Biology Raven/Johnson 8e
Chapter 16 Answers
1. Control of gene expression can occur at which of the following steps? a. Splicing of pre-mRNA into mature mRNA b. Initiation of translation c. Initiation of transcription d. All of the above The correct answer is d A. Answer a is incorrect. Because alternative splicing can produce different gene products, it can be considered a possible mechanism of control. It is not the only possibility. The correct answer is d B. Answer b is incorrect. Although not the most common form of control, the initiation of translation is controlled. It is not the only possibility. The correct answer is d C. Answer c is incorrect. Control of the initiation of transcription is a common form of control in all cell types. It is not the only possibility. The correct answer is dAll of the above D. Answer d is correct. All of the options listed are potential modes of control of gene expression. 2. Regulatory proteins interact with DNA by a. unwinding the helix and changing the pattern of base-pairing b. the sugarphosphate backbone of the double helix c. unwinding the helix and disrupting-base pairing d. binding to the major groove of the double helix interacting with base-pairs The correct answer is d A. Answer a is incorrect. Regulatory proteins can bind without unwinding the DNA strands, and they never change the pattern of base-pairing. The correct answer is d B. Answer b is incorrect. The sugarphosphate backbone does not participate in the binding of regulator proteins. This region of the DNA is always the same, independent of the sequence of nucleotides. The correct answer is d C. Answer c is incorrect. Regulatory proteins bind without unwinding the helix. The correct answer is dbinding to the major groove of the double helix interacting with basepairs D. Answer d is correct. The major groove accommodates a number of regulatory proteins. The base pairs are accessible within the major groove. 3. The two protein subunits of a leucine zipper are held together a. by -sheet domains b. by the interactions of hydrophobic amino acids c. by two -helical domains separated by a turn
Raven/Johnson Biology 8e d. the interaction of atoms of zinc
Chapter 16 Answers
The correct answer is b A. Answer a is incorrect. Beta-sheet domains are associated with zinc-finger motifs. The correct answer is bby the interactions of hydrophobic amino acids B. Answer b is correct. Hydrophobic leucine amino acids interact to zip the two proteins together. The correct answer is b C. Answer c is incorrect. Alpha-helical domains are common to many DNA-binding motifs. This is a description of the helix-turn-helix motif. The correct answer is b D. Answer d is incorrect. The zinc-finger motif is associated with zinc atoms. In this motif the Zn atoms do not hold protein subunits together. 4. Which domain of a helix-turn-helix protein is directly involved with binding a specific DNA sequence? a. The recognition helix b. The homeodomain c. The zinc finger d. The leucine zipper The correct answer is aThe recognition helix A. Answer a is correct. The recognition helix fits into the major groove allowing, it to bind to a specific domain. The correct answer is a B. Answer b is incorrect. The homeodomain is a special class of helix-turn-helix proteins that function in development. The correct answer is a C. Answer c is incorrect. The zinc finger is its own motif and not part of a helix-turn-helix motif. The correct answer is a D. Answer d is incorrect. The leucine zipper is it own motif and not part of a helix-turn-helix protein. 5. Negative control of transcription in a prokaryotic cell involves _________ molecules that alter the conformation of ________ proteins that bind to DNA and prevent transcription. a. operator; repressor b. activator; RNA polymerase c. activator; operator d. effector; repressors The correct answer is d A. Answer a is incorrect. An operator is a region of DNA, not a molecule.
Raven/Johnson Biology 8e The correct answer is d B. Answer b is incorrect. An activator enhances transcription.
