Biology of the Cell Exam 2
The amino terminal presequence on proteins imported into the mitochondrial inner membrane is a. cleaved by mitochondrial matrix processing peptidase (MPP). b. transported back to the intermembrane space by Hsp70 chaperones. c. cleaved and translocate laterally into the inner membrane. d. cleaved during transit from Tom to Tim23.
Correct Answer: a
The nuclear basket protein complex is a component of the nuclear pore complex found a. inside the nucleus. b. buried in the nuclear envelope. c. on the cytoplasmic face of the pore. d. lining the central channel of the pore.
Correct Answer: a
The process by which cells degrade their own components by enclosing them in a cytosolic membrane is a. autophagy. b. ER-mediated endocytosis. c. phagocytosis. d. pinocytosis.
Correct Answer: a
The major model of vesicle fusion holds that actual fusion of a vesicle with its target membrane is driven by the interaction of pairs of proteins called vesicle and target a. SNAREs. b. SNAPs. c. COPs. d. tethers
Correct Answer: a Answer A Feedback: According to the hypothesis, SNAREs on vesicles bind to corresponding SNAREs on the target membrane and initiate fusion of the two
The major site at which membrane lipids are synthesized is the a. cytosolic side of the ER membrane. b. cytosol. c. lumenal side of the ER membrane. d. cytosolic side of the Golgi membrane.
Correct Answer: a Answer A Feedback: Because lipids are hydrophobic in nature, they are synthesized in association with already existing membranes, and the synthetic enzymes are located in the cytosol.
A deficiency in cardiolipin would lead to a. increased proton flow across the inner mitochondrial membrane. b. decreased proton flow across the inner mitochondrial membrane. c. increased proton flow across the outer mitochondrial membrane. d. decreased proton flow across the outer mitochondrial membrane.
Correct Answer: a Answer A Feedback: Cardiolipin is located in the inner mitochondrial membrane and restricts proton flow across the membrane, thus helping maintain the energy gradient. If it were deficient, the proton flow would increase.
Cofilin plays a role in the a. disassembly of actin filaments. b. stimulation of actin filament formation. c. nucleation of microfilaments. d. disassembly of microtubules.
Correct Answer: a Answer A Feedback: Cofilin contributes to the disassembly of actin filaments by severing them and by enhancing the rate of dissociation of actin monomers from the minus end.
Calreticulin assists in folding glycoproteins that contain one _______ residue. a. glucose b. mannose c. N-acetylglucosamine d. sialic acid
Correct Answer: a Answer A Feedback: Glucose is present on N-linked oligosaccharide sidechains of glycoproteins in the ER, but not elsewhere.
Like myosins, kinesins and dyneins are both families of proteins. Which of the following statements is true of all kinesins and dyneins? a. They are microtubule-dependent motors. b. They are minus-end-directed motors. c. The motor activity of the proteins resides in their light chains. d. They are plus-end-directed motors.
Correct Answer: a Answer A Feedback: Kinesins and dyneins are all microtubule-dependent motors.
What triggers lipid-containing etioplasts to undergo complete development into chloroplasts? a. Light b. Hormonal signaling c. Darkness d. Pigment
Correct Answer: a Answer A Feedback: Light is what triggers the conversion.
For many yeast proteins, posttranslational translocation of proteins targeting the ER is a. a process affecting many newly synthesized proteins. b. much less common than in mammalian cells. c. rare. d. impossible, because all translocation into the ER is driven by the process of protein synthesis.
Correct Answer: a Answer A Feedback: Many newly synthesized proteins in yeast are posttranslationally translocated into the ER.
Where do phospholipids in mitochondrial membranes such as phosphatidylcholine and phosphatidylethanolamine originate? a. In the ER b. In the intermembrane space c. On the cytosolic side of the outer membrane d. On the lumenal side of the inner membrane
Correct Answer: a Answer A Feedback: Phosphatidylcholine and phosphatidylethanolamine are synthesized in the ER membrane and then transported to the mitochondria via phospholipid transfer proteins. The mitochondria can then synthesize phosphatidylserine and cardiolipin using the ER-derived phospholipids as raw materials.
Which of the following processes does not require the assistance of chaperones? a. Protein transport into the nucleus b. Protein transport into the ER c. Folding during protein synthesis d. Assembly of proteins consisting of multiple polypeptide chains
Correct Answer: a Answer A Feedback: Proteins do not need to be in an unfolded state to be transported into the nucleus, and hence do not require chaperones.
The directionality of nuclear protein import is determined by a. an unequal distribution of Ran/GTP. b. importin. c. an ion gradient. d. the nuclear lamina.
