03_DNA_and_Mutations (1)

03_DNA_and_Mutations (1) - BI SC 002 LECTURE 3DNA AND...

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BI SC 002 LECTURE 3—DNA AND MUTATIONS Draft: October 27, 2010 So, what really IS a gene? We gave a functional definition of gene in the last lecture, but, structurally (and chemically) a gene is really a section of DNA that codes for a functional protein. Proteins are molecules in a living thing that “do stuff”—in other words, just about any activity that takes place in a living thing, whether it’s allowing chemical reactions to occur, defense, structure, transport, you name it, a protein does it. It’s the structure of proteins that lead to their function, and it’s the function of proteins that basically make every living thing what they are. In other words, the proteins lead (either directly or indirectly) to the traits of an organism. A gene can be thought of as a recipe for a protein. A DNA molecule looks something like this: The DNA molecule looks sort of like a ladder twisted around on itself (almost like a spiral staircase). The shape of a DNA molecule is called a double helix , that is, there are two strands of DNA, and both twisted in a corkscrew manner. The poles of the DNA ladder are made up of repeating units of sugars and phosphate groups (called the backbone). The rungs of the ladder are made up of chemicals called nitrogenous bases . There are 4 nitrogenous bases—adenine (A), guanine (G), cytosine (C), and thymine (T). In any situation, if, in one strand of the DNA there is an A, there will always be a T opposite it in the other strand. If in one strand there is a C,
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there will always be a G opposite. The A, G, C, and T make up a 4-letter alphabet of instructions for building protein. Much like the order of letters in a word gives the word its meaning, the order of bases in a DNA sequence determines the protein structure. So, how does DNA serve as the instructions for making protein? The protein production process is a 2-step process. The first step is called transcription . A cell would not want its master plans of DNA going to the area where proteins are made—this could lead to permanent damage to the DNA. Rather, working copies of the DNA molecule in the forms of another molecule called
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03_DNA_and_Mutations (1) - BI SC 002 LECTURE 3DNA AND...

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