Molecular Biology of the Cell Chapter 012 Test QUIZ BANK
Revised Questions & Answers 2023/2024
Half correct answers In eukaryotes, the endoplasmic reticulum can make up how much the total
membrane in a cell?
1. Lipid and protein biosynthesis
2. The intracellular storage of calcium correct answers Give two functions of the ER
Bound ribosomes correct answers Rough ER is studded with what?
Steroid/hormone production correct answers Ribosomes are absent on smooth ER which are specialized
for the production of what?
Interconnected correct answers An individual cell contains both smooth and rough ER membranes that
are often what?
The specialized function of that cell correct answers The percentage of the ER that is smooth versus
rough will vary depending on what?
Co-translationally correct answers The bulk of the proteins targeted are what targeted to the ER
explaining the association of ribosomes with ER?
Post-translationally correct answers A small subset of proteins are targeted to the ER what?
Post-translationally correct answers In mammalian cells, proteins less than 70 amino acids in size are
transported when?
Co-translationally correct answers In mammalian cells, proteins less than 70 amino acids in size are
transported post-translationally; the remaining are transported when?
A: Co-translational translocation
B: Post-translational translocation correct answers Identify A and B
1. Endoplasmic Reticulum
2. Mitochondrial-encoded proteins
3. Chloroplast-encoded proteins correct answers Give some examples of co-translational transport
pathways
1. Endoplasmic Reticulum
2. Nuclear-encoded mitochondrial proteins
3. Nuclear-encoded chloroplast proteins
4. Peroxisomes
5. Nucleus correct answers Give some examples of post-translational transport pathways
Isolated ER vesicles correct answers The idea of a targeting signal originated in the 1970's by scientists
working on protein transport into what?
,Signal hypothesis correct answers What is the idea that proteins destined for the ER had a stretch of
amino acids that were recognized by receptors found only in the ER?
Translocation correct answers The ER has a stretch of amino acids that were recognized by recepters
found only in the ER. This signal peptide was then removed after what was complete?
Other organelles correct answers The signal hypothesis has expanded to include transport into what?
Cytoplasmic ribosomes correct answers Proteins destined for the ER (or later organelles in the secretory
pathway) begin translation on what?
Signal recognition particle (SRP) correct answers When their amino terminal signal sequence emerges
from the ribosome it is recognized by a what
ER membrane correct answers SRP binding halts translation and targets the ribosome with its bound
mRNA and polypeptide chain to the what?
ER translocation channel correct answers The SRP engages with the what?
ER lumen correct answers SRP is released and ribosome mediated translation continues with the
elongating polypeptide entering the what?
The cytosol for another round of protein synthesis correct answers When translation is completed, the
ribosome is released from the ER into what? For what reason?
Amino acid sequence conservation correct answers The ER targeting signal has no what?
Charged and nonpolar amino acids correct answers ER targeting signals share a common distribution of
what? (2 answers)
The signal sequence binding pocket on SRP correct answers The centrally located hydrophobic stretch of
amino acids is recognized by what?
The ER signal peptidase correct answers Following signal sequence binding pocket on SRP is a peptidase
cleavage site that is recognized by what?
Both soluble and transmembrane proteins correct answers What two kinds of proteins are targeted to
the ER using similarly organized targeting signals?
1. No amino acid sequence conservation
2. 20-30 amino acids in length
3. At the amino terminus of ER targeted proteins
4. Shared pattern of charged and nonpolar amino acids (+ charged N terminus, central hydrophobic
domain, polar C-terminus)
5. Peptidase cleavage site follows the signal sequence correct answers Give 5 characteristics of the ER
targeting sequence:
,1. Soluble proteins (lumenal ER, Golgi, secreted proteins)
2. Transmembrane proteins (ER, Golgi, plasma membrane) correct answers What are the two types of
proteins targeted to the ER?
1. Provides time for targeting to the ER before translation is completed.
2. Prevents the release of potentially dangerous proteins into the cytosol. correct answers What are the
two purposes of translational pausing?
ER targeting correct answers Recognition of the amino terminal ER signal sequence soon after its
translation by cytosolic SRP is a key step for what?
The eukaryotic SRP correct answers What is a ribonucleoprotein composed of 6 protein subunits and 1
7S RNA molecule?
An elongated, flexible rod shaped structure correct answers What does the eukaryotic SRP look like?
A: Signal Recognition Particle (SRP)
B: Signal sequence bound by SRP
C: Small ribosomal subunit
D: Large ribosomal subunit
E: Signal sequence on nascent polypeptide chain correct answers Identify A - E
Wraps around the large ribosomal subunit to block the elongation factor binding site. correct answers
The SRP allows one end to bind to the emerging
polypeptide chain while the other end does what?
Translational arrest correct answers SRP binding causes what?
~1 (cytosolic SRP molecule for every ribosome) correct answers There are how many SRPs for every
ribosome?
A signal sequence correct answers SRP affinity for a ribosome increases 1,000 fold when what is present?
An ER targeting signal correct answers Predicted that SRP acts by cycling on and off cytosolic ribosomes
searching for what?