Correct Answer: a Answer A Feedback: Ran/GTP is more abundant in the nucleus than in the cytoplasm, and this factor is thought to give directionality to protein import.
Initial steps in the functional assembly of snRNPs occur in the a. cytoplasm. b. nucleolus. c. nucleoplasm. d. PML body.
Correct Answer: a Answer A Feedback: The RNA associates with proteins to form functional snRNPs in the cytoplasm.
The outer nuclear membrane is contiguous with the a. endoplasmic reticulum. b. nuclear lamina. c. Golgi apparatus. d. plasma membrane.
Correct Answer: a Answer A Feedback: The endoplasmic reticulum is a large membrane-bounded structure that is contiguous with the outer membrane of the nucleus.
The nucleolus is the site where a. ribosomal RNA is transcribed and ribosomes are partially assembled. b. DNA replication occurs. c. proteins recently imported from the cytoplasm are deposited. d. translation occurs.
Correct Answer: a Answer A Feedback: The nucleolus is a non-membrane-bounded structure where the many copies of the three largest RNA subunits are transcribed. These are then processed and complexed with ribosomal proteins and the smallest ribosomal RNA to form the ribosomal subunits.
Which statement correctly describes the difference between the proton gradient across the inner mitochondrial membrane and the proton gradient across the thylakoid membrane of chloroplasts? a. The first creates an electrochemical gradient, while the second is largely a chemical gradient. b. There is no proton gradient across the thylakoid membrane; magnesium ions create the gradient instead. c. It is generated by an electron transport system in the first case but not the second. d. It drives the synthesis of ATP in the first case but not in the second.
Correct Answer: a Answer A Feedback: The thylakoid membrane is permeable to chloride and magnesium ions, so electrical neutrality is maintained. Despite the smaller electrical component, the same amount of energy is stored in the gradient, due to the higher pH differential across it: 3.5 pH units, as opposed to 1.4 pH units for the inner mitochondrial membrane.
What is the average number of replication forks per DNA replication cluster in a mammalian cell nucleus? a. Approximately 20 b. Approximately 200 c. Approximately 4,000 d. Approximately 8,000
Correct Answer: a Answer A Feedback: There are several hundred discrete clusters and about 4,000 origins of replication, each of which produces two replication forks.
The function of aminoacyl tRNA synthetases is to a. covalently attach amino acids to their corresponding tRNA molecules. b. synthesize tRNA molecules. c. catalyze the formation of the aminoacyl-ATP intermediate during amino acid attachment to tRNAs. d. catalyze the formation of a peptide bond between amino acids.
Correct Answer: a Answer A Feedback: These enzymes attach amino acids to the 3′ ends of tRNA molecules, the first step in the inclusion of amino acids in a polypeptide chain.
Signal sequences are sequences of hydrophobic amino acids that target membrane translocation. Signal sequences are found a. at the amino terminus of the secreted protein. b. at the carboxy terminus of the secreted protein. c. on the cytoplasmic side of the membrane channel proteins. d. on the amino terminus of the 28S of certain ribosomal complexes associated with secretion.
Correct Answer: a Answer A Feedback: These sequences are usually about 20 amino acids long
Which of the following statements about translational initiation is false? a. In prokaryotes, ribosomes often bind the mRNA and can scan 5′ or 3′ until recognizing a Shine-Dalgarno sequence. b. Viral mRNAs contain internal ribosome entry sites that allow ribosomes to bind to an internal site of the mRNA. c. Initiation codons in prokaryotic cells are preceded by Shine-Dalgarno sequences. d. 5′ 7-methylguanosine caps serve as the point of recognition and binding for ribosomes in eukaryotic cells.
Correct Answer: a Answer A Feedback: This is a false statement.
Which of the following statements about translational regulation of ferritin is false? a. The iron response element is a unique sequence of amino acids near the amino terminus of the growing polypeptide. b. In the absence of iron, an iron regulatory protein binds the IRE, preventing translation. c. In the presence of iron, the ferritin protein is translated. d. The iron binding protein must bind the mRNA within about 70 bases of the 5′ mRNA cap.
Correct Answer: a Answer A Feedback: This is a false statement. The IRE is a unique sequence of bases located near the 5′ cap of the mRNA.
Which of the following is not involved in the specific targeting of proteins to lysosomes? a. N-linked glycosylation in the ER b. Mannose phosphorylation c. The mannose-6-phosphate receptor d. Signal patches
Correct Answer: a Answer A Feedback: This modification can be made on all classes of proteins (lysosomal, ER, plasma membrane, etc.) passing through the ER. However, it does not specifically target a protein to the lysosomes.
