Module 5 Lesson 2 - 6

Going Beyond

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Knowing that certain chemicals interfere with the process of cell division, researchers set out to find drugs to help in curing cancer. This has led to a cancer treatment method called chemotherapy, or treatment by chemical drugs. Because cancer cells divide rapidly and continually, any chemical that blocks cell division or kills cells while they are dividing has 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.

Now, more than two dozen anticancer drugs 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 enzyme from doing their job. Without these enzymes, new molecules of DNA cannot be synthesized. If cell division does not occur among these drug-damaged cells, none of the newly formed cells will survive. Generally, methotrexate is quite successful at first, but similar to other such 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 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 laborious process requiring 1000 kg of periwinkle stems is used. Chemists have synthesized the substances successfully, but this is even more expensive. At present, new methods of culturing the plants are being developed to speed the production of these drugs. The medical potential of Madagascar periwinkle is a good example of why conserving plant diversity is so important.

Watch and Listen

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Watch and listen for information related to the following questions:

• How does radiation help combat cancer?
• What are some of the considerations when using radiation?
• What are some of the side effects of radiation treatment?

Discussion

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Much attention has been given recently to telomeres and their role in the aging process. Cancer cells seem to be attentive to them, too, as most cancers turn on a gene to create telomerase, an enzyme that prevents the shortening of telomeres. However, they are not the single factor in aging as you have read. Instead, excess sugar, alcohol, smoke, and processed foods full of chemicals seem to cause far greater damage to our DNA and our life span.

Given that research and funding is limited, should more focus be given to finding gene treatments to lengthen our telomeres, or should focus and funding go to programs that encourage us to reduce behaviours that put our health and longevity at risk?

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Check your understanding. Complete the following exercise on mitosis and cellular division; then, check your answers. If any of these questions give you difficulty, ask your teacher for clarification about that concept.

1. Label the following terms on the flow chart below: mother cell, daughter cells, chromosomes, S phase, metaphase, separation of chromatids, prophase.
   
   



2. A skin cell taken from a chimpanzee contains forty-eight chromosomes.
   
   
   1. How many chromosomes are in the nerve cells or bone cells of this animal?
      
      
   2. If a skin cell of the chimpanzee underwent cell division, how many chromosomes would be in each daughter cell?
   
   
   3. A sample of chimpanzee nerve cells underwent a radiation therapy, and now the cells have mutated to become tetraploids. How many chromosomes are present in each irradiated nerve cell?
      



3. What role do centrioles have in cell division of animal cells?
   
   
4. Match the event described in each statement with the correct stage of mitosis labelled on the diagram.
   
   

   A. Interphase	B. Prophase	C. Metaphase	D. Anaphase	E. Telephase
   	Event
   ________ a.	Normal growth and functioning of the cell occurs here.
   ________ b.	Chromosomes replicate to produce two sets of chromosomes in preparation for cell division.
   ________ c.	Chromosomes with their duplicates still attached shorten by coiling, thus becoming visible under the microscope.
   ________ d.	Centrioles migrate to opposite sides of the cell, and the nuclear membrane dissolves.
   ________ e.	Spindle fibres grow from each centriole and attach to the centromere of each chromatid pair.
   ________ f.	Chromatid pairs still joined at the centromere line up along the middle of the cell, called the metaphase plate.
   ________ g.	Chromatids are pulled apart by shortening of the spindle fibres. One complete set of chromosomes is pulled to each pole.
   ________ h.	Chromosomes uncoil, spindle fibres dissolve, and cytoplasm divides. Two daughter cells are formed.

   
   



5. Name the process of cytoplasmic division and describe how it is different in plant cells and animal cells.


6. What three functions does mitosis serve in your body?
   
   
7. In which phase of mitosis does each of the following events occur?
   
   
   1. Migration of sister chromatids to opposite poles
   2. Condensation of chromatin into compact chromosomes
   3. Formation of a nuclear membrane
   4. Alignment of chromosomes along the cell equator
      
      
8. Sketch the four phases of mitosis. Include labels to explain what is happening in each phase.
   
   
9. What role does the spindle apparatus have in cell division?
   
   
10. Explain briefly the link between cell cycle regulation and cancer.
    
    
11. Scientists in a lab have isolated a substance that prevents cells from synthesizing microtubules. What effect might this substance have on cell division? Explain.
    
    
12. A scientist studying a group of somatic cells notices that, upon the completion of the cell cycle, half the daughter cells have no chromosomes and the other half have 92 chromosomes. In what phase of mitosis likely did an error occur? Explain your reasoning.