The integral membrane receptor SR, for SRP receptor. correct answers Translationally paused SRP-
ribosome-mRNA-nascent polypeptide complex is targeted to the ER membrane by its association with
what?
Free cytosolic ribosomes and membrane bound ribosomes correct answers Ribosome binding to the ER
sets up what two populations of ribosomes?
Protein they are synthesizing correct answers Free cytosolic ribosomes and membrane bound ribosomes
are structurally and functionally identical and only differ at any one time in the what?
A: Free ribosome cycle
B: Membrane-bound ribosome cycle
, C: SRP cycle correct answers Identify the three cycles:
The ER receptor SR correct answers The SRP bound ribosome is targeted to the cytoplasmic side of the
ER membrane through its association with what?
Sec61 channel correct answers The SRP bound ribosome associated with SR promotes ribosome binding
to the ER membrane where it binds to an available what?
A conformational change correct answers The transfer of the amino-terminal sequence from SRP to the
Sec61 channel triggers hydrolysis of the GTP bound to both the SRP and the SR receptor, inducing what?
Reinitiation of translation correct answers After the hydrolysis of the GTP bound to both the SRP and the
SR receptor, SRP is released allowing for the what?
Ribosome correct answers The ER translocon is an unopened position until the what is bound?
The ER lumen correct answers The protein conducting channel in the large ribosomal subunit is
positioned directly over the Sec61 channel such that the elongating polypeptide chain has direct access
to what?
1. Cytosolic SRP,
2. A membrane bound SRP receptor,
3. A translationally arrested ribosome,
4. The Sec61 translocon correct answers Co-translational transport into the ER requires what
components?
The ribosome correct answers For co-translational protein transport the driving force for protein
translocation is provided by what?
Protein translation correct answers Because of the close contact between the ribosome and the channel,
the elongating polypeptide has no where to go but through the ER channel. Therefore, for co-
translational transport the energy driving the translocation is what?
Protein folding and aggregation correct answers Proteins are translated on cytosolic ribosomes and are
bound by cytosolic chaperones to prevent what?
An unknown mechanism correct answers They are targeted to the Sec61 translocon by what?
1. 4 additional membrane proteins: Sec62, 62, 71 and 72
2. 1 additional ER lumenal protein: Bip/Kar2. correct answers Polypeptide translocation through the
Sec61 channel requires what components?
BiP correct answers What is an ATP dependent chaperone which grabs the polypeptide as it emerges
into the ER lumen?
The unidirectional transport of polypeptides into the ER correct answers Rounds of ATP dependent
binding and release by BiP mediates what?
Revised Questions & Answers 2023/2024
Half correct answers In eukaryotes, the endoplasmic reticulum can make up how much the total
membrane in a cell?
1. Lipid and protein biosynthesis
2. The intracellular storage of calcium correct answers Give two functions of the ER
Bound ribosomes correct answers Rough ER is studded with what?
Steroid/hormone production correct answers Ribosomes are absent on smooth ER which are specialized
for the production of what?
Interconnected correct answers An individual cell contains both smooth and rough ER membranes that
are often what?
The specialized function of that cell correct answers The percentage of the ER that is smooth versus
rough will vary depending on what?
Co-translationally correct answers The bulk of the proteins targeted are what targeted to the ER
explaining the association of ribosomes with ER?
Post-translationally correct answers A small subset of proteins are targeted to the ER what?
Post-translationally correct answers In mammalian cells, proteins less than 70 amino acids in size are
transported when?
Co-translationally correct answers In mammalian cells, proteins less than 70 amino acids in size are
transported post-translationally; the remaining are transported when?
A: Co-translational translocation
B: Post-translational translocation correct answers Identify A and B
1. Endoplasmic Reticulum
2. Mitochondrial-encoded proteins
3. Chloroplast-encoded proteins correct answers Give some examples of co-translational transport
pathways
1. Endoplasmic Reticulum
2. Nuclear-encoded mitochondrial proteins
3. Nuclear-encoded chloroplast proteins
4. Peroxisomes
5. Nucleus correct answers Give some examples of post-translational transport pathways
Isolated ER vesicles correct answers The idea of a targeting signal originated in the 1970's by scientists
working on protein transport into what?
,Signal hypothesis correct answers What is the idea that proteins destined for the ER had a stretch of
amino acids that were recognized by receptors found only in the ER?
Translocation correct answers The ER has a stretch of amino acids that were recognized by recepters
found only in the ER. This signal peptide was then removed after what was complete?
Other organelles correct answers The signal hypothesis has expanded to include transport into what?
Cytoplasmic ribosomes correct answers Proteins destined for the ER (or later organelles in the secretory
pathway) begin translation on what?
Signal recognition particle (SRP) correct answers When their amino terminal signal sequence emerges
from the ribosome it is recognized by a what
ER membrane correct answers SRP binding halts translation and targets the ribosome with its bound
mRNA and polypeptide chain to the what?
ER translocation channel correct answers The SRP engages with the what?
ER lumen correct answers SRP is released and ribosome mediated translation continues with the
elongating polypeptide entering the what?