_______ different translocon systems are used for protein import from the chloroplast stroma into the thylakoid lumen or membrane. a. Two b. Three c. Five d. Ten
Correct Answer: b
The ribosomes of the rough endoplasmic reticulum are targeted to the cytoplasmic side of the membrane via a. a signal sequence within the 28S ribosomal RNA. b. a sequence within the protein being synthesized. c. the S6 ribosomal protein. d. the cap sequence at the 5′ end of the mRNA being translated.
Correct Answer: b Answer B Feedback: A signal sequence, typically at the amino terminus of the protein being synthesized, directs the ribosome to the rough ER.
Ferritin expression is stimulated by iron because iron a. stimulates a protein to bind the ferritin mRNA and inhibit its degradation. b. stimulates the dissociation of a translational inhibitor from the ferritin mRNA. c. binding stabilizes the ferritin protein. d. stimulates the extension of poly-A tails of ferritin mRNAs.
Correct Answer: b Answer B Feedback: A translational inhibitor bound to the 5′ end of the ferritin mRNA is released upon binding iron.
Actin may be cross-linked into either parallel or _______ bundles. a. antiparallel b. contractile c. networks of actin filament d. orthogonal filament
Correct Answer: b Answer B Feedback: Actin filaments in parallel bundles are cross-linked by fimbrin and in contractile bundles by α-actinin
Which of the following is a possible intermediate during pre-rRNA processing? a. An RNA molecule containing 18S + 5.8S rRNAs b. An RNA molecule containing 5.8S + 28S rRNAs c. An RNA molecule containing 5S + 28S rRNAs d. An RNA molecule containing 18S + 28S rRNAs
Correct Answer: b Answer B Feedback: After cleavage of some noncoding 5′ sequence from the pre-rRNA, the next cleavage step occurs between the 18S and 5.8S rRNAs, yielding a molecule containing the 5.8S and 28S rRNAs as an intermediate
Which of the following is not true of the assembly of actin filaments? a. It begins with the formation of an aggregate of three actin monomers. b. It requires ATP. c. Polymerization occurs from both the plus and minus ends. d. Polymerization is faster from the plus end than from the minus end.
Correct Answer: b Answer B Feedback: Although polymerization occurs more readily in the presence of ATP, it is not absolutely required.
Ribosomes contain one copy each of 5.8S, 18S, and 28S rRNA. The major mechanism ensuring that each is produced in equal molar amounts is a. the selective degradation of excess transcripts of the respective rRNAs. b. the grouping of the DNA sequences encoding each rRNA into a single rRNA gene. c. the outcome of coevolution of the separate genes. d. the existence of common sequence features for transcription initiation for the genes encoding each rRNA.
Correct Answer: b Answer B Feedback: Because the sequences encoding the three rRNAs are grouped together into a single gene, all three are then transcribed as a single pre-rRNA that is then processed to produce each in equal amounts.
Clathrin-coated vesicles are involved in a. the retrieval of ER resident proteins from the cis Golgi or the ER-Golgi intermediate compartment. b. the uptake of extracellular molecules by endocytosis and the transport of molecules from the trans-Golgi network to the lysosomes. c. transport from the ER to the Golgi. d. the recycling of Golgi resident proteins during cisternal maturation.
Correct Answer: b Answer B Feedback: Clathrin-coated vesicles mediate both of these processes.
N-linked glycosylation at an Asn-X-Ser/Thr consensus sequence adds _______ sugar(s) in a single step to the protein. a. 1 b. 14 c. 30 d. 100
Correct Answer: b Answer B Feedback: Fourteen sugars are added in one step from a preformed precursor oligosaccharide linked to a dolichol pyrophosphate
The location of Ran GAP in association with the cytoplasmic filaments of the nuclear pore results in the conversion of the nucleotide bound to Ran in the nucleus to a. ADP. b. GDP. c. ATP. d. GTP.
Correct Answer: b Answer B Feedback: GDP is bound to Ran in the cytoplasm. Ran GAP activates Ran to cleave GTP to GDP as export into the cytoplasm occurs.
Kinesin and dynein are a. intermediate filament proteins. b. microtubule motor proteins. c. proteins within centrosomes that mediate nucleation of microtubules. d. microfilament motor proteins.
Correct Answer: b Answer B Feedback: Kinesin moves vesicles and organelles toward the plus end of microtubules, and dynein moves them toward the minus end
Which of the following are not subcompartments of the nucleus? a. Clustered DNA replication sites b. Lipid droplets c. PML bodies d. Speckles enriched in RNA splicing components
Correct Answer: b Answer B Feedback: Lipid droplets are found in the cytoplasm, not in the nucleus.
Mitochondria differ from other organelles such as lysosomes and the Golgi apparatus in that they a. contain enzymes specific to their function. b. contain their own genomes. c. do not contain proteins that are imported from the cytosol. d. are not membrane-bounded.
