Lecture 2 Answers - Lecture 2: Study Questions and Answers...

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Lecture 2 : Study Questions and Answers 1. What is the difference between somatic mutations and germ-line mutations? Germ-line mutations are changes in the DNA of germ (reproductive) cells and may be passed to offspring. Somatic mutations are changes in the DNA of an organism’s somatic tissue cells and cannot be passed to offspring. 2. What is the difference between a transition and a transversion? Which type of base substitution is usually more common? Transition mutations are base substitutions in which one purine (A or G) is changed to the other purine, or a pyrimidine (T or C) is changed to the other pyrimidine Transversions are base substitutions in which a purine is changed to a pyrimidine or vice versa. Although transversions would seem to be statistically favored because there are eight possible transversions and only four possible transitions, about twice as many transition mutations are actually observed in the human genome. 3. What is the difference between a missense mutation and a nonsense mutation? A silent mutation and a neutral mutation? A base substitution that changes the sequence and the meaning of a mRNA codon, resulting in a different amino acid being inserted into a protein, is called a missense mutation. Nonsense mutations occur when a mutation replaces a sense codon with a stop (or nonsense) codon. A nucleotide substitution that changes the sequence of a mRNA codon, but not the meaning is called a silent mutation. In neutral mutations, the sequence and the meaning of a mRNA codon are changed. However, the amino acid substitution has little or no effect on protein function. 4. A codon that specifies the amino acid Gly undergoes a single-base substitution to become a nonsense mutation. In accord with the genetic code given in Figure 15.10 (Pierce Extra), is this mutation a transition or a transversion? At which position of the codon does the mutation occur? By examining the four codons that encode for Gly, GGU, GGC, GGA, and GGG, and the three nonsense codons, UGA, UAA, and UAG, we can determine that only one of the Gly codons, GGA, could be mutated to a nonsense codon by the single substitution of a U for a G at the first position: GGA to UGA. Because uracil is a pyrimidine and guanine is a purine, the mutation is a transversion. 5. If a single transition occurs in a codon that specifies Leu, what amino acids could be specified by the mutated sequence?
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6. Hemoglobin is a complex protein that contains four polypeptide chains. The normal hemoglobin found in adults—called adult hemoglobin—consists of two α and two β polypeptide chains, which are encoded by different loci. Sickle-cell hemoglobin, which causes sickle-cell anemia, arises from a mutation in the β chain of adult hemoglobin. Adult hemoglobin and sickle-cell hemoglobin differ in a single amino acid: the sixth amino acid from one end in adult hemoglobin is glutamic acid, whereas sickle-cell hemoglobin has valine at this position. After consulting the genetic code provided in Figure 15.10, indicate the type and location of the mutation
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This note was uploaded on 05/07/2011 for the course BIOLOGY 352 taught by Professor Townsend during the Spring '08 term at San Diego State.

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Lecture 2 Answers - Lecture 2: Study Questions and Answers...

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