Challenge Questions

28. In what ways are spliceosomes and ribosomes similar? In what ways are they different? Can you suggest some possible reasons for their similarities.

29. Several experiments were conducted to obtain information about how the eukaryotic ribosome recognizes the AUG start codon. In one experiment, the gene that codes for methionine initiator tRNA (tRNAiMet) was located and changed. The nucleotides that specify the anticodon on tRNAiMet were mutated so that the anticodon in the tRNA was 5' - CCA - 3' instead of 5' - CAU - 3'. When this mutated gene was placed into a eukaryotic cell, protein synthesis took place, but the proteins produced were abnormal. Some of the proteins produced contained extra amino acids, and others contained fewer amino acids.

(a) What do these results indicate about how the ribosome recognizes the starting point for translation in eukaryotic cells? Explain your reasoning.

(b) If the same experiment had been conducted on bacterial cells, what results would you expect?


Agrawal, R. K., P. Penczek, R. A. Grassucci, Y. Li, A. Leith, K. H. Nierhaus, and J. Frank. 1996. Direct visualization of A-, P-, and E-site transfer RNAs in the Escherichia coli ribosome. Science 271:1000 -1002.

A three-dimensional reconstruction of the location of tRNAs in the three sites of the ribosome during translation.

Beadle, G. W., and E. L. Tatum. 1942. Genetic control of biochemical reactions in Neurospora. Proceedings of the National Academy of Sciences 27:499 - 506. Seminal paper in which Beadle and Tatum outline their basic methodology for isolating auxotrophic mutants.

Cech, T. R. 2000. The ribosome is a ribozyme. Science 289:878-879. A brief commentary on research indicating that RNA in the ribosome is responsible for catalyzing peptide-bond formation in protein synthesis.

Dever, T. E. 1999. Translation initiation: adept at adapting. Trends in Biochemical Science. 24:398-403. A discussion of factors that play a role in eukaryotic translation initiation.

Fox, T. D. 1987. Natural variation in the genetic code. Annual Review of Genetics 21:67 - 91.

A review of exceptions to the universal genetic code. Gualerzi, C. O., and C. L. Pon. 1990. Initiation of mRNA translation in prokaryotes. Biochemistry 29:5881 - 5889. A good, although fairly technical, review of the process of translational initiation in prokaryotic cells.

Ibba, M., and D. Söll. 1999. Quality control mechanisms during translation. Science 286:1893 -1897.

A review of mechanisms that prevent errors in translation.

Iborra, F. J., D. A. Jackson, and P. R. Cook. 2001. Coupled transcription and translation within nuclei of mammalian cells. Science 293:1139-1142.

A report of experiments demonstrating that some translation takes place within the eukaryotic nucleus. Khorana, H. G., H. Buchi, H. Ghosh, N. Gupta, T. M. Jacob, H. Kossel, R. Morgan, S. A. Narang, E. Ohtsuka, and R. D. Wells.

1966. Polynucleotide synthesis and the genetic code. Cold Spring Harbor Symposium on Quantitative Biology 31:39 -49. The use of repeating RNA polymers in solving the code. Nirenberg, M., and P. Leder. 1964. RNA code words and protein synthesis I: the effect of trinucleotides upon the binding of sRNA to ribosomes. Science 145:1399-1407. A description of the tRNA binding technique for solving the code. Nirenberg, M. W., O. W. Jones, P. Leder, B. F. C. Clark, W. S. Sly, and S. Pestka. 1963. On the coding of genetic information. Cold Spring Harbor Symposium on Quantitative Biology 28:549 - 557. The use of random copolymers in solving the code.

Nakamura, Y., K. Ito, and L. A. Isaksson. 1996. Emerging understanding of translational termination. Cell 87:147-150. A review of translational termination. Noller, H. F. 1991. Ribosomal RNA and translation. Annual Review of Biochemistry 60:191 -227. A good review of RNA- RNA interactions in translation. Preiss, T., and M. W. Hentze. 1998. Dual function of the messenger RNA cap structure in poly(A)-tail-promoted translation in yeast. Nature 392:516 -519. Research article describing evidence that the poly(A) tail plays a role in translational initiation.

Sachs, A. B., P. Sarnow, and M. W. Hentz. 1997. Starting at the beginning, middle, and end: translation initiation in eukaryotes. Cell 89:831 -838.

A good review of different ways in which translation is initiated in eukaryotic mRNAs. Wickner, S., M. R. Maurizi, and S. Gottesman. 1999. Postranslational quality control: folding, refolding, and degrading proteins. Science 286:1888-1893. A review of posttranslation modifications of proteins. Yusupov, M. M., G. Z. Yusupova, A. Baucom, K. Lieberman, T. N. Earnest, J. H. D. Cate, and H. F. Noller. 2001. Crystal structure of the ribosome at 5.5 A resolution. Science 292:883 -896. A report of the structure of the complete ribosome with mRNA and bound tRNAs at high resolution.

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