Correct Answer: b Answer B Feedback: Mitochondria contain their own genes, including genes for proteins involved in the electron transport system, as well as some rRNA and tRNA genes.
The proteins shaped like a "double chamber" that are involved in protein folding are called a. Hsp70 proteins. b. protein disulfide isomerases. c. chaperonin proteins. d. peptidyl prolyl isomerases.
Correct Answer: c Answer C Feedback: These proteins form a "double-chamber structure" in which proteins are sequestered from the cytosol until they are properly folded
The human mitochondrial genome encodes only 22 tRNAs. This limited array of tRNAs can read the 64 possible triplet codons through extreme wobble in base pairing at the third codon position and the use of a(n) _______ genetic code. a. chloroplast-mitochondrial-specific b. nonuniversal mitochondrial c. peroxisomal-mitochondrial-specific d. universal
Correct Answer: b Answer B Feedback: Mitochondria use a variation on the universal genetic code that permits all 64 codon combinations to be read.
An important step in the import of the transcription factor NF-κB into the nucleus is regulated by a a. nuclease. b. protease. c. phospholipase. d. plasma membrane receptor tyrosine kinase that directly phosphorylates IκB.
Correct Answer: b Answer B Feedback: NF-κB is normally associated with IκB in the cytoplasm. Upon appropriate signaling, IκB is phosphorylated and cleaved by a protease. The nuclear localization signal of NF-κB is exposed
Which of the following statements about the attachment of amino acids to tRNAs is false? a. The amino acid is first joined to AMP, forming an aminoacyl AMP intermediate. b. Two molecules of ATP are required for the process, one at each step. c. Aminoacyl tRNA synthetases catalyze the reaction. d. The amino acid is transferred to the 3′ end of the tRNA.
Correct Answer: b Answer B Feedback: Only one ATP is required, and it is used in the formation of the aminoacyl AMP intermediate.
Peroxisomes are involved in all of the following reactions except a. the biosynthesis of the amino acid lysine. b. the biosynthesis of the enzyme catalase, which breaks down hydrogen peroxide. c. the biosynthesis of lipids. d. oxidative reactions leading to the production of hydrogen peroxide.
Correct Answer: b Answer B Feedback: Peroxisomes have no genome. Therefore, they cannot synthesize a polypeptide.
Protein phosphatases a. catalyze the addition of phosphate residues to proteins. b. catalyze the removal of phosphate residues from proteins. c. catalyze the addition of glycosylphosphatidylinositol to proteins. d. are proteins that specifically bind phosphorylated proteins.
Correct Answer: b Answer B Feedback: Phosphatases can remove phosphate residues from serine, threonine, and tyrosine residues.
Which of the following plastids is a precursor to all other plastids? a. Chromoplast b. Proplastid c. Etioplast d. Amyloplast
Correct Answer: b Answer B Feedback: Proplastids are precursors to all plastids
Which of the following is not a contiguous membrane domain? a. Rough ER b. Secretory granules c. Smooth ER d. ER exit sites (ERES)
Correct Answer: b Answer B Feedback: Secretory vesicles are not part of the ER. They form by budding from the Golgi apparatus
The sequence Lys-Asp-Glu-Leu (KDEL) serves to retain proteins in the ER by a. preventing their packaging into vesicles destined for the Golgi. b. binding to receptors within the membranes of the ERGIC and Golgi, which retain them or return them to the ER. c. binding the SRP receptor in the ER membrane. d. associating with the lipids in the ER membrane.
Correct Answer: b Answer B Feedback: The KDEL sequence does not prevent the transport of proteins to the Golgi, but it does serve to retrieve them.
In terms of its role in the generation of metabolic energy, the inner membrane in mitochondria is equivalent to which of the following in chloroplasts? a. The inner membrane b. The thylakoid membrane c. The outer membrane d. The stroma
Correct Answer: b Answer B Feedback: The electron transport system is located in the thylakoid membrane of chloroplasts and thus is analogous to the inner membrane of mitochondria.
A proteasome is a a. vesicle containing proteolytic enzymes. b. multisubunit protease complex that degrades proteins marked for destruction. c. complex of a proteolytic enzyme and the protein that is being degraded. d. precursor to lysosomes.
Correct Answer: b Answer B Feedback: The proteasome degrades ubiquitinated proteins.
In cell fractionation experiments in which subcellular organelles are separated on the basis of their density, the smooth endoplasmic reticulum is _______ the rough endoplasmic reticulum. a. denser than b. lighter than c. the same density as d. the same density as lipid droplets and much denser than
Correct Answer: b Answer B Feedback: The smooth endoplasmic reticulum is lighter. The rough endoplasmic reticulum is denser because of the attached ribosomes.
