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?
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A three-dimensional reconstruction of the location of tRNAs in the three sites of the ribosome during translation.
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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.
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A review of mechanisms that prevent errors in translation.
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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.
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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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