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FY13CE Biology study materials
FY13CE Biology study resources

FY13CE Biology practice questions

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Question 1Molecules and Cells

Which organelle is the primary site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Rough endoplasmic reticulum
  2. Golgi apparatus
  3. Lysosome
  4. Mitochondrion Correct answer.

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, so oxidative phosphorylation occurs there. The other listed organelles perform entirely different roles in the endomembrane system.

Question 2Molecules and Cells

A polypeptide is synthesized and then transported through the endomembrane system to the cell exterior. Which sequence of compartments matches this secretory pathway?

  1. Golgi → rough ER → lysosome → plasma membrane
  2. Rough ER → Golgi → secretory vesicle → plasma membrane Correct answer.
  3. Smooth ER → nucleus → Golgi → plasma membrane
  4. Cytosol → mitochondrion → Golgi → plasma membrane

Explanation

Proteins destined for secretion are co-translationally translocated into the rough ER, then move to the Golgi for modification and sorting, and finally leave in secretory vesicles that fuse with the plasma membrane. The order of compartments is fixed by vesicle trafficking routes.

Question 3Molecules and Cells

Which structural feature distinguishes a eukaryotic chromosome from a bacterial chromosome?

  1. Eukaryotic DNA is wrapped around histone proteins to form nucleosomes Correct answer.
  2. Eukaryotic chromosomes are circular and lack associated proteins
  3. Eukaryotic chromosomes are always present in multiple identical copies
  4. Eukaryotic chromosomes contain RNA as their genetic material

Explanation

Eukaryotic chromatin consists of DNA wound around histone octamers, producing nucleosomes that compact into higher-order structures. Bacterial chromosomes are typically circular and use histone-like proteins rather than true nucleosomes.

Question 4Molecules and Cells

During which phase of the cell cycle does DNA replication occur?

  1. G2 phase
  2. M phase
  3. G1 phase
  4. S phase Correct answer.

Explanation

Replication takes place in S phase, which lies between the two gap phases. G1 and G2 are growth and checkpoint phases, and M phase is when chromosomes are segregated and the cell divides.

Question 5Molecules and Cells

Which property of the plasma membrane is best described by the fluid mosaic model?

  1. Lipids are covalently cross-linked to form an impermeable sheet
  2. The membrane is a static sandwich of protein layers surrounding a lipid core
  3. Proteins are embedded in or attached to a dynamic lipid bilayer Correct answer.
  4. The bilayer is a rigid, protein-free crystal lattice

Explanation

The fluid mosaic model describes a fluid phospholipid bilayer in which proteins and other components diffuse laterally. The mosaic refers to the varied proteins embedded in or bound to the bilayer.

Question 6Molecules and Cells

Which process moves water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration?

  1. Active transport
  2. Facilitated diffusion of ions
  3. Osmosis Correct answer.
  4. Phagocytosis

Explanation

Osmosis is the passive movement of water down its own concentration gradient, which means from lower to higher solute concentration. It requires no energy input and occurs through the bilayer or aquaporins.

Question 7Molecules and Cells

Which enzyme catalyzes the synthesis of RNA from a DNA template during transcription?

  1. DNA ligase
  2. Peptidyl transferase
  3. RNA polymerase Correct answer.
  4. Helicase

Explanation

RNA polymerase binds a promoter, unwinds the DNA, and polymerizes ribonucleotides complementary to the template strand. It is the central enzyme of transcription in both prokaryotes and eukaryotes.

Question 8Molecules and Cells

Which molecule serves as the principal energy currency that couples exergonic and endergonic reactions in cells?

  1. ATP Correct answer.
  2. Pyruvate
  3. NADH
  4. Glucose

Explanation

ATP hydrolysis releases free energy that drives many endergonic cellular processes, and ATP is regenerated by catabolism. NADH and glucose are energy-rich but are not the direct coupling currency.

Question 9Molecules and Cells

A cell is placed in a hypertonic solution. What is the expected immediate response of an animal cell?

  1. Water enters the cell and the cell swells
  2. Solutes move out of the cell until isotonicity is reached
  3. There is no net water movement because the membrane is impermeable
  4. Water leaves the cell and the cell shrinks Correct answer.