Proteins are translocated into the ER lumen a. by flippases. b. through an aqueous channel created by the Sec61 protein. c. by being pushed by translation through the lipid bilayer of the ER membrane. d. by being pulled by BiP across the lipid bilayer of the ER membrane.
Correct Answer: b Answer B Feedback: The translocon Sec61 forms an aqueous channel through which proteins are translocated into the ER lumen.
Proteins that span the membrane multiple times have multiple _______ that alternate with multiple transmembrane stop-transfer sequences. a. glycosylation sites b. internal signal sequences c. phosphorylation sites d. sulfation sites
Correct Answer: b Answer B Feedback: These mediate repeated rounds of reopening of the translocon and insertion into the membrane.
Which of the following is not associated with translational regulation? a. Dephosphorylation of 4E-BPs that bind eIF4E and prevent its interaction with eIF4G b. Autolytic degradation of the mRNA by folding back on itself c. Cleavage of mRNA by miRNA/RISC complexes d. Phosphorylation of eIF2, which inhibits GDP/GTP exchange
Correct Answer: b Answer B Feedback: This is not a form of translational regulation.
Which of the following does not take place in the nucleus? a. DNA replication b. Translation c. RNA processing d. Transcription
Correct Answer: b Answer B Feedback: Translation is carried out by ribosomes in the cytoplasm.
In the "pulse-chase" experiment, Palade and colleagues studied the pathway taken by newly secreted proteins in pancreatic acinar cells by labeling them with radioactive amino acids and then determining their progressive location within the cells. Which of the following represents the correct order in which the proteins were identified? a. Secretory vesicles → Golgi apparatus → rough ER → nucleus b. Golgi apparatus → rough ER → nucleus → secretory vesicles c. Rough ER → Golgi apparatus → secretory vesicles d. Rough ER → secretory vesicles → Golgi apparatus
Correct Answer: c
The half-lives of proteins in the cell vary widely, ranging from a. milliseconds to seconds. b. three to seven minutes. c. minutes to days. d. days to weeks.
Correct Answer: c Answer C Feedback: A regulatory protein, such as an inducible transcription factor, may have a half-life as short as a few minutes, whereas structural proteins or other proteins necessary for the day-to-day maintenance of cell physiology may have longer half-lives, up to a few days.
The primary function of rRNAs in the ribosome is a. to serve as a scaffold for the ribosomal proteins. b. to assist in the proper positioning of tRNAs along the mRNA template. c. to catalyze peptide bond formation. d. to assist in the proper folding of ribosomal proteins.
Correct Answer: c Answer C Feedback: Although this fundamental reaction in protein synthesis was once thought to be carried out by proteins, it has recently been found to be catalyzed by rRNAs
Chloroplast genomes contain approximately how many genes? a. None b. 40 c. 150 d. 80,000-100,000
Correct Answer: c Answer C Feedback: Chloroplast genes encode both protein and RNA molecules that are involved in a variety of functions.
Which of the following statements about cilia is false? a. They are about 10 micrometers in length, and cells that have them usually have many of them. b. They can be used to move fluids over the cell surface or to move the cell through fluids. c. They are projections of the plasma membrane supported by microfilaments. d. Their movement relies on the motor activity of axonemal dynein.
Correct Answer: c Answer C Feedback: Cilia are projections of the plasma membrane, but they are supported by microtubules, not microfilaments.
Altered gene expression is one possible explanation for the phenotypic consequences of nuclear lamina diseases. The other is a. ablative shearing. b. degradative acceleration. c. mechanical stress. d. targeting insufficiency.
Correct Answer: c Answer C Feedback: Defective nuclear lamina and/or defective connections to emerin or lamin B receptor may lead to a nucleus insufficiently resistant to mechanical stress.
The initiator codon in prokaryotes is a. the first codon located at the 5′ end of the mRNA. b. recognized by scanning of the ribosome downstream of the 5′ methylguanosine cap. c. recognized via the Shine-Dalgarno sequence. d. the first 5′ AUG of the mRNA.
Correct Answer: c Answer C Feedback: Initiation codons in prokaryotes are preceded by a specific sequence called the Shine-Dalgarno sequence, which correctly aligns them on the ribosome for initiation of translation.
Which of the following microtubules are attached to chromosomes? a. Astral microtubules b. Interphase microtubules c. Kinetochore microtubules d. Polar microtubules
Correct Answer: c Answer C Feedback: Kinetochore microtubules radiate out from the centrosomes in mitotic cells and attach to chromosomes at their centromeres, which are associated with specific proteins to form the kinetochore.
