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Chapter 7

chapter 7

3 Pages

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Biology 1225
Michael Butler

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Test # 2 covers chapters 7-14 with the exclusion of chapter 10 Chapter 7: Gene Expression and Control Before we go any further, in RNA there is NO thymine nucleotide (T in the genetic code) as there is in DNA. Instead, there is a similar base called Uracil (U). So, in RNA, U replaces T and thus U pairs with A. You can be sure, if you see a sequence of bases mentioned and a U is present (such as AUCGAAUU) that it is RNA and NOT DNA that is being discussed, uracil is unique to RNA. What is a Gene? How is it possible that a single-celled zygote can contain the information necessary to form the complete individual? The answer lies in the nature of the DNA molecule. This unique chemical has a structure that allows it to replicate prior to each cell division so that the daughter cells have the same chromosomes (and genes) as the original zygote had. The structure of DNA also allows it to store information. The language used to record the information has an alphabet of only 4 “letters”. These are the bases, adenine, guanine, thymine and cytosine. Sequences of these 4 bases along the length of the chromosomes form the genes. But what is the significance of a gene? In the 1940’s, many years before Watson and Crick described the structure of the DNA molecule, two scientists named George Beadle and Edward Tatum received the Nobel Prize for developing the first clear definition of a gene. They said “one gene, one enzyme”. In other words, a gene is a section of a DNA molecule that codes for an enzyme in one of the chemical processes of a cell. This is a very profound statement, and although the definition of a gene has been expanded and refined over the years, the basic principle remains constant. Remember that enzymes are catalysts at each stage of biochemical pathways. They bring the reactants together in the right way so that the reactions occur quickly. It is believed that, given the correct enzymes in the correct order, and the raw materials or substrates, all of the chemical reactions of the cell occur, and therefore all of the characteristics of the organism are possible. What we actually inherit from our parents is the information to make enzymes plus the other proteins of the cell. Your text refers to “one gene, one protein” to encompass proteins other than enzymes, like hormones, membrane proteins etc., in the definition of a gene. How Proteins are Made (a simplified overview) You could consider the DNA to be like a cookbook. It contains the recipes for all of the proteins that the cell can possibly make over its lifetime. It also contains instructions for making the molecules that carry out protein synthesis. (You could think of these molecules as the machinery necessary for the process.) DNA also contains vast amounts of information that is not yet understood. If the DNA is the cookbook, then a messenger RNA or (mRNA) molecule is a copy of the recipe for one protein. It is a strip of RNA complementary to a fairly short section of one side of a chromosome. mRNA is made by transcription and then it leaves the nucleus. Once in the cytoplasm it may be used over and over again to make many molecules of the same protein. In general, each set of three bases on the mRNA stra
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