Explanation

In a hypertonic environment the external solute concentration exceeds the internal one, so water moves out by osmosis and the cell loses volume. Animal cells lack a wall and may crenate under severe shrinkage.

Question 10Molecules and Cells

An inhibitor binds to the active site of an enzyme and can be displaced by raising the substrate concentration. This inhibition is best classified as:

  1. Irreversible covalent inhibition
  2. Competitive inhibition Correct answer.
  3. Noncompetitive inhibition
  4. Allosteric activation

Explanation

Competitive inhibitors occupy the active site and compete with substrate, so increased substrate outcompetes the inhibitor and restores reaction rate. The other modes are not reversed by adding substrate.

Question 11Molecules and Cells

In the absence of oxygen, a muscle cell converts pyruvate to lactate. What is the main purpose of this reaction?

  1. Generating acetyl-CoA for the citric acid cycle
  2. Producing large amounts of ATP directly from lactate
  3. Reversing the proton gradient of the inner mitochondrial membrane
  4. Regenerating NAD+ so glycolysis can continue Correct answer.

Explanation

Lactate fermentation oxidizes NADH back to NAD+, which is required as an electron acceptor in the glyceraldehyde-3-phosphate dehydrogenase step of glycolysis. Without this regeneration, glycolysis would halt under anaerobic conditions.

Question 12Molecules and Cells

A mutation changes a single base pair in a coding region but the encoded polypeptide is unchanged. This mutation is described as:

  1. A silent mutation Correct answer.
  2. A frameshift mutation
  3. A missense mutation
  4. A nonsense mutation

Explanation

Silent mutations alter a codon without changing the amino acid specified, owing to the redundancy of the genetic code. The protein sequence, and usually its function, remains the same.

Question 13Molecules and Cells

Which statement correctly describes the role of tRNA in translation?

  1. It carries amino acids and pairs its anticodon with the mRNA codon Correct answer.
  2. It transports mRNA from the nucleus to the cytoplasm
  3. It serves as the template for DNA synthesis during replication
  4. It forms the catalytic core of the ribosome that joins amino acids

Explanation

Each tRNA is charged with a specific amino acid and delivers it to the ribosome by base-pairing its anticodon with the complementary codon on mRNA. This ensures the genetic message is decoded accurately.

Question 14Molecules and Cells

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate, with net production of:

  1. 4 ATP and 4 NADH
  2. 1 ATP and 2 NADPH
  3. 2 ATP and 4 FADH2
  4. 2 ATP and 2 NADH Correct answer.

Explanation

Glycolysis consumes two ATP in the investment phase and produces four ATP in the payoff phase, for a net gain of two ATP, while two NAD+ are reduced to two NADH. These yields are per glucose molecule.

Question 15Molecules and Cells

A protein that functions in the cytosol is synthesized on free ribosomes, whereas a secreted protein is synthesized on ribosomes bound to the rough ER. What best explains this difference?

  1. Only proteins with an ER signal sequence are targeted to the rough ER Correct answer.
  2. Bound ribosomes contain a different set of ribosomal RNA genes
  3. Free ribosomes can synthesize only short polypeptides
  4. Secreted proteins lack a signal sequence and are captured by the ER by chance

Explanation

An N-terminal signal peptide directs the ribosome-mRNA complex to the ER membrane, where the nascent chain is translocated into the lumen. Proteins without such a signal remain on free ribosomes and fold in the cytosol.

Question 16Molecules and Cells

Which feature of the genetic code allows a single tRNA species to recognize more than one codon for the same amino acid?

  1. Proofreading by DNA polymerase during replication
  2. Methylation of the mRNA cap
  3. Wobble base pairing at the third codon position Correct answer.
  4. Alternative splicing of the tRNA gene transcript

Explanation

Wobble pairing permits nonstandard hydrogen bonding between the anticodon's first base and the codon's third base, so one tRNA can read several synonymous codons. This flexibility reduces the number of tRNA species needed.

Question 17Molecules and Cells

In a cell cycle checkpoint at the end of G2, which condition must be satisfied before the cell enters mitosis?

