Module 5 Cell Division

3.5.3 page 3

Reflect and Connect

By creating copies of our DNA in S phase, and then carefully separating those identical sets of DNA into new cells during Mitosis, our cells ensure that the next generation has all it needs to continue life. Each new cell has the same number of chromosomes as its parent cell. Consistency seems assured.

However, as we considered in the introduction, our bodies do change over time. For example, our skin is not as elastic when we’re old as it is when we’re young. What could be causing this? The source of aging seems to be two-fold. One factor is built-in to the process of copying of our DNA, while the other is linked to environmental stress.

Our environment is harsh. There are all kinds of chemicals and radiation that can break up our DNA or cause changes known as mutations. One common class of these nasty chemicals is known as oxidants. These are highly reactive substances containing oxygen that are present always, but increase with infection, as well as with consumption of alcohol, cigarettes and highly processed foods. Another concern is high levels of glucose, which can bind to our DNA and cause it to stop functioning. Mutation, oxidants and high glucose can all cause our cells to die. Even if they escape death, the cell line may be forever reduced in function or may end up with cancerous growth.

In addition to environmental stress, our DNA faces challenges from within. Each time our DNA is copied in S phase, it is not perfectly copied. Instead, the ends of our chromosomes, known as telomeres, are shortened just a bit. Telomeres protect the chromosome in much the same fashion a plastic tip protects a shoe lace. When these ends are too short, the chromosomes can no longer be copied, nor do they function properly, and as a result the cell and its line die. Cells can at best divide 50 times, no more.

Discuss

A lot of attention has recently been given to telomeres and their role in the aging process. Cancer cells seem to pay attention to them too, as most cancers turn on a gene to create telomerase, an enzyme that will prevent the shortening of telomeres. However, they are not the single factor in aging, as you’ve 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? Take a position on this statement and engage your classmates and your instructor in discussion.

Self-Check

Try reviewing this material by answering the following questions:

1. What three functions does mitosis serve in your body?
2. 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
3. Sketch the four phases of mitosis. Include labels to explain what is happening in each phase.
4. What role does the spindle apparatus play in cell division?
5. Briefly explain the link between cell cycle regulation and cancer.
6. The scientists in a lab have isolated a substance that prevents cells from synthesizing microtubules. What impact would this substance have on cell division? Explain.
7. A scientist studying a group of somatic cells notices that upon the completion of the cell cycle, half of the daughter cells have no chromosomes and the other half have 92 chromosomes. In what phase of mitosis did an error likely occur? Explain your reasoning.

Discuss your answers with your instructor and save them to your course folder for review. These questions are also found on page 561 of your text.

Going Beyond

Pop a pill or change your life style.

In this lesson you were introduced to two forces that cause aging. One is built in to our cells, but the other was environmental. Environmental factors have been linked to at risk behaviour like overeating, alcohol consumption or smoking. However, a lot of research goes in to creating medications or treatments that combat the effects of these behaviours. Using the Internet, your local library, or other community resources you might have access to, conduct your own research to find out about new treatments or medications that are being introduced to the market to combat the negative effects of today’s lifestyle choices. Organize your findings into a presentation style of your choice and share it with your classmates.

Lesson Summary

During this lesson you explored the following focusing questions:

• Identify and describe the phases of mitosis.

• How does mitosis maintain consistency in plants and animals?

Mitosis is the orderly separation of doubled chromosomes into new daughter cells. Through prophase, metaphase, anaphase, and telophase the cell carefully ensures each new cell has a complete set of chromosomes. By this division,  plant or animal tissue lines are continued faithfully until time and exposure to the environment cause a breakdown with age.