Chapter 16 Answers
The correct answer is d C. Answer c is incorrect. Activators enhance transcription, and operators are regions of DNA, not protein. The correct answer is deffector; repressors D. Answer d is correct. Effectors are molecules that bind to repressors (or activators) and control the ability of these regulatory proteins to bind to DNA. 6. What is an operon? a. A region of DNA involved in regulation of transcription b. A cluster of genes that are expressed as a single unit c. A DNA-binding motif d. A regulator protein that enhances transcription The correct answer is b A. Answer a is incorrect. A regulatory region of DNA is an operator. The correct answer is bA cluster of genes that are expressed as a single unit B. Answer b is correct. An operon is a collection of related genes. The correct answer is b C. Answer c is incorrect. An operon is a collection of related genes. The correct answer is b D. Answer d is incorrect. An operon is a collection of related genes. 7. What effect would the presence of lactose have on a lac operon? a. The repressor would bind to the operator site of the operon b. Lactose will bind to the operator site of the operon c. The lac operon would be transcribed d. It would have no effect The correct answer is c A. Answer a is incorrect. Lactose triggers a reaction that leads to the removal of the repressor from the operator. The correct answer is c B. Answer b is incorrect. Lactose is converted to allolactose, which will bind to the repressor molecule. The correct answer is cThe lac operon would be transcribed C. Answer c is correct. The presence of lactose triggers the release of the repressor from the operator, allowing the operon to be transcribed. The correct answer is c
Raven/Johnson Biology 8e
Chapter 16 Answers
D. Answer d is incorrect. The lac operon allows for prokaryotic cells to respond to varying levels of lactose in the environment. 8. How does the presence of glucose influence the regulation of the lac operon? a. Glucose reduces the amount of cAMP, which CAP requires for action. b. Glucose prevents the transport of lactose into the cell. c. Glucose binds to the repressor protein. d. Both a and b. The correct answer is d A. Answer a is incorrect. The presence of glucose reduces the amount of cAMP in the cell, which alters the activity of CAP. This is probably not the primary mechanism of glucose repression, but it does occur. The correct answer is d B. Answer b is incorrect. By preventing lactose transport, glucose prevents induction of the lac operon. This is probably the primary mechanism of glucose repression, but it is not the only effect of glucose. The correct answer is d C. Answer c is incorrect. Glucose does not interact with the repressor protein. The correct answer is dBoth a and b. D. Answer d is correct. Both a and b are effects of glucose on the lac operon. 9. What effect does the amino acid tryptophan have on the trp operon? a. Tryptophan binds to the repressor, resulting in transcription of the trp operon. b. Tryptophan binds to the repressor and prevents transcription. c. Tryptophan increases cAMP, activating catabolite activator protein (CAP). d. Tryptophan interacts with repressor, leading to derepression of the trp operon. The correct answer is b A. Answer a is incorrect. Tryptophan binding to the repressor triggers a conformation change in the repressor, allowing it to bind to the operator and prevent transcription. The correct answer is bTryptophan binds to the repressor and prevents transcription. B. Answer b is correct. Tryptophan alters the conformation of the repressor allowing it to bind to the operator, preventing RNA polymerase binding. The correct answer is b C. Answer c is incorrect. Tryptophan influences the activity of the repressor directly. The correct answer is b D. Answer d is incorrect. If tryptophan is present, then there is no need to express the proteins in the trp operon. 10. How do specific transcription factors differ from general transcription factors? Specific a. transcription factors increase the rate of transcription. b. Specific transcription factors bind to the TATA box sequence.
Raven/Johnson Biology 8e
Chapter 16 Answers
c. Specific transcription factors form an initiation complex. d. Specific transcription factors bind to RNA polymerase. The correct answer is aSpecific transcription factors increase the rate of transcription. A. Answer a is correct. Special transcription factors increase transcription over basal levels in a tissue- and time-dependent fashion. The correct answer is a B. Answer b is incorrect. TFIID is the general transcription factor that binds to the TATA box sequence. The correct answer is a C. Answer c is incorrect. The collection of general transcription factors is responsible for the formation of the initiation complex. The correct answer is a D. Answer d is incorrect. Specific transcription factors function by binding to DNA. 11. DNA methylation a. inhibits transcription by blocking base-pairing of cytosine and guanine b. inhibits transcription by blocking base-pairing of uracil and adenine c. prevents transcription by blocking the TATA box sequence d. is correlated with genes that are turned off The correct answer is d A. Answer a is incorrect. Methylation does not interfere with base-pairing. The correct answer is d B. Answer b is incorrect. Methylation does not interfere with base-pairing. The correct answer is d C. Answer c is incorrect. Methylation does not affect the TATA box sequence. The correct answer is dis correlated with genes that are turned off D. Answer d is correct. Proteins bind to methylated cytosine and block the ability of activators to bind to DNA. 12. What is the function of small interfering RNAs? a. They bind to mRNA and block translation. b. They block transcription of complementary mRNAs. c. They trigger the destruction of complementary mRNAs. d. They compete with transfer RNAs during translation. The correct answer is c A. Answer a is incorrect. RNA interference inhibits gene expression by eliminating mRNAs. The correct answer is c B. Answer b is incorrect. RNA interference regulates gene expression by destroying mRNAs before they are translated.
