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2 Scientific Foundations
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Scientific Foundations 2 In the nuclei of the cells in our body are chromosomes. These are long strands of a molecule called DNA (short for deoxyribonucleic acid), wound up tightly to fit in the confined space of the nucleus. Most normal cell nuclei contain two sets of these chromosomes, one set inherited from the mother and one set from the father. Each set normally contains twenty-three chromosomes. Twentytwo of these chromosomes are referred to by number—chromosome 4, chromosome 21, and so on. The remaining chromosome in each set is a sex chromosome, either an “X” or a “Y.” A person who has two X chromosomes is female; a person who has an X and a Y is a male. Human chromosomes can be viewed and photographed through a microscope.The resulting image is called a karyotype (see fig. 1). Occasionally, people have an extra chromosome in their cell nuclei. This occurs when something goes wrong in the way the cells divide during reproduction. A person who has an extra copy of chromosome 21, for example, has Down syndrome.This is why Down syndrome is also called “Trisomy 21”: “trisomy” means “three 12 Wondergenes copies.” Some males have an extra Y chromosome. At one time, this was thought to make them prone to violent criminal behavior.1 If you took a chromosome, unwound the DNA, and looked through a powerful microscope, you would see the remarkable physical structure that James Watson and Francis Crick discovered in 1953. The DNA resembles a twisted ladder, a shape called a “double-helix.” The ladder is composed of a double string of chemicals called “nucleotides,” each of which is made up of a sugar, a phosphate, and something called a “base.” To understand the structure of DNA, think of the ladder cut in half down the middle of the rungs. The sugars and phosphates of the nucleotides form the long strip which is the side of the halfladder . Sticking out from the side are the bases, each forming a half of a rung. The bases are the important part of the DNA molecule. There are only four bases: adenine, guanine, cytosine, and thymine, known by their initials A, G, C, andT.They can connect or “bond” to one another only according to a strict set of rules. “A” bonds only with “T” and vice versa. “G” bonds only with “C.” In humans and other higher organisms, the two halves of the DNA molecule in the chromosomes spend most of their time joined together, like what you would get if you reunited the two halves of the ladder. Each of the bases that make up a half-rung is connected to the base of the other half-rung according to the bonding rules: an A with aT, a G with a C. Each rung thus consists of a pair of bases, known as a “base pair.” The ladder twists around a central axis, forming the famous double helix shape (fig. 2). If you unwound the DNA in the chromosomes in a human cell nucleus and laid it in a straight line, you would get a double strand more than five feet long. If you added up the base pairs, there would be about three billion. The DNA of lower organisms has fewer base pairs. The DNA of the bacterium E. coli has about four million. Yeast has about fifteen million. The bases are the important part of the DNA molecule because the order in which they occur, or their “sequence,” forms the genetic code of the organism. Along certain stretches of DNA, the sequence of base pairs contains instructions for making proteins. Proteins consist of long, complex chains of chemicals called amino [54.242.75.224] Project MUSE (2024-03-19 11:07 GMT) 13 Scientific Foundations Figure 1. From an actual photograph. Source: U.S. Department of Energy Human Genome Program, http://www.ornl.gov/hgmis 14 Wondergenes Figure 2. Source: National Human Genome Research Institute, Division of Intramural Research, http://www.genome.gov [54.242.75.224] Project MUSE (2024-03-19 11:07 GMT) 15 Scientific Foundations acids, and they form the structural components of cells and tissue and the chemicals that control biochemical processes, called “enzymes .” The stretches of DNA that contain the instructions for making proteins are called genes. (The rest of the DNA was originally thought to have no function, and was referred to as “junk DNA.”2 Scientists are now discovering that the non-gene stretches...