Exam 3 Study Questions Essay

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1. What was Crick’s “adapter molecule” and how was it discovered? What is an “anticodon”?
The adapter molecule are tRNA's, as they act as a the interpreter during translation. It was discovered by attaching radioactive leucine molecles to tRNA, which were then added to in vitro translation system, and the fate was followed. It turned out that in the beginning of the experiment, all the leucine was attached to tRNA, but by the end, most was attached to the protein. An anticodon is a set of three ribonucleotides that forms base pairs with the mRNA codon.
p. 325

2. How many different codons are there (Chap 16)? How many different anticodons are there? [Hint: How many types of tRNA are there?] Explain what accounts for the difference.
There are 64 different codons (61 not including start and stop). There are 45 types of tRNA. The difference is alright as tRNAs can base-pair with more than one type of codon.
p.328

3. There must be at least _?_ different “aminoacyl tRNA snythetases.” What do these enzymes do? What would be the effect of a mutation in the gene for a particular aminoacyl tRNA synthetase that caused the enzyme to recognize lysine if the unmutated synthetase recognized the amino acid serine? There are 20 aminoacyl tRNA synthesases, one for each amino acid. These enzymes catalyze the addition of amino acids to tRNA's. Each synthetase has the ability to recognize any of the tRNAs that carry an anticodon for the amino acid in question. The amino acid would not correctly form.
p.327

4. What are the three phases of protein synthesis (translation)? Briefly describe each phase.
The three phases of protein synthesis are: (1) initiating translation: (a) the mRNA binds to a small ribosomal subunit, (b) The initiator aminoacyl tRNA bearing f-met binds to the start codon, and (c) the large ribosomal subunit binds, completing the complex; (2) elongation of the polypeptides during translation: after aan mRNA codon has been exposed to the A site, elongation proceeds when an aminoacyl tRNA binds to a codon in the A site, and when both the P and A sites are occupied, peptide-bond formation ocurs, and (3) termination of translation, or when the translating ribosome reaches one of the stop codons , and a protein called a release factor recognizes it and fills the A site.
p.330-331

5. How do mRNAs and ribosomes get together to start the process of translation in bacteria? In eukaryotes?
In bacteria, mRNAs and ribosomes get together to start translation by binding the mRNA to a small subunit in the ribosome, with the help of the complementary sequence in an RNA molecule in the ribosome. In eukaryotes, the initiating tRNA carries a normal methionine, whereas in bacteria its a form of methionine called N-formylmethionine.
p. 329-330

6. What is a “ribozyme” and how do we know that a ribozyme catalyzes peptide bond formation? A ribozyme is any RNA molecule that can act as a catalyst, or speed up the chemical reaction. We know that it catalyzes peptide bond formation becasue the models confirmed that the active site consists solely of ribosomal RNA.
p. 330

7. How does protein synthesis conclude when a ribosome reaches the end of the translated region? Is the process different in bacteria and eukaryotes? If so, how do they differ? Protein synthesis concludes when a ribosome reaches the end of the translated region by reaching a stop codon, and as release factors do not contain an amino acid, the proteins active site catalyzes the hydrolysis of the bond that links tRNA in the P site to the polypeptide chain, which frees the polypeptide. Termination is very similar in both bacteria and eukaryotes.
p.331

8. What processes take place after the translation of mRNA into a linear sequence of amino acids (i.e., into a polypeptide or protein) has been completed? In addition to protein folding and attachment of small chemical groups (such as sugar-based sorting signals), describe at least one other type
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