Module 5
1. Module 5
1.10. Page 3
Module 5—Cell Division: The Processes of Mitosis and Meiosis
Reflect on the Big Picture
In addition to ignoring clues or checkpoints that essentially control cellular growth, cancer cells also never stop dividing. Probably the best example of this comes from cancer cells removed from Henrietta Lacks in 1951. Those cells are still in existence today. They are alive in a variety of laboratories around the world, and were even included in the Discover 17 satellite and launched into space. Though Henrietta died eight months after the cells were removed, her cancer cells have provided opportunity for research and in one way helped her memory live on.
Healthy cells have a built-in countdown timer. At best, a cell can divide approximately 50 times. After that, the cell will cause its own death, or self-destruct. Many cosmetic companies and advertisements would like you to believe that you can cheat this number. The advertisements suggest that you can keep your cells healthy and dividing as usual for longer and longer periods. Is this a reasonable claim?
mutagenic agent: a chemical or physical agent that has the ability to mutate DNA, affecting the timing of the cell cycle; increasing the rate of mitosis
cellular clock: a property of cells that allows them to go through a set number of cell divisions and then stop, whereupon the cell line dies out; sometimes called apoptosis
Cancer cells do not have a normal cell clock so they do not apoptose.
Earlier in the lesson, you learned about the S phase of the cell cycle. During this phase the DNA is duplicated for the next generation of cells. This is crucial, since it is DNA that contains all of the instructions necessary for cellular function. The problem is that DNA accumulates errors over time. Errors occur in the duplication process itself or from exposure to environmental mutagenic agents. Over time, somatic cells’ DNA gets pretty beaten up. These errors in DNA can cause serious problems or diseases like cancer. To reduce the impact of these errors, every cell contains specific instructions to self destruct after about 50 divisions. It seems the cellular clock exists for a good reason.
Discuss
Now that you have gained further knowledge on the cell cycle and its regulations or limits on cell growth, do you believe that research should continue to be focused on finding a fountain of youth? Is it time to accept our limits and focus on the quality of our lives now rather than focus on the potential length of our lives? Discuss your opinion with your classmates and your instructor. Summarize the important points of the discussion both for and against this research. Post your summary for others to view and store a copy in your course folder.
Module 5: Lesson 2 Assignment
chemotherapy: the use of cytotoxic drugs that inhibit cell division, usually by preventing DNA replication or interfering with the spindle mechanism of mitosis or by interfering with the supply of blood and nutrients to the tumour; applied systemically (into the bloodstream); targets cancerous cells but may also affect rapidly dividing normal cells to some degree
Before you begin your Lesson 2 Assignment involving research on chemotherapy, you may wish to do the questions on page 555 of your textbook. Discuss your responses with your teacher.
Knowing that certain chemicals interfere with the process of cell division, researchers endeavoured to find drugs that would help in curing cancer. This led to a cancer treatment method called chemotherapy, or treatment by chemical drugs. Since cancer cells divide rapidly and continually, any chemical that blocks cell division or kills cells while they are dividing will have a much greater effect on cancerous cells than on normal cells. However, these drugs also destroy other fast-growing cells in the body, such as hair follicles. This explains the loss of hair by cancer patients on chemotherapy.
There are now more than two dozen different anticancer drugs that can be used to treat cancer. One drug used in chemotherapy is methotrexate, which attaches to certain enzymes involved in chromosome (DNA) replication and prevents these enzymes from doing their job. Without these enzymes, new molecules of DNA cannot be synthesized. If cell division does not take place among these drug-damaged cells, none of the newly formed cells will survive.
Methotrexate can initially be quite successful, but like other similar drugs, it loses its effectiveness over time. Studies show that the cancer cells become resistant to these anticancer drugs. Researchers believe that resistance to methotrexate occurs because the drug-treated cancer cells produce multiple copies of the specific gene that is affected by the drug. Methotrexate alters the DNA molecules in cancer cells so that some genes begin to multiply uncontrollably. One of these genes directs the synthesis of the DNA-replicating enzyme, the exact enzyme that the drug inhibits. Multiple copies of this gene cause a pronounced increase in the production of the DNA-replicating enzyme, which in turn causes a dramatic increase in the rate of DNA replication within the cancer cells. This leads to an increase in the rate of cell division. Daughter cells from these altered cells also show multiple genes and a more rapid rate of cell division. Ironically, the very drug that stops cancer cells from dividing also has the effect of making these cells more resistant. Eventually, the chemical's inhibition of cell division in cancerous cells becomes ineffective and essentially useless.
Two other drugs used in the treatment of certain cancers are vinblastine and vincristine. These two drugs were discovered in the Madagascar periwinkle plant, Catharanthus roseus. Vincristine is very effective in the treatment of leukemia, and vinblastine in the treatment of Hodgkin's disease. Vinblastine doubles the chance of surviving Hodgkin's disease. Currently the only practical source of the two drugs is from this plant. However, to produce 5.0 g of vincristine, an expensive and labourious process requiring 1000 kg of periwinkle stems is used. Chemists have successfully synthesized the substances, but this is even more expensive. New methods of culturing the plants are currently being developed to speed up the production of these drugs. The medical potential of Madagascar periwinkle is a good example of why conserving plant diversity is so important.
Retrieve your copy of Module 5: Lesson 2 Assignment that you saved to your computer earlier in this lesson. Complete the assignment and submit it to your teacher.