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Bio301 Biochemistry Midterm Examination II, December 5, 2005 Question 1 It is plausible that RNA was utilized to store genetic information early in the history of life. However, in modern organisms, with the exception of some viruses DNA performes this function. a) What is the selective advantage of DNA over RNA as the genetic material? b) What is the selective advantage of replacing uracil with thymine during the transformation from RNA to DNA as the genetic material? Question 2 Explain how a bacteria could express a new set of genes when subjected to a rapid increase in temperature so that it could protect itself under extreme conditions. Question 3 There are 64 different codons but only approximately thirty different transferRNA molecules in E. coli. Discuss the advantages or disadvantages of this fact for the fitness of the bacteria. Question 4 True or false? Enzyme catalysis of a chemical reaction a) decreases G’ so that the reaction can proceed spontaneously. b) does not change Go’, but it changes the ratio of products to reactants. c) decreases the entropy of the reaction. d) decreases the energy of the transition state. e) increases the forward and reverse reaction rates equally. Question 5 Rearrange the Michaelis-Menten equation to give Vo as a function of Vo/[S].([Eadie-Hofstee Plot), so that you can convert it into an equation of a line. Sketch an Eadie-Hofstee Plot (a hypothetical one) in the absence of an inhibitor. On the same plot, show how the same experiment would look like in the presence of a competitive inhibitor. How would you calculate the Vmax and the KM for each case on this plot. Question 6 What would be the value of Keq’ for a reaction that correspond to a free energy change of Go’= -10 kcal/mol. Assume that this reaction takes place at 25 oC. (R=1.98x10-3 kcal. mol-1 oK- 1) Question 7 The following initial velocitiy data were obtained for an enzyme: [S] (mM) V (M/s) x 107 0.20 0.96 0.25 1.12 0.334 1.35 0.50 1.66 1.0 2.22 2.0 2.63 Each assay at the indicated substrate concentration was initiated by adding enzyme to a final concentration of 0.01 nM. Derive KM, Vmax, kcat and the specificity constant kcat /KM. Question 8 What are the two new amino acids that are recently added to the list of 20? How they are specified in the genetic code and selected to ribosomes?
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