Most snoRNAs function as a. catalytic RNAs. b. self-replicating RNAs. c. guide RNAs. d. self-splicing RNAs.
Correct Answer: c Answer C Feedback: Most snRNAs function as guide RNAs to direct the specific base modifications of pre-rRNA.
"Classical" nuclear localization signals are composed primarily of a. hydrophobic amino acids. b. acidic amino acids. c. basic amino acids. d. sulfur-containing amino acids.
Correct Answer: c Answer C Feedback: Nuclear localization signals are often composed of basic amino acids such as arginine and lysine.
Protein transport into the nucleus occurs by a. diffusion through the nuclear envelope membranes. b. passive diffusion of large proteins through the nuclear pores. c. selective transport through the nuclear pores. d. selective transport through individual subsets of nuclear pore complexes that are specific for individual classes of proteins.
Correct Answer: c Answer C Feedback: The entry of most proteins into the nucleus is through sequence mediated selective binding to importins and then entry through the pore.
Which of the following would you expect to find at high concentrations in lysosomes? a. Proteins destined for secretion b. Glycosylation enzymes c. Degradative enzymes d. Recycling endosomes
Correct Answer: c Answer C Feedback: The function of lysosomes is to degrade, at an acid pH, substances taken up from outside the cell, as well as components of the cell that have outlived their usefulness.
The lumen of the ER is equivalent topologically to the a. cytoplasm. b. cytoplasmic face of the plasma membrane. c. extracellular space. d. nucleoplasm.
Correct Answer: c Answer C Feedback: The lumen of the ER is topologically equivalent to the extracellular space, and hence the inner leaflet of the ER membrane is topologically equivalent to the cell surface.
Human diseases caused by mutations in mitochondrial genomes a. are inherited from both parents. b. are inherited from the father. c. are inherited from the mother. d. do not exist, because the mutation is always complemented by the normal gene copy in the nucleus.
Correct Answer: c Answer C Feedback: The oocyte contributes many more mitochondria to the fertilized egg than the sperm does, and thus mitochondrial diseases are inherited maternally.
As they emerge from the ribosome, signal sequences are recognized and bound by a(n) a. tRNA. b. signal peptidase. c. signal recognition particle (SRP). d. SRP receptor.
Correct Answer: c Answer C Feedback: The signal recognition particle, which consists of seven polypeptides and one RNA molecule, binds the signal sequence and targets it to the ER membrane.
The trans-Golgi network is a. the intermediate compartment between the ER and the Golgi. b. the part of the Golgi where fusion of vesicles from the ER occurs. c. the exit part of the Golgi where sorting of proteins to the lysosomes, plasma membrane, and cell exterior occurs. d. the network of vesicles that transport resident Golgi proteins between cisternae.
Correct Answer: c Answer C Feedback: The trans-Golgi network is the last part of the Golgi through which proteins pass. It serves as a distribution center.
In some proteins the only transmembrane sequence is located C-terminally in the protein. These proteins are recognized by a. the SRP. b. the Sec61 translocon. c. TRC40 (GET3). d. the GET1-GET2 complex embedded in the ER membrane.
Correct Answer: c Answer C Feedback: The transmembrane domains of these proteins are recognized posttranslationally by TRC40, which then escorts the transmembrane protein to the ER membrane, where it is inserted via the GET1-GET2 complex.
Which of the following statements about heterochromatin is false? a. It is highly condensed chromatin. b. There are two forms—constitutive and facultative heterochromatin. c. It is transcriptionally active. d. It is largely localized to the nuclear periphery.
Correct Answer: c Answer C Feedback: This is a false statement. Heterochromatin, due to its highly condensed nature, is inaccessible to the transcriptional machinery and is thus transcriptionally inactive.
Which of the following statements regarding tRNAs is false? a. tRNAS are approximately 70-80 bases long and form a cloverleaf structure. b. All tRNAs have a CCA sequence at their 3′ terminus. c. tRNAs differ in sequence only at the anticodon. d. There are several modified bases present in mature tRNAS.
Correct Answer: c Answer C Feedback: This is not true. While there are many regions of shared sequence identity between tRNAs, they are not identical.
Which of the following is not one of the functions of the cytoskeleton? a. To provide a structural framework for the cell b. Cell locomotion c. Protein translocation into the ER d. Intracellular movement of organelles and other structures
Correct Answer: c Answer C Feedback: This process is carried out by the SRP and its receptor; it is not mediated by the cytoskeleton.
