Question

Chapter 15: 1. What is the significance of the fact that many synonymous codons differ in the third nucleotide position? 2. Define the following terms as they apply to the genetic code: a. Reading frame b. Overlapping code C. Nonoverlapping code d. Initiation codon e. Termination codon f. Sense codon 8. Nonsense codon h. Universal code i. Nonuniversal code 3. What role do the initiation factors play in protein synthesis? 4. Compare and contrast the process of protein synthesis in bacterial and eukaryotic cells, giving similarities and differences in the process of translation in these two types of cells. 5. What are some types of posttranslational modifications of proteins? 6. Referring to the genetic code presented in Figure 15.10, give the amino acids specified by the following bacterial mRNA sequences: a. 5'-AUGUUUAAAUUUAAAUUUUGA-3 b. 5-AGGGAAAUCAGAUGUAUAUAUAUAUAUGA-3 c. 5-UUUGGAUUGAGUGAAACGAUGGAUGAAAG AUUUCUCGCUUGA-3 d. GUACUAAGGAGGUUGUAUGGGUUAGGGGACAUCAUUUUGA-3 The following amino acid sequence is found in a tripeptide: Met-Trp-His. Give all possible nucleotide sequences on the mRNA, on the template strand of DNA, and on the nontemplate strand of DNA that can encode this tripeptide. 7. The redundancy of the genetic code means that some amino acids are specified by more than one codon. For example, the amino acid leucine is specified by six different codons. Within a genome, synonymous codons are not present in equal numbers; some synonymous codons appear much more frequently than others, and the preferred codons differ among species. For example, in one species, the codon UUA might be used most often to specify leucine whereas in another species, the codon CUU might be used most often. Speculate on a reason for this bias in codon usage and why the preferred codons are not the same in all organisms. 8. In what ways are spliceosomes and ribosomes similar? In what ways are they different? Can you suggest some possible reasons for their similarities? 9. In humans, there may be three times as many proteins as genes. If each gene encodes a protein, how can there be more proteins than genes?

Answer 1

1. According to the Wobble hypothesis, we know that one tRNA can recognise and bind to more than one codon as a less prcise base pairing take place between the 3rd base of the codon and the 1st base of the anticodon of the tRNA.