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Biological Sciences
BISC 202
Timothy Swartz

9Proteins and Their Synthesis WORKING WITH THE FIGURESn in Figure 93a Where in the mRNA 1 The primary protein structure is shownear the 5 or 3 end would a mutation in R be encoded2Answer As mRNA is translated 5 to 3polypeptides are assembled fromamino end to carboxyl end the carboxyl end of the growing polypeptide chainis bound to the amino end of theincoming amino acidSince R is near the 2 amino end of the protein a mutation for R would be near the5 end of the 2 mRNA 2 In this chapter you were introduced to nonsense suppressor mutations in tRNA genes However suppressor mutations also occur in proteincoding genes Using the tertiary structure of thesubunit of hemoglobin shown in Figure 93c explain in structural terms how a mutation could cause the loss of globin protein function Now explain how a mutation at a second site in the same protein could suppress this mutation and lead to a normal or nearnormal proteinAnswer Proper folding is dependent on amino acid sequence and necessary for protein functionAn amino acid replacement that disrupted folding of thesubunit would cause a loss of function for the protein because the correct tertiary structure could not formIf a subsequent mutation in another region ofcomplemented the first mutation by at least partially reestablishing the normal folding pattern adequate tertiary structure could form and the first mutation would be suppressedFor example if bonding between points A and B resulted in proper folding a mutation that changed A would disrupt folding and cause loss of function A subsequent mutation that changed B so it could bond with mutant A would reestablish the normal folding and normal function 3 Using the quarternary structure of hemoglobin shown in Figure 93d explain in structural terms how a mutation in thesubunit protein could be suppressed by a mutation in thesubunit geneChapter Nine 283 Answer A mutation in thesubunit that prevented bonding ofandsubunits would prevent formation of the quaternary structure and block protein functionA complementary mutation in thesubunit that established the capacity to bond with the mutantsubunit would allow formation of a normal quaternary structure and effectively suppress the mutation in4 Transfer RNAs tRNAs are examples of RNA molecules that do not encode protein Based on Figures 96 and 98 what is the significance of the sequence of tRNA molecules What do you predict would be the impact on translation of a mutation in one of the bases of one of the stems in the tRNA structure On the mutant organismAnswer The sequence of the tRNA molecules determines the three dimensional structure producing the characteristic L shapeThe conservation of the L shape among the different tRNAs implies an important functionIf one of the bases in one of the stems were mutant the formation of the L shape would likely be impaired reducing or eliminating the capacity of the tRNA to act as an adapter molecule For example a mutant tRNA might not be able to bind with the synthetase molecule to become charged or it might not form a sufficiently stable complex with the ribosome during translation In the first case the mutant tRNA would not participate in translation at all in the second it could disrupt translation when it was insertedThe overall effect on the mutant organism would probably be minimal because there are several copies of tRNA genes and the normal function would be served by the remaining normal copies of the gene 5 Ribosomal RNAs rRNAs are another example of a functional RNA molecule Based on Figure 911 what do you think is the significance of the secondary structure of rRNAAnswer The amount of double stranded pairing in the rRNA indicates that a large part of the molecule will have a doublehelical structureThe double helical regions could potentially interact with ribosomal proteins via their major groovesrRNA could also interact with other RNAsThe size of the rRNA indicates a complex functionThe 16S rRNA contains the ShineDalgarno sequence which directs the 30S subunit to the start codon and it could also stabilize bonding between the 30S subunit and mRNA and between the 30S and 50S subunits 6 The components of prokaryotic and eukaryotic ribosomes are shown in Figure 910 Based on this figure do you think that the large prokaryotic ribosomal RNA 23S rRNA would be able to substitute for the eukaryotic 28S rRNA Justify your answer
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