  1. Chromosomes must already be separated into two nuclei
  2. DNA replication must be complete and DNA damage repaired Correct answer.
  3. The spindle must have attached to kinetochores and aligned chromosomes
  4. Cytokinesis must have produced two daughter cells

Explanation

The G2 checkpoint verifies that replication has finished and that any DNA damage has been repaired before mitosis begins. Spindle attachment is monitored later, at the metaphase checkpoint.

Question 18Molecules and Cells

A cell is treated with a drug that blocks the electron transport chain. Which immediate consequence is most likely?

  1. The proton gradient across the inner mitochondrial membrane collapses and ATP synthesis falls Correct answer.
  2. The citric acid cycle accelerates to compensate for the lost ATP
  3. Glycolysis stops immediately because glucose cannot enter the cell
  4. NADH is oxidized more rapidly at complex I

Explanation

Electron transport pumps protons to build the gradient that drives ATP synthase, so blocking the chain dissipates the gradient and halts oxidative phosphorylation. Glycolysis can continue briefly by fermentation because it does not require the chain directly.

Question 19Molecules and Cells

A segment of DNA has the template strand sequence 3'-TAC GGA-5'. What mRNA sequence is transcribed from it?

  1. 5'-AUG CCU-3' Correct answer.
  2. 5'-TAC GGA-3'
  3. 5'-AUG GGU-3'
  4. 5'-UAC GGA-3'

Explanation

RNA polymerase reads the template strand antiparallel and inserts complementary ribonucleotides, so T pairs with A, A with U, C with G, and G with C. The resulting mRNA is 5'-AUG CCU-3', which also encodes methionine followed by proline.

Question 20Molecules and Cells

A diploid organism with genotype AaBb produces gametes. Assuming the two genes assort independently, what proportion of gametes carries the Ab combination?

  1. 3/4
  2. 1/2
  3. 1/4 Correct answer.
  4. 1/8

Explanation

Independent assortment gives four equally likely gamete types (AB, Ab, aB, ab), so each occurs with probability 1/4. The calculation multiplies the 1/2 chance of A by the 1/2 chance of b.

Question 21Molecules and Cells

A researcher finds that a metabolic enzyme has a much lower Km for its substrate after a single amino acid substitution. What does this indicate about the mutant enzyme?

  1. It reaches half-maximal velocity at a lower substrate concentration, indicating higher substrate affinity Correct answer.
  2. It has a lower maximal velocity because the catalytic step is slower
  3. It requires a higher substrate concentration to saturate because affinity decreased
  4. It has lost all catalytic activity and binds substrate irreversibly

Explanation

Km is the substrate concentration at which the reaction proceeds at half of Vmax, so a lower Km reflects tighter substrate binding. The substitution appears to improve affinity without necessarily altering Vmax.

Question 22Molecules and Cells

A cell is shifted from a 15N-labeled medium to a 14N medium, and DNA is replicated for exactly two generations before analysis. Which distribution of DNA molecules is expected under semi-conservative replication?

  1. Half of the molecules are hybrid and half contain only 14N Correct answer.
  2. All molecules contain only 14N because the label is diluted out
  3. One quarter are fully 15N and three quarters are fully 14N
  4. All molecules are hybrid, each with one 15N and one 14N strand

Explanation

After one generation every duplex is hybrid; after the second, each hybrid duplex separates into one hybrid and one light daughter, giving a 1:1 ratio of hybrid to fully light molecules. This pattern is the classic evidence for semi-conservative replication.

Question 23Molecules and Cells

In a signal transduction cascade, a ligand binds a G protein-coupled receptor and activates adenylyl cyclase. Which outcome directly follows from this activation?

  1. Direct phosphorylation of target proteins by the receptor's kinase domain
  2. Hydrolysis of GTP bound to the receptor, terminating the signal immediately
  3. Conversion of ATP to cyclic AMP, which activates protein kinase A Correct answer.
  4. Opening of ligand-gated ion channels in the plasma membrane

Explanation

Activated adenylyl cyclase cyclizes ATP into cAMP, a second messenger that binds the regulatory subunits of protein kinase A and unleashes its catalytic activity. Amplification at each step makes the response large even at low ligand concentration.

