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C H A P T E R 8 R E V I E W Summary of Expectations Language of Biology Briefly explain each of the following points. Write a sentence including each of the following words or terms. Use any six terms in a concept map to show your understanding of how they are related. • The four different nucleotides found in DNA make up the genetic “words” that code for all 20 different amino acids. (8.1) • The two-step process of gene expression has both costs and benefits for cells. (8.1, 8.4) • The genetic code is redundant but not ambiguous. (8.1) • Many transcripts can be simultaneously produced from a single gene. (8.2) • Several types of processing are required before a pre-mRNA transcript can meet the needs of a eukaryotic cell. (8.2, 8.4) • Four different types of RNA are involved in the main steps of protein synthesis. (8.2, 8.3) • In prokaryotes, translation can begin before transcription is done. (8.2) • The process of translation involves enzymes both inside and outside the ribosomes. (8.3) • An active ribosome complex has at least four RNA binding sites. (8.3) • An E. coli cell will only produce lactosemetabolizing enzymes if lactose is present in the cell’s environment. (8.4) • Crick’s “central dogma” does not fully capture the process of gene expression. (8.1, 8.4) • • • • • • • • • • • • • • • • • • • • codon triplet hypothesis transcription translation reading frame initiation elongation messenger RNA (mRNA) termination processing sense strand anti-sense strand promoter sequence RNA polymerase precursor mRNA (pre-mRNA) 5′ cap poly-A tail spliceosome small nuclear RNA (snRNA) transfer RNA (tRNA) • anticodon • amino acid attachment site • amino-acyl tRNA (aa-tRNA) • activating enzyme • ribosome • ribosomal RNA (rRNA) • translocation • release factor • polyribosome • operon • operator • repressor • negative gene regulation • inducer • activator • positive gene regulation • co-repressor • transcription factors UNDE RSTAN DIN G CON CEPTS 1. A given organism has many different tissues, yet its cells all carry the same genetic information. Explain how this is possible. 2. Identify the significance of the discovery that the specific arrangement of amino acids is directly related to the structure of specific proteins. 3. Describe how Crick tested the triplet hypothesis. 4. What is the codon concept? 5. What was the term Crick coined to describe the process by which he believed genes were expressed? Why did he choose this term? 6. How is it possible for information on DNA that is confined to the nucleus of a eukaryotic cell to be expressed as protein products outside the nucleus? 7. How did Nirenberg and Matthaei begin working out the genetic code? 8. Identify the amino acids coded for by the following codons: AGC, GUU, UAU, AUG. 9. Name three characteristics of the genetic code and explain why they are important. 10. One species of bacteria manufactures 37 different tRNA molecules. Explain how these bacteria can still match anticodons to all of the 64 different potential mRNA codons. 11. RNA transcription in eukaryotes consists of four steps. Identify and describe each. 12. Define the term “sense strand.” What is its counterpart called, and why? 13. Explain how activating enzymes work. 14. What causes RNA polymerase to cease transcription? 15. Draw a diagram that illustrates the main steps in the elongation cycle of translation. What enzymes play a role in maintaining this cycle? 16. Identify two different mechanisms that help a single gene produce very high levels of a polypeptide product in a short time. Chapter 8 Protein Synthesis • MHR 281 17. Our bodies often benefit from antibiotics when fighting bacterial infections. Explain what this might have to do with ribosomes. 18. What functions are performed (a) by the polypeptide component of a ribosome? (b) by the rRNA component of a ribosome? 19. What is an operator, and how does it relate to operons? In what kind of cells are operons found? 20. Prokaryotic promoter sequences are sometimes described as being asymmetrical. (a) To what property of the promoter sequences does this term refer? (b) What is the significance of this feature? 21. List the main control steps in the protein synthesis pathway of prokaryotes, and briefly describe why each step is important. 22. Name the five control points available to eukaryotic cells and briefly describe each one. 23. How does the regulation of gene expression enable a cell to conserve energy? Give an example of a case in which another advantage might outweigh the need to conserve energy. 24. Explain what will happen to a pre-mRNA strand that lacks a 3′ poly-A tail or 5′ cap. How will this affect translation? 