If a drug blocked the activity of the mitochondrial inner membrane proteins Tim9 and Tim10, which of the following would most likely not be found in the outer mitochondrial membrane? a. SAM complex b. α-helix membrane proteins c. β-barrel proteins d. Tom
Correct Answer: c Answer C Feedback: Tim9/10 are chaperones that bring the β-barrel polypeptides to the SAM complex, which then processes it into the mature transported in the outer membrane. Without Tim9/10 the polypeptides would not be escorted to SAM.
Zellweger syndrome is caused by a defect in a. mitochondrial protein import. b. the electron transport system in mitochondria. c. protein import into peroxisomes. d. the synthesis of peroxisomal proteins.
Correct Answer: c Answer C Feedback: Zellweger syndrome can be caused by mutations in a number of different proteins involved in peroxisome protein import and is lethal within the first ten years of life.
Chromosomes are distributed in the nucleus a. as cleavage fragments. b. as condensed mitotic chromosomes. c. randomly. d. in distinct territories.
Correct Answer: d
Antibiotics are powerful medications that inhibit the growth of bacteria. They work at a variety of levels, but many target the process of protein synthesis in the bacterial cell. In the developing of an antibiotic, which of the following would be an effective strategy or target for the drug? a. Inhibition of translational initiation b. Induction of premature polypeptide chain termination c. Inhibition of aminoacyl tRNA binding d. All of the above
Correct Answer: d Answer D Feedback: All of these would be effective strategies or targets.
Which of the following cytosolic proteins in red blood cells is the link between the plasma membrane and the spectrin/actin network beneath the cell surface? a. Band 3 b. Glycophorin c. Dystrophin d. Ankyrin
Correct Answer: d Answer D Feedback: Ankyrin binds both spectrin and the cytoplasmic portion of band 3, a plasma membrane protein, thus linking the actin network to the plasma membrane.
The carotenoids, which give many plants their yellow, orange, and red colors, are located in a. vacuoles. b. etioplasts. c. leucoplasts. d. chromoplasts.
Correct Answer: d Answer D Feedback: Chromoplasts are the sites of carotenoid storage; their function, however, is still unclear.
Which of the following factors contributes to the comparative ease of determining how pre-rRNA is processed? a. The abundance of rRNA genes b. The abundance of ribosomes (5-10 million) that need to be synthesized per cell cycle c. The grouping of rRNA genes into discrete nuclear subcompartments, the nucleoli d. All of the above
Correct Answer: d Answer D Feedback: Correct. All of these factors contribute to the ease of study.
Which of the following lipids is/are synthesized in the Golgi apparatus? a. Phospholipids b. Cholesterol c. Ceramide d. Glycolipids
Correct Answer: d Answer D Feedback: Glycolipids are synthesized from ceramide in the Golgi.
Which of the following is not a common lipid modification to proteins? a. N-myristoylation b. Prenylation c. GPI anchor addition d. Glycosylation
Correct Answer: d Answer D Feedback: Glycosylation is the addition of an oligosaccharide, not a lipid, to a protein.
Assuming that human mitochondria contain about 1,500 different proteins, approximately what percentage of the mitochondrial proteome is encoded by mitochondrial DNA? a. 0.05% b. 0.1% c. 0.50% d. 1.00%
Correct Answer: d Answer D Feedback: Human mitochondria encode 13 polypeptides (13/1500 × 100 = 0.86%).
Which of the following is an example of posttranslational modification? a. Glycosylation b. Proteolysis c. Palmitoylation d. All of the above
Correct Answer: d Answer D Feedback: It is important to realize that many modifications are made to proteins and affect their function.
What is the function of karyopherins? a. They attach to chromosomes in order to activate heterochromatin. b. They coat chromosomes. c. They define the structure of Cajal bodies. d. They transport macromolecules into or out of the nucleus.
Correct Answer: d Answer D Feedback: Karyopherins are nuclear transport receptors that belong to the importin or exportin class; they bind to cargo and promote its import or export.
The fibrous proteins underlying the inner nuclear membrane are a. band 3s. b. collagens. c. keratins. d. lamins.
Correct Answer: d Answer D Feedback: Lamins are the intermediate filament family proteins from which the nuclear lamina underlying the nuclear envelope is made.
A centrosome is a. a cylindrical structure made up of nine triplets of microtubules. b. a chromosomal region that connects sister chromatids during mitosis and attaches them to the spindle. c. a protein structure that binds centromeres and mediates the attachment of chromosomes to the spindle. d. the major microtubule-organizing center in animal cells.
Correct Answer: d Answer D Feedback: Microtubules emanate from the centrosome in nondividing cells, and during mitosis they are critical to the formation of the mitotic spindle.