The cytosol for another round of protein synthesis correct answers When translation is completed, the
ribosome is released from the ER into what? For what reason?
Amino acid sequence conservation correct answers The ER targeting signal has no what?
Charged and nonpolar amino acids correct answers ER targeting signals share a common distribution of
what? (2 answers)
The signal sequence binding pocket on SRP correct answers The centrally located hydrophobic stretch of
amino acids is recognized by what?
The ER signal peptidase correct answers Following signal sequence binding pocket on SRP is a peptidase
cleavage site that is recognized by what?
Both soluble and transmembrane proteins correct answers What two kinds of proteins are targeted to
the ER using similarly organized targeting signals?
1. No amino acid sequence conservation
2. 20-30 amino acids in length
3. At the amino terminus of ER targeted proteins
4. Shared pattern of charged and nonpolar amino acids (+ charged N terminus, central hydrophobic
domain, polar C-terminus)
5. Peptidase cleavage site follows the signal sequence correct answers Give 5 characteristics of the ER
targeting sequence:
,1. Soluble proteins (lumenal ER, Golgi, secreted proteins)
2. Transmembrane proteins (ER, Golgi, plasma membrane) correct answers What are the two types of
proteins targeted to the ER?
1. Provides time for targeting to the ER before translation is completed.
2. Prevents the release of potentially dangerous proteins into the cytosol. correct answers What are the
two purposes of translational pausing?
ER targeting correct answers Recognition of the amino terminal ER signal sequence soon after its
translation by cytosolic SRP is a key step for what?
The eukaryotic SRP correct answers What is a ribonucleoprotein composed of 6 protein subunits and 1
7S RNA molecule?
An elongated, flexible rod shaped structure correct answers What does the eukaryotic SRP look like?
A: Signal Recognition Particle (SRP)
B: Signal sequence bound by SRP
C: Small ribosomal subunit
D: Large ribosomal subunit
E: Signal sequence on nascent polypeptide chain correct answers Identify A - E
Wraps around the large ribosomal subunit to block the elongation factor binding site. correct answers
The SRP allows one end to bind to the emerging
polypeptide chain while the other end does what?
Translational arrest correct answers SRP binding causes what?
~1 (cytosolic SRP molecule for every ribosome) correct answers There are how many SRPs for every
ribosome?
A signal sequence correct answers SRP affinity for a ribosome increases 1,000 fold when what is present?
An ER targeting signal correct answers Predicted that SRP acts by cycling on and off cytosolic ribosomes
searching for what?
The integral membrane receptor SR, for SRP receptor. correct answers Translationally paused SRP-
ribosome-mRNA-nascent polypeptide complex is targeted to the ER membrane by its association with
what?
Free cytosolic ribosomes and membrane bound ribosomes correct answers Ribosome binding to the ER
sets up what two populations of ribosomes?
Protein they are synthesizing correct answers Free cytosolic ribosomes and membrane bound ribosomes
are structurally and functionally identical and only differ at any one time in the what?
A: Free ribosome cycle
B: Membrane-bound ribosome cycle
, C: SRP cycle correct answers Identify the three cycles:
The ER receptor SR correct answers The SRP bound ribosome is targeted to the cytoplasmic side of the
ER membrane through its association with what?
Sec61 channel correct answers The SRP bound ribosome associated with SR promotes ribosome binding
to the ER membrane where it binds to an available what?
A conformational change correct answers The transfer of the amino-terminal sequence from SRP to the
Sec61 channel triggers hydrolysis of the GTP bound to both the SRP and the SR receptor, inducing what?
Reinitiation of translation correct answers After the hydrolysis of the GTP bound to both the SRP and the
SR receptor, SRP is released allowing for the what?
Ribosome correct answers The ER translocon is an unopened position until the what is bound?
The ER lumen correct answers The protein conducting channel in the large ribosomal subunit is
positioned directly over the Sec61 channel such that the elongating polypeptide chain has direct access
to what?
1. Cytosolic SRP,
2. A membrane bound SRP receptor,
3. A translationally arrested ribosome,
4. The Sec61 translocon correct answers Co-translational transport into the ER requires what
components?
The ribosome correct answers For co-translational protein transport the driving force for protein
translocation is provided by what?
Protein translation correct answers Because of the close contact between the ribosome and the channel,
the elongating polypeptide has no where to go but through the ER channel. Therefore, for co-
translational transport the energy driving the translocation is what?
Protein folding and aggregation correct answers Proteins are translated on cytosolic ribosomes and are
bound by cytosolic chaperones to prevent what?
An unknown mechanism correct answers They are targeted to the Sec61 translocon by what?
1. 4 additional membrane proteins: Sec62, 62, 71 and 72
2. 1 additional ER lumenal protein: Bip/Kar2. correct answers Polypeptide translocation through the
Sec61 channel requires what components?
BiP correct answers What is an ATP dependent chaperone which grabs the polypeptide as it emerges
into the ER lumen?
The unidirectional transport of polypeptides into the ER correct answers Rounds of ATP dependent
binding and release by BiP mediates what?