25. Choose the correct completion for the following statement: In a eukaryotic cell, most of the genome (a) remains unexpressed most of the time. (b) is expressed most of the time. (c) is repressed by a repressor most of the time. (d) is expressed by a promoter most of the time. IN QU IRY 26. Suppose you are provided with a sample of DNA. After you have analyzed its base composition, you divide the sample into three separate reaction mixtures in order to transcribe mRNA. Once the transcription is complete, you analyze the base composition of the mRNA from each reaction mixture. You obtain the following results. Based on these findings, answer the questions below. A G C T U DNA strand I 19.1 26.0 31.0 23.9 0 DNA strand II 24.2 30.8 25.7 19.3 0 mRNA type A (reaction mixture A) 19.0 25.9 30.8 0 24.3 mRNA type B (reaction mixture B) 23.2 27.6 22.9 0 26.3 mRNA type C (reaction mixture C) 36.0 19.1 0 21.9 23.0 (a) Which strand of the DNA is the sense strand that serves as the template for the synthesis of mRNA? Explain your reasoning. (b) How can you explain the composition of mRNA type B? (c) What substance(s) might have been added to reaction mixture B in order to obtain mRNA type B? 282 MHR • Unit 3 Molecular Genetics (d) What might have happened in reaction mixture C in order to produce mRNA type C? 27. A researcher studying protein synthesis in E. coli uses an electron microscope to record the image below. (a) What structure is shown in this image? What is its significance? (b) The same researcher puts a new sample of E. coli into a medium containing a substance that destroys activating enzymes. She then records another image using the electron microscope. Assuming that this substance has no other effect on the cell, how would you expect the image to differ? Explain. 28. In a fictional species of bacterium, the caf operon contains genes that code for an enzyme that breaks down caffeine into its component molecules. The cell uses one of these molecules as a source of energy. (a) Would you expect this operon to be repressible or inducible? Explain. (b) Draw a diagram that illustrates how the presence of caffeine in the medium in which the cell is kept regulates the expression of the caf genes. 29. A particular yeast cell reacts to an increase in glucose in its medium by producing more of enzyme X. (a) Design an experiment to determine whether the main control point for the synthesis of enzyme X is found during transcription or translation. (b) Would the same experiment work if you were studying a bacterial cell? Explain. COMMU N ICATIN G 30. “Proteins, not DNA, are the key to cell specialization.” In a report, explain what is meant by this statement. 31. Write a short essay that compares and contrasts the roles of DNA polymerase and RNA polymerase. 32. Create a table that could be used by others to distinguish between the various types of RNA and their roles in transcription and translation. 33. Assume for a moment that you want to author a paper on transfer RNA. With pen and paper, illustrate the basic structure of a tRNA molecule and label it accordingly. Write a caption to go with it that explains its functions. 34. With an opponent, debate ways in which the mechanisms involved in the control of the lac operon can be compared to traffic signals, and ways in which this analogy is not helpful. 35. When Crick first described the “central dogma”, he had little experimental evidence to support his theory. In groups of three or four, brainstorm ways in which more recent evidence both supports and challenges the central dogma. 36. Form a debate team to discuss whether prokaryotes or eukaryotes, based on their methods of gene expression, have the evolutionary advantage. 37. Gene regulation is an important part of the growth and development of living organisms. Explain this statement in a short essay. M A KIN G CON N ECTION S 38. Researchers have the ability to add a number of A nucleotides to the end of a strand of mRNA. What could be some practical applications of this procedure? 39. A molecular biologist creates a form of RNA polymerase that has the same proofreading ability as DNA polymerase. Explain what some of the advantages and disadvantages of this form of RNA polymerase could be: (a) for researchers in a laboratory setting (b) for living organisms 40. In small groups, interview people in your community who work in science, medicine, technology, or the environment to discover their thoughts on dogma (either current or historical) in their field. Do they believe this dogma has been largely helpful or misleading? Based on their answers, evaluate what role dogma should play in scientific discovery. 41. How could you use the information from this chapter to find ways to fight bacterial infections in humans? Write a short report (up to one page) identifying some processes that might be significant in the development of treatments. Could the same processes be applied to fight infections by eukaryotic cells such as yeast? Why or why not? 42. The study of the structure of genomes, including projects such as the sequencing of the human genome, is often referred to as “genomics.” Many researchers claim that genomics is not nearly as significant as “proteonomics,” the study of protein structure and function. Based on the information in this chapter, which field would you argue holds the greatest promise for advances in medicine and in understanding human development? Chapter 8 Protein Synthesis • MHR 283