The trans-Golgi network is a hub within the secretory pathway for protein sorting to various destinations. Mitochondria and chloroplasts are major examples of organelles that are not part of the secretory pathway and hence not destinations for transport vesicles arising from the trans-Golgi network hub. Which of the following is the most plausible explanation for their absence from the secretory pathway? a. The role they play in cellular energetics b. Their double membrane systems c. The presence of ribosomes in the mitochondrial and chloroplast lumen d. Their evolutionary origin through endosymbiosis
Correct Answer: d Answer D Feedback: Mitochondria and chloroplasts have a different evolutionary origin from that of other parts of the cell. Because of this, their route of delivery of proteins to the organelle is different from that of secretory pathway organelles.
Which of the following is not a destination for vesicles leaving the Golgi apparatus? a. The plasma membrane b. The exterior of the cell c. Lysosomes d. Mitochondria
Correct Answer: d Answer D Feedback: Proteins destined for the mitochondria are transported via a mechanism that does not involve vesicular transport or the Golgi apparatus.
What is the major site of energy production in the form of ATP in human cells? a. The mitochondrial matrix b. The cytoplasm c. The outer mitochondrial membrane d. The inner mitochondrial membrane
Correct Answer: d Answer D Feedback: The inner mitochondrial membrane is the site of oxidative phosphorylation, which produces close to 90% of the ATP derived from glucose metabolism.
The nuclear pore complex of proteins is estimated to have a mass a. smaller than that of a ribosome. b. about the same as that of a ribosome. c. about five times that of a ribosome. d. about 30 times that of a ribosome.
Correct Answer: d Answer D Feedback: The nuclear pore complex is very large, with a diameter of about 120 nm and a mass of about 125 million daltons.
The most prominent nuclear body is the a. mitochondrion. b. endoplasmic reticulum. c. histone locus body. d. nucleolus.
Correct Answer: d Answer D Feedback: The nucleolus is the most prominent subcompartment in the nucleus and can be readily recognized by light microscopy.
cAMP activates cAMP-dependent protein kinase by a. stimulating its phosphorylation. b. stimulating the dimerization of kinase subunits. c. stimulating the release of a translational inhibitory protein bound to its mRNA. d. binding regulatory subunits and inducing their release from the catalytic subunits.
Correct Answer: d Answer D Feedback: The regulatory subunits inhibit the activity of the catalytic subunits until they are bound by cAMP, which induces a conformational change that causes dissociation of the complex.
Which of the following statements about transmembrane proteins is true? a. The signal sequences are always cleaved off. b. They are always inserted with the amino terminus on the lumenal side and the carboxyl terminus in the cytosol. c. They cross the membrane only once. d. They usually have one or more α helices spanning the membrane bilayer.
Correct Answer: d Answer D Feedback: The transmembrane portions are usually α helices of approximately 20-25 amino acids. α helices are especially stable in the nonpolar environment of the lipid bilayer.
Cargo proteins are transported through the Golgi apparatus a. by forward (anterograde) moving vesicles. b. by backward (retrograde) moving vesicles. c. by cisternal maturation in which the cisternae themselves are the carriers for cargo transport through the Golgi apparatus. d. by a process(es) that remain controversial and may include elements of both vesicular transport and cisternal maturation.
Correct Answer: d Answer D Feedback: This is an area of molecular cell biology in which what the correct answer is remains controversial.
Which of the following is not a protein translocon/translocase found in the mitochondrial inner or outer membrane? a. Oxa1 b. Tim23 c. Tom40 d. Toc75
Correct Answer: d Answer D Feedback: Toc75 is a component of the outer chloroplast membrane.
The first amino acid of eukaryotic polypeptides is a. the amino acid encoded by the first 5′ codon. b. valine. c. N-formylmethionine. d. methionine.
Correct Answer: d Answer D Feedback: Translation of eukaryotic messages always begins with an AUG codon, which codes for methionine.
Which of the following is not involved in protein transport into mitochondria? a. A positively charged presequence of 15-55 amino acids located at the N-terminus b. The Tom complex c. The proton gradient across the inner mitochondrial membrane d. Vesicular transport
Correct Answer: d Answer D Feedback: Vesicular transport mediates many cellular transport steps, but it does not mediate transport into the mitochondria
mRNA molecules are exported from the nucleus to the cytoplasm via a. a consensus sequence located at the 3′ terminus. b. the 7-methylguanosine cap structure. c. importin. d. a recruited protein complex.
Correct Answer: d Answer D Feedback: mRNAs are associated with a large number of proteins during their processing and after processing an mRNA export complex is recruited
snRNPs that are responsible for pre-mRNA splicing are assembled and stored in a. Cajal bodies. b. histone locus bodies. c. speckles. d. both Cajal bodies and speckles.
Correct Answer: d Answer D Feedback: snRNPs are assembled and stored in both Cajal bodies and nuclear speckles