Types of Cycles (Genetics & Inherited Conditions)
Biological clocks control a number of physiological functions, including sexual behavior and reproduction, hormonal levels, periods of activity and rest, body temperature, and other activities. In humans, phenomena such as jet lag and shift-work disorders are thought to result from disturbances to the innate biological clock.
The most widely studied cycles are circadian rhythms. These rhythms have been observed in a variety of animals, plants, and microorganisms and are involved in regulating both complex and simple behaviors. Typically, circadian rhythms are innate, self-sustaining, and have a cyclicity of nearly, but not quite, twenty-four hours. Normal temperature ranges do not alter them, but bursts of light or temperature can change the rhythms to periods of more or less than twenty-four hours. Circadian rhythms are apparent in the activities of many species, including humans, flying squirrels, and rattlesnakes. They are also seen to control feeding behavior in honeybees, song calling in crickets, and hatching of lizard eggs.
What is known about the nature of the biological clock? The suprachiasmatic nucleus (SCN) consists of a few thousand neurons or specialized nerve cells that are found at the base of the hypothalamus, the part of the brain that controls the nervous and endocrine systems. The SCN appears to play a major role in the regulation of circadian rhythms in mammals and affects cycles of sleep,...
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Biological Clocks and Aging (Genetics & Inherited Conditions)
Genes present in the fertilized egg direct and organize life processes from conception until death. There are genes whose first effects may not be evident until middle age or later. Huntington’s disease (also known as Huntington’s chorea) is such a disorder. An individual who inherits this autosomal dominant gene is “programmed” around midlife to develop involuntary muscle movement and signs of mental deterioration. Progressive deterioration of body functions leads to death, usually within fifteen years. It is possible to test individuals early in life before symptoms appear, but such tests, when no treatment for the disease is available, are controversial.
Alzheimer’s disease (AD) is another disorder in which genes seem to program processes to occur after middle age. AD is a progressive, degenerative disease that results in a loss of intellectual function. Symptoms worsen until a person is no longer able to care for himself or herself, and death occurs on an average of eight years after the onset of symptoms. AD may appear as early as forty years of age, although most people are sixty-five or older when they are diagnosed. Age and a family history of AD are clear risk factors. Gene mutations associated with AD have been found on human chromosomes 1, 14, 19, and 21. Although these genes, especially the apolipoprotein e4 gene, increase the likelihood of a person getting AD, the complex nature of...
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Impact and Applications (Genetics & Inherited Conditions)
Evidence has accumulated that human activities are regulated by biological clocks. It has also become evident that many disorders and diseases, and even processes that are associated with aging, may be affected by abnormal clocks. As understanding of how genes control biological clocks develops, possibilities for improved therapy and prevention should emerge. It may even become possible to slow some of the harmful processes associated with normal aging.
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Further Reading (Genetics & Inherited Conditions)
Finch, Caleb Ellicott. Longevity, Senescence, and the Genome. Reprint. Chicago: University of Chicago Press, 1994. Provides a comparative review of research on organisms from algae to primates, expanding traditional gerontological and geriatric issues to intersect with behavioral, developmental, evolutionary, and molecular biology. Illustrated.
Foster, Russell G., and Leon Kreitzman. Rhythms of Life: The Biological Clocks That Control the Daily Lives of Every Living Thing. London: Profile Books, 2004. Explains the workings of biological clocks in human beings and other creatures. Describes how these clocks are controlled by nerve cells and genes.
Hamer, Dean, and Peter Copeland. Living with Our Genes: Why They Matter More than You Think. New York: Doubleday, 1998. Links DNA and behavior and contains a good chapter on biological clocks and aging.
Koukkari, Willard L., and Robert B. Sothern. Introducing Biological Rhythms: A Primer on the Temporal Organization of Life, with Implications for Health, Society, Reproduction, and the Natural Environment. New York: Springer, 2006. Provides a comprehensive overview of circadian, tidal, lunar, and other biological rhythms and describes the application and implications of these rhythms in daily life.
Medina, John J. The Clock of Ages: Why We Age, How We Age—Winding Back the Clock. New York: Cambridge University Press,...
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Web Sites of Interest (Genetics & Inherited Conditions)
Learn Geneticism, The Time of Our Lives. http://learn.genetics.utah.edu/content/begin/DNA/clockgenes. Well-written and illustrated discussion of biological clocks and the mechanisms of their genes. Discusses the implications of these clocks for human sleep and health.
National Institute of General Medical Sciences. http://www.nigms.nih.gov/Publications/Factsheet_CircadianRhythms.htm. A fact sheet providing questions and answers about circadian rhythms.
Time Matters: Biological Clockworks. http://www.hhmi.org/biointeractive/clocks/museum.html. A virtual museum exhibit exploring the inputs, outputs, and mechanisms of biological clocks. Focuses on circadian rhythms, with information on the genetic sources of these rhythms.
Web MD, Sleep and Circadian Rhythm Disorders. http://www.webmd.com/sleep-disorders/guide/circadian-rhythm-disorders-cause. Discusses the causes of circadian rhythm disorders and their relationship to sleep.
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