CH.YBW.CTS.C05.Final.q 6/21/03 1:05 PM Page 64
64
CELLS, TISSUES, AND SKIN
chromatin must be folded and twisted into condensed structures called chromosomes. Chromosomes greatly compact the already condensed genetic material in chromatin, preventing entanglement and breakage of delicate strands during the movements that occur during cell division. A chromosome packages genetic material in an analogous way as a skein packages a great length of yarn. In other words, a skein makes it possible for you to transport yarn from the store without it tangling or breaking. AGING
Aging of individual cells is responsible for many of the problems associated with old age of an organism. Cell aging is a complicated phenomenon that is not completely understood. Some researchers suggest that it results from continual small challenges from toxins, which over time lead to permanent cell damage. For instance, pesticides, alcohol, certain environmental pollutants, and bacterial toxins may damage cell membranes, change the activity of enzymes, and cause mistakes in DNA replication (mutations). In addition, free radicals produced by mitochondria
YOUR HEALTH: Mutations A mutation refers to a change in the structure of DNA. This, in turn, causes a gene to code for one or more inappropriate amino acids, thereby producing an abnormal protein, which may not function properly. A consequence of a mutation can be a specific disease. For example, since most enzymes and many structural components are composed of proteins, mutations can lead to a variety of enzyme deficiency diseases or disorders affecting membrane transport or regulation of metabolism. Sickle cell anemia is an example of an inherited mutation that is caused by an inappropriate substitution of one amino acid in the entire hemoglobin molecule. Mutations also can cause cancer. Excessive exposure to ultraviolet light, X-rays, and some chemicals, such as tars in cigarette smoke, alcohol, and certain environmental pollutants, can cause mutations.