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Gene Expression Objectives 1. Be able to describe the types of information coded in DNA. 2. Be able to identify the structural components of RNA. 3. Be able to distinguish RNA from DNA both structurally and functionally. 4. Be able to describe, in order, the steps involved in producing a functional mRNA molecule, starting with transcription of a protein-coding gene. 5. Be able to explain the roles of rRNA, mRNA, and tRNA in translation. 6. Be able to describe, in order, the steps of translation. 7. Be able to explain what the genetic code is and to explain the significance of key features of the genetic code. 8. Be able to use a codon chart to relate a DNA sequence to an amino acid sequence (and vice versa) and to decode mRNA. 9. Be able to distinguish between substitution, deletion, and insertion mutations and explain the consequences for the polypeptide specified by the mutated DNA. 10. Be able to relate gene expression to an organism’s phenotype. Terms N-base adenine (A) guanine (G) thymine (T) cytosine (C) uracil (U) pentose deoxyribose ribose phosphate nucleotide complementary bases DNA RNA mRNA tRNA rRNA ribosome transcription template gene promoter coding region terminator amino acid protein/polypeptide genetic code codon anticodon translation initiation elongation termination mutation substitution deletion insertion genotype phenotype enzyme biosynthetic pathway 1 Procedure 1. Basic Gene Expression 1. Obtain an envelope of molecules and a ribosome from the prep area. 2. The white molecule is a double stranded DNA molecule. Use the chain marked with an “X” as a template to construct an mRNA molecule with the yellow bases. (This simulation does not include introns or other control elements.) 3. Get a codon chart from the prep table or use the one in your textbook. Match the seven blue tRNA anticodons with the appropriate purple amino acids to make charged tRNA molecules. Remember that the codon chart gives codons, and that you are matching based on anticodons! 4. Align the small ribosomal subunit with the start codon. Match the appropriate charged tRNA with the anticodon that complements the start codon. Attach the large ribosomal subunit such that the charged tRNA is in the P site. 5. Find the charged tRNA whose anticodon complements the second codon. Align the tRNA with the second codon; the tRNA and its amino acid should be in the A site. 6. Gently detach the amino acid from the first tRNA and attach it to left tab of the amino acid in the A site. Remove the first tRNA from the ribosome. Shift the ribosome so that the remaining tRNA is now in the P site. 7. Align the third tRNA with the codon that is now in the A site. Gently detach the two amino acids from the tRNA in the P site and attach them to the left tab of the tRNA in the A site. Remove the now empty tRNA from the P site of the ribosome, and shift the ribosome to bring a new codon into the A site. 8. Repeat step 7 until all of the charged tRNA molecules have been matched to codons on the mRNA. At this point, a stop codon should be in the A site. Match the release factor with the stop codon to terminate translation. 9. Write your mRNA and amino acid sequences in the space below. 2 Review Questions 1. List three structural differences between DNA and RNA. 2. Distinguish between transcription and translation. 3. Describe how a functional mRNA molecule is produced. 4. List the three primary types of RNA molecules and give the function of each. 5. Describe the events of the three stages of translation: initiation, elongation, and termination. (space to continue on next page) 3 6. What is meant by the term genetic code? What are some of the features of the genetic code? 7. Given below is the sequence of a short wild-type protein as well as various mutant forms of the protein. a. Describe the general type of mutation in each case. b. Indicated the specific base-pair change that occurred for each mutation. Assume these mutations resulted from single base-pair changes. (There may be more than one correct answer). c. A fourth mutation is identified. The strain carrying this mutation produces abnormally low amounts of wild-type protein. In what part of the gene is this mutation likely to be located? Wild-type: Mutant#1: Mutant#2: Mutant#3: met met met met trp trp cys trp tyr arg gly ser pro thr ile his val arg val gly leu ser gln pro thr 4