Lehninger Principles of Biochemistry: Chapter 28 Flashcards


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1

“Housekeeping genes” in bacteria are commonly expressed constitutively, but not all of these genes are expressed at the same level (the same number of molecules per cell). The primary mechanism responsible for variations in the level of constitutive enzymes from different genes is that:

A) all constitutive enzymes are synthesized at the same rate, but are not degraded equally.

B) their promoters have different affinities for RNA polymerase holoenzyme.

C) some constitutively expressed genes are more inducible than others.

D) some constitutively expressed genes are more repressible than others.

E) the same number of mRNA copies are made from each gene, but are translated at different rates.

B) their promoters have different affinities for RNA polymerase holoenzyme.

2

Which of the following statements correctly describes promoters in E. coli?

A) A promoter may be present on either side of a gene or in the middle of it.

B) All promoters have the same sequence that is recognized by RNA polymerase holoenzyme.

C) Every promoter has a different sequence, with little or no resemblance to other promoters.

D) Many promoters are similar and resemble a consensus sequence, which has the highest affinity for RNA polymerase holoenzyme.

E) Promoters are not essential for gene transcription but can increase its rate by two- to three-fold.

D) Many promoters are similar and resemble a consensus sequence, which has the highest affinity for RNA polymerase holoenzyme.

3

The operator region normally can be bound by:

A) attenuator.

B) inducer.

C) mRNA.

D) repressor.

E) suppressor tRNA.

D) repressor.

4

Small signal molecules that regulate transcription are not known to:

A) cause activator proteins to bind DNA sites.

B) cause repressor proteins to bind DNA sites.

C) directly bind to DNA sites.

D) prevent activator proteins from binding to DNA sites.

E) release repressor proteins from DNA sites.

C) directly bind to DNA sites.

5
card image

The diagram below represents a hypothetical operon in the bacterium E. coli. The operon consists of two structural genes (A and B), which code for the enzymes A-ase and B-ase, respectively, and also includes P (promoter) and O (operator) regions as shown.

When a certain compound (X) is added to the growth medium of E. coli, the separate enzymes A-ase and B-ase are both synthesized at a 50-fold higher rate than in the absence of X. (X has a molecular weight of about 200.) Which of the following statements is true of the operon described above?

A) All four genes (A, B, O, and P) will be transcribed into an mRNA that will then be translated into four different proteins.

B) The 3' end of the mRNA from the operon will correspond to the left end of the operon as drawn.

C) The 5' end of the messenger from this operon will correspond to the right end of the operon as drawn.

D) The repressor for this operon binds just to the right of A.

E) When RNA polymerase makes mRNA from this operon, it begins RNA synthesis just to the left of gene A.

E) When RNA polymerase makes mRNA from this operon, it begins RNA synthesis just to the left of gene A.

6
card image

The diagram below represents a hypothetical operon in the bacterium E. coli. The operon consists of two structural genes (A and B) that code for the enzymes A-ase and B-ase, respectively, and also includes P (promoter) and O (operator) regions as shown.

When a certain compound (X) is added to the growth medium of E. coli, the separate enzymes A-ase and B-ase are both synthesized at a 50-fold higher rate than in the absence of X (which has a molecular weight of about 200). Which one of the following statements is true of such an operon?

A) Adding X to the growth medium causes a repressor protein to be released from the O region.

B) Adding X to the growth medium causes a repressor protein to bind tightly to the O region.

C) Synthesis of the mRNA from this operon is not changed by the addition of compound X.

D) The mRNA copied from this operon will be covalently linked to a short piece of DNA at the 5' end.

E) Two mRNA molecules are made from this operon, one from gene A the other from gene B.

A) Adding X to the growth medium causes a repressor protein to be released from the O region.

7

Transcription of the lactose operon in E. coli is stimulated by:

A) a mutation in the repressor gene that strengthens the affinity of the repressor for the operator.