Raven/Johnson Biology 8e
Chapter 16 Answers
The correct answer is cThey trigger the destruction of complementary mRNAs. C. Answer c is correct. RNA interference occurs after mRNAs have been transcribed by destroying the message before translation occurs. The correct answer is c D. Answer d is incorrect. RNA interference prevents translation by destroying the mRNAs. 13. RNA editing is a consequence of a. base-pair substitutions caused by a mutation in the DNA b. splicing of a pre-mRNA c. methylation of the mRNA d. modifications of a base within the mRNA The correct answer is d A. Answer a is incorrect. RNA editing influences mRNA, not DNA. The correct answer is d B. Answer b is incorrect. Alternative splicing links together different combinations of exons. RNA editing changes the base-pairing of the mRNA. The correct answer is d C. Answer c is incorrect. Methylation is a modification associated with DNA, not RNA. The correct answer is dmodifications of a base within the mRNA D. Answer d is correct. Deamination of RNA nucleotides alter their base-pairing properties, altering the codon:anticodon binding that occurs during translation. 14. What is ubiquitin? a. A type of protease b. A posttranslational modification that targets proteins for destruction c. A protein involved in the transport of mRNAs out of the nucleus d. A posttranslational modification that degrades mRNAs The correct answer is b A. Answer a is incorrect. Ubiquitin targets proteins for destruction by proteases. The correct answer is bA posttranslational modification that targets proteins for destruction B. Answer b is correct. Ubiquinated proteins are targeted to the proteosome where they are degraded. The correct answer is b C. Answer c is incorrect. mRNA transport out of the nucleus involves the nuclear pores. The correct answer is b D. Answer d is incorrect. mRNA degradation involves specific sequences that are targeted by endonucleases. 15. Which of the following is NOT a true statement about proteosomes?
Raven/Johnson Biology 8e a. b. c. d. They are membrane-bounded organelles. They break proteins into amino acids. They do not degrade ubiquitin. They remove ubiquitin from proteins.
Chapter 16 Answers
The correct answer is aThey are membrane-bounded organelles. A. Answer a is correct. The proteosome is not membrane-bounded, although it is considered to be a cellular compartment. The correct answer is a B. Answer b is incorrect. Proteosomes function to degrade proteinsbreaking them into their amino acid subunits. The correct answer is a C. Answer c is incorrect. Ubiquitin is not destroyed by the proteosome. The correct answer is a D. Answer d is incorrect. Ubiquitin is recycled by the cell. Challenge Questions 1. Examples of positive and negative control of transcription can be found in the regulation of expression of the prokaryotic operons lac and trp. Use these two operon systems to describe the difference between positive and negative regulation. AnswerNegative control of transcription occurs when the ability to initiate transcription is reduced. Positive control occurs when the ability to initiate transcription is enhanced. The lac operon is regulated by the presence or absence of lactose. The proteins encoded within the operon are specific to the catabolism (breakdown) of lactose. For this reason, operon expression is only required when there is lactose in the environment. Allolactose is formed when lactose is present in the cell. The allolactose binds to a repressor protein, altering its conformation and allowing RNA polymerase to bind. In addition to the role of lactose, there is also a role for the activator protein CAP in regulation of lac. When cAMP levels are high then CAP can bind to DNA and make it easier for RNA polymerase to bind to the promoter. The lac operon is an example of both positive and negative control. The trp operon encodes protein manufacture of tryptophan in a cell. This operon must be expressed when cellular levels of tryptophan are low. Conversely, when tryptophan is available in the cell, there is no need to transcribe the operon. The tryptophan repressor must bind tryptophan before it can take on the right shape to bind to the operator. This is an example of negative control. 2. How is an operator different from a promoter in the regulation of prokaryotic gene expression? AnswerOperators are DNA sequences that bind repressor proteins. The presence of the repressor physically blocks the ability of RNA polymerase to bind to the promoter region. The promoter is the site where transcription begins. 3. What forms of eukaryotic control of gene expression are unique to eukaryotes? Could
Raven/Johnson Biology 8e
Chapter 16 Answers
prokaryotes use the mechanisms, or are they due to differences in these cell types? AnswerForms that control gene expression that are unique to eukaryotes include alternative splicing, control of chromatin structure, control of transport of mRNA from the nucleus to the cytoplasm, control of translation by small RNAs, and control of protein levels by ubiquitindirected destruction. Of these, most are obviously part of the unique features of eukaryotic cells. The only mechanisms that could work in prokaryotes would be translational control by small RNAs and controlled destruction of proteins. 4. The number and type of proteins found in a cell can be influenced by genetic mutation and regulation of gene expression. Discuss how these two processes differ. AnswerMutation is a permanent change in the DNA. Regulation is a short-term change controlled by the cell. Like mutations, regulation can alter the number of proteins in a cell, change the size of a protein, or eliminate the protein altogether. The key difference is that gene regulation can be reversed in response to changes in the cells environment. Mutations do not allow for this kind of rapid response.