Question

1. Error Rate in Protein Synthesis It is often said that protein complexes are made from subunits (that is,individually synthesized proteins) rather than as one long protein because it is more likely to give a correct final structure.

a. Assuming an error rate of 1/10,000, what fraction of bacterial ribosomes would be constituted correctly if the proteins were synthesized from one large protein versus assembled from individual proteins? For sake of calculation assume that the ribosome is composed of 50 proteins, each 200 amino acids in length, and that the subunits – correct and incorrect – areassembled with equal likelihood into the complete ribosome. Note that the probability of a correct sequence, Pc, equals the fraction correct for each operation, fc, raised to a power equal to the number of operations, n:Pc=(fc)n. For an error rate 1/10,000, fC=0.9999.

b. Is the assumption that correct and incorrect subunits assemble equally well likely to be true? Why or why not? How would a change in that assumption affect the calculation in Part a?

2. Much important confirmatory evidence on the genetic code has come from assessing changes in the amino acid sequence of mutant proteins after a single base has been changed in the gene that encodes the protein. Which of the following amino acid replacements would be consistent with the genetic code if the replacements were caused by a single base change? Which cannot be the result of a single-base mutation? Why? (a) Phe-Leu (e) Ile->Leu (b) Lys->Ala (f) His->Glu (c) Ala->Thr (g) Pro->Ser (d) Phe->Lys

Answer 1