B) a mutation in the repressor gene that weakens the affinity of the repressor for the operator.

C) a mutation in the repressor gene that weakens the affinity of the repressor for the inducer.

D) binding of the repressor to the operator.

E) the presence of glucose in the growth medium.

B) a mutation in the repressor gene that weakens the affinity of the repressor for the operator.

8

Protein amino acid side chains can hydrogen bond in the major groove of DNA, and discriminate between each of the four possible base pairs. In which one of the following groups of amino acids can all three members potentially be used in such DNA-protein recognition?

A) Ala,Asn,Glu

B) Arg, Gln, Leu

C) Asn, Gln, Trp

D) Asn, Glu, Lys

E) Glu, Lys, Pro

D) Asn, Glu, Lys

9

The DNA binding motif for many prokaryotic regulatory proteins, such as the lac repressor, is:

A) helix-turn-helix.

B) homeobox.

C) homeodomain.

D) leucine zipper.

E) zinc finger.

A) helix-turn-helix.

10

Protein structural motifs often have general functions in common. Which one of the following motifs is known to be involved in protein dimer formation, but not in direct protein-DNA interactions?

A) β-barrel

B) helix-turn-helix

C) homeodomain

D) leucine zipper

E) zinc finger

D) leucine zipper

11

Which of the following statements about regulation of the lac operon is true?

A) Glucose in the growth medium decreases the inducibility by lactose.

B) Glucose in the growth medium does not affect the inducibility by lactose.

C) Glucose in the growth medium increases the inducibility by lactose.

D) Its expression is regulated mainly at the level of translation.

E) The lac operon is fully induced whenever lactose is present.

A) Glucose in the growth medium decreases the inducibility by lactose.

12

The binding of CRP (cAMP receptor protein of E. coli) to DNA in the lac operon:

A) assists RNA polymerase binding to the lac promoter.

B) is inhibited by a high level of cAMP.

C) occurs in the lac repressor region.

D) occurs only when glucose is present in the growth medium.

E) prevents repressor from binding to the lac operator.

A) assists RNA polymerase binding to the lac promoter.

13

Consider the lac operon of E. coli. When there is neither glucose nor lactose in the growth medium:

A) CRP protein binds to the lac operator.

B) CRP protein displaces the Lac repressor from the lac promoter.

C) repressor is bound to the lac operator.

D) RNA polymerase binds lac promoter and transcribes the lac operon.

E) the operon is fully induced.

C) repressor is bound to the lac operator.

14

A regulon is a(n):

A) group of related triplet codons.

B) network of operons with a common regulator.

C) operon that is subject to regulation.

D) protein that regulates gene expression.

E) ribosomal protein that regulates translation.

B) network of operons with a common regulator.

15

The tryptophan operon of E. coli is repressed by tryptophan added to the growth medium. The tryptophan repressor probably:

A) binds to RNA polymerase when tryptophan is present.

B) binds to the trp operator in the absence of tryptophan.

C) binds to the trp operator in the presence of tryptophan.

D) is a DNA sequence.

E) is an attenuator.

C) binds to the trp operator in the presence of tryptophan.

16

Which one of the following statements about the transcription attenuation mechanism is true?

A) In some operons (e.g., the his operon), attenuation may be the only regulatory mechanism.

B) Sequences of the trp operon leader RNA resemble an operator.

C) The leader peptide acts by a mechanism that is similar to that of a repressor protein.

D) The leader peptide gene of the trp operon includes no Trp codons.

E) The leader peptide is an enzyme that catalyzes transcription attenuation.

A) In some operons (e.g., the his operon), attenuation may be the only regulatory mechanism.

17

Which of the following statements is true of the attenuation mechanism used to regulate the tryptophan biosynthetic operon in E. coli?

A) Attenuation is the only mechanism used to regulate the trp operon.

B) One of the enzymes in the Trp biosynthetic pathway binds to the mRNA and blocks translation when tryptophan levels are high.

