by chemicals from the adjacent damaged tissue. Or the dividing cells may be stem cells that are among the specialized cells. (A region of the brain called the hip- pocampus, which is necessary to form new memories, seems to have cells capable of division.) At present we do not have definitive knowledge, but we do know that for most people with heart damage or central nervous system injury, mitosis does not take place, or not suf- ficiently enough to replace the cells that have died and preserve or restore normal functioning of the organ.
Research is continuing, and may eventually find the stimulus necessary to produce extended mitosis that would bring about true tissue repair.
MEIOSIS
Meiosisis a more complex process of cell division that results in the formation of gametes, which are egg and sperm cells. In meiosis, one cell with the diploid number of chromosomes divides twice to form four BOX 3–3 ABNORMAL CELLULAR FUNCTIONING—CANCER
the trigger is believed to be infection with certain viruses that cause cellular mutations. Carriers of hepatitis B virus, for example, are more likely to develop primary liver cancer than are people who have never been exposed to this virus. Research has discovered two genes, one on chromosome 2 and the other on chromosome 3, that contribute to a certain form of colon cancer. Both of these genes are the codes for proteins that correct the “mis- takes” that may occur when the new DNA is syn- thesized. When these repair proteins do not function properly, the mistakes (mutations) in the DNA lead to the synthesis of yet other faulty pro- teins that impair the functioning of the cell and pre- dispose it to becoming malignant.
Once cells have become malignant, their func- tioning cannot return to normal, and though the immune system will often destroy such cells, some- times it does not, especially as we get older.
Therefore, the treatments for cancer are directed at removing or destroying the abnormal cells. Surgery to remove tumors, radiation to destroy cells, and chemotherapy to stop cell division or interfere with other aspects of cell metabolism are all aspects of cancer treatment.
New chemotherapy drugs are becoming more specific, with very precise targets. For example, the cells of several types of solid-tumor cancers have been found to have mutations in the gene for the cell membrane receptor for a natural growth factor (epidermal growth factor receptor, or EGFR). These altered receptors, when triggered by their usual growth factor, then cause the cell to divide uncon- trollably, an abnormal response. Medications that target only these altered receptors have already been developed for some forms of lung cancer and breast cancer. Not only do they show promise in treating the cancer, they do not have the side effects of other forms of chemotherapy.
There are more than 200 different types of cancer, all of which are characterized by abnormal cellular functioning. Normally, our cells undergo mitosis only when necessary and stop when appropriate. A cut in the skin, for example, is repaired by mitosis, usually without formation of excess tissue. The new cells fill in the damaged area, and mitosis slows when the cells make contact with surrounding cells.
This is called contact inhibition, which limits the new tissue to just what is needed. Malignant (cancer) cells, however, are characterized by uncon- trolled cell division. Our cells are genetically pro- grammed to have particular life spans and to divide or die. One gene is known to act as a brake on cell division; another gene enables cells to live indefi- nitely, beyond their normal life span, and to keep dividing. Any imbalance in the activity of these genes may lead to abnormal cell division. Such cells are not inhibited by contact with other cells, keep dividing, and tend to spread.
A malignant tumor begins in a primary site such as the colon, then may spread or metastasize. Often the malignant cells are carried by the lymph or blood to other organs such as the liver, where sec- ondary tumors develop. Metastasisis characteris- tic only of malignant cells; benigntumors do not metastasize but remain localized in their primary site.
What causes normal cells to become malignant?
At present, we have only partial answers. A malig- nant cell is created by a mutation, a genetic change that brings about abnormal cell functions or responses and often leads to a series of muta- tions. Environmental substances that cause muta- tions are called carcinogens. One example is the tar found in cigarette smoke, which is definitely a cause of lung cancer. Ultraviolet light may also cause mutations, especially in skin that is overex- posed to sunlight. For a few specific kinds of cancer,
cells, each with the haploid number (half the usual number) of chromosomes. In women, meiosis takes place in the ovaries and is called oogenesis. In men, meiosis takes place in the testes and is called sper- matogenesis. The differences between oogenesis and spermatogenesis will be discussed in Chapter 20, The Reproductive Systems.
The egg and sperm cells produced by meiosis have
the haploid number of chromosomes, which is 23 for humans. Meiosis is sometimes called reduction divi- sion because the division process reduces the chromo- some number in the egg or sperm. Then, during fertilization, in which the egg unites with the sperm, the 23 chromosomes of the sperm plus the 23 chro- mosomes of the egg will restore the diploid number of 46 in the fertilized egg. Thus, the proper chromo- 62 Cells
Interphase
Nucleus
Prophase
Centrioles
Pair of chromatids Centromere Equator of cell
Spindle fibers
Metaphase
Anaphase Nuclear membrane re-forms
Telophase Cytoplasm divides
Figure 3–5. Stages of mitosis in a cell with the diploid number of four. See Table 3–4 for description.
QUESTION:In prophase, what is a pair of chromatids made of?
some number is maintained in the cells of the new individual.