. Malignant tumors have the ability to invade other tissues and potentially spread to another part of the body, a process known as metastasis. Regulators promoting cell growth and division may be overactive in cancer cells. One class of gene that has been damaged by mutation and can transform a normal cell into a tumor cell is called an oncogene.

Development of Cancer

Cancer begins with a cell that undergoes a mutation, causing it to proliferate when normal cells would not. As the cells continue to divide, they form a tumor. Once some of the tumor cells enter the bloodstream or lymphatic vessels, metastasis has occurred.