C) The leader peptide plays a direct role in causing RNA polymerase to attenuate transcription.

D) Trp codons in the leader peptide gene allow the system to respond to tryptophan levels in the cell.

E) When tryptophan levels are low, the trp operon transcripts are attenuated (halted) before the operon’s structural genes are transcribed.

D) Trp codons in the leader peptide gene allow the system to respond to tryptophan levels in the cell.

18

Attenuation in the trp operon of E. coli:

A) can adjust transcription of the structural genes upwards when tryptophan is present.

B) can fine-tune the transcription of the operon in response to small changes in Trp availability.

C) is a mechanism for inhibiting translation of existing (complete) trp mRNAs.

D) results from the binding of the Trp repressor to the operator.

E) results from the presence of short leader peptides at the 5' end of each structural gene.

B) can fine-tune the transcription of the operon in response to small changes in Trp availability.

19

RecA protein provides the functional link between DNA damage and the SOS response by displacing the LexA protein from its operator sites on the SOS genes. RecA does so by:

A) associating with polymerase holoenzyme to help it remove LexA from operator.

B) bending LexA operator DNA to force dissociation of LexA repressor.

C) binding to LexA protein to weaken directly its affinity for operator sites.

D) causing self-cleavage of LexA, thus inactivating its binding to operator.

E) competitively binding to LexA operators and serving as an activator.

D) causing self-cleavage of LexA, thus inactivating its binding to operator.

20

An example of coordinate control is the down-regulation of ribosomal RNA synthesis in response to amino acid starvation, which will cause synthesis of ribosomal proteins to be limited. What is the correct order of the following events that participate in the signaling process?

1. Binding of stringent factor to the ribosome.
2. Formation of the unusual nucleotide ppGpp.
3. Formation of the unusual nucleotide pppGpp. 4. Uncharged tRNA binds in the ribosomal A-site.

A) 1,4,2,3

B) 1,4,3,2

C) 4,1,2,3

D) 4,1,3,2

E) 4,2,1,3

D) 4,1,3,2

21

Which one of the following statements about eukaryotic gene regulation is correct?

A) Large polycistronic transcripts are common.

B) Most regulation is positive, involving activators rather than repressors.

C) Transcription and translation are mechanistically coupled.

D) Transcription does not involve promoters.

E) Transcription occurs without major changes in chromosomal organization.

B) Most regulation is positive, involving activators rather than repressors.

22

Which of the following is a DNA sequence?

A) Coactivator

B) Corepressor

C) Enhancer

D) Inducer

E) Transactivator

C) Enhancer

23

Which one of the following types of eukaryotic regulatory proteins interact with enhancers?

A) Basal transcription factors

B) Coactivators

C) Repressors

D) TATA-binding proteins

E) Transactivators

E) Transactivators

24

Which one of the following is not involved in steroid hormone action?

A) Cell surface receptors

B) Hormone-receptor complexes

C) Specific DNA sequences

D) Transcription activation and repression

E) Zinc fingers

A) Cell surface receptors

25

Which one of the following classes is expressed in the unfertilized egg and is involved in directing the spatial organization of the Drosophila embryo early in development?

A) Gap genes

B) Homeotic genes

C) Maternal genes

D) Segment polarity genes

E) Segmentation genes

C) Maternal genes

26

Which one of the following classes of genes is involved in specifying the localization of organs in the Drosophila embryo?

A) Gap genes

B) Homeotic genes

C) Maternal genes

D) Segment polarity genes

E) Segmentation genes

B) Homeotic genes

27

In the development of the fly Drosophila, homeotic genes:

A) are transcribed during egg production; their mRNAs lie dormant in the egg until it is fertilized.

B) determine the number of body segments that will form.

C) are expressed late and determine the detailed structure of each body segment.

D) generally have no introns.

E) are not translated into proteins.

C) are expressed late and determine the detailed structure of each body segment.