sci230r6_week2_reading2 - c05.qxd 9:25 AM Page 115 5...

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115 Chromosomes: Where Are the Genes Found? If we consider that into these chromosomes are packed from the beginning all that preordains, if not our fate and fortunes, at least our bodily characteristics down to the color of our eyelashes, it becomes a question whether the virtues of the nucleic acids may not rival those of the amino acids in their importance. —J. B. Leathes, 1926 Chapter opening photo Human chromosomes. This color-enhanced electron micrograph shows chromosomes in the condensed form just prior to cell division. (magnified 2,280 times) Overview Consider a complex, many- celled creature such as yourself. You began life as a single cell formed by the fusion of an egg cell from your mother and a sperm cell from your father. This fertilized egg, called a zygote , then divided, making two cells, then four, then eight, and so on, until it had given rise to the trillions 5
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116 C HAPTER 5 Chromosomes: Where Are the Genes Found? of cells that make up the complex adult that you are today. Even if you have stopped growing, most of your cells continue to divide. They will do so for as long as you live, replacing old, worn cells and repairing damaged tissue. All of the instructions, or hereditary units, for building a human being were there at the start, when you were a single-celled zygote. Those instructions influenced what kind of organism you are, the color of your eyes, the way your nerve cells connect in your brain, the length of your legs—every physical trait that you possess. We call those hereditary units your genes . With each cycle of cell division, all of the genes are carefully copied and passed on to each new cell so that each of the trillions of cells of your body has its own complete set. Cell theory brought new understanding to the nature of life, but in the late 1800s, the nature of the genes was not yet known. What are they made of? What structures, small enough to be contained in a single zygote, can harbor enough information to build an entire organism? How are they faithfully copied and passed on in full with each cell division? Genes, scientists reasoned, must be very tiny, indeed. The search for genes began under the microscope, with careful observations of cells in the process of dividing. 5-1 What Cellular Structure Holds the Genetic Information? The earliest clues to the physical nature of genes came in the 1880s from the German anatomist, Walther Flemming. Flemming had at his disposal some of the finest micro- scope lenses of the day.These oil immersion lenses could provide up to 1000-fold mag- nification while still maintaining a great deal of resolution. Resolution is the term that describes the amount of detail that can be seen in a magnified image. Flemming used a variety of chemical dyes that preferentially cling to certain cellular structures, staining them distinctive colors and improving the contrast between tiny cellular parts. Unfor- tunately, staining cells usually kills them. His observations were restricted to dead cells, killed in the act of dividing.
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