Question 24Molecules and Cells

A frameshift mutation occurs early in a gene, yet a truncated but partially functional protein is produced. Which explanation is most consistent with this observation?

  1. The new reading frame introduced a premature stop codon downstream of the shift Correct answer.
  2. The ribosome corrected the frameshift by proofreading the mRNA
  3. The mutation was a silent substitution that preserved the amino acid sequence
  4. The mutation deleted an entire exon without altering the reading frame

Explanation

A frameshift repositions all downstream codons, and one of the shifted triplets is often a stop codon, producing a shortened polypeptide. The truncated protein may retain some function if the lost region is not essential.

Question 25Molecules and Cells

A mitochondrial uncoupler such as dinitrophenol increases the permeability of the inner membrane to protons. What is the predicted effect on a respiring cell?

  1. Glycolysis is inhibited because cytosolic NADH cannot be oxidized
  2. Electron transport accelerates but ATP synthesis decreases and heat is released Correct answer.
  3. Both electron transport and ATP synthesis increase in parallel
  4. The proton gradient increases and ATP synthesis rises

Explanation

Uncouplers let protons leak back across the inner membrane without passing through ATP synthase, so the gradient is dissipated and ATP synthesis falls. The respiratory chain speeds up in response to the reduced gradient, releasing energy as heat.

Question 26Molecules and Cells

Which type of bond links the phosphate group of one nucleotide to the sugar of the next nucleotide in a single DNA strand?

  1. Hydrogen bond
  2. Peptide bond
  3. Phosphodiester bond Correct answer.
  4. Glycosidic bond

Explanation

The backbone of a DNA strand is formed by covalent phosphodiester bonds between the 3' hydroxyl of one deoxyribose and the 5' phosphate of the next nucleotide. Hydrogen bonds instead hold complementary bases together across the two strands, and peptide and glycosidic bonds are not part of the DNA backbone.

Question 27Molecules and Cells

Which property of phospholipids allows them to form a bilayer in aqueous environments?

  1. They contain a rigid steroid ring structure.
  2. They are completely hydrophobic molecules.
  3. They are completely hydrophilic molecules.
  4. They have a hydrophilic head and hydrophobic tails. Correct answer.

Explanation

Phospholipids are amphipathic: the phosphate-containing head is polar and interacts with water, while the fatty acid tails are nonpolar and cluster away from water. This arrangement spontaneously produces a bilayer with tails facing inward and heads facing the aqueous surroundings.

Question 28Molecules and Cells

Which statement best describes the role of ATP hydrolysis in driving an endergonic reaction in a cell?

  1. ATP hydrolysis directly lowers the activation energy of the reaction.
  2. ATP hydrolysis provides a phosphate group that always remains attached to the product.
  3. ATP hydrolysis releases free energy that can be coupled to the unfavorable reaction. Correct answer.
  4. ATP hydrolysis raises the equilibrium constant of the reaction.

Explanation

Cells couple exergonic ATP hydrolysis to endergonic processes so that the overall free-energy change is negative. Enzymes, not ATP itself, lower activation energy, and phosphorylation of a substrate is a mechanism of coupling rather than a universal fate of the phosphate.

Question 29Molecules and Cells

Which organelle is the primary site of lipid synthesis in a eukaryotic cell?

  1. Lysosome
  2. Smooth endoplasmic reticulum Correct answer.
  3. Rough endoplasmic reticulum
  4. Golgi apparatus

Explanation

The smooth endoplasmic reticulum lacks bound ribosomes and houses enzymes for lipid and steroid synthesis as well as detoxification. The rough ER is specialized for protein synthesis, while the Golgi modifies and sorts proteins and lipids, and lysosomes carry out degradation.

Question 30Molecules and Cells

Which statement correctly distinguishes prokaryotic from eukaryotic chromosomes?

  1. Prokaryotic chromosomes are typically circular and lack histone-based nucleosomes, whereas eukaryotic chromosomes are linear and wrapped around histones. Correct answer.
  2. Prokaryotic chromosomes are linear and wrapped around histones, whereas eukaryotic chromosomes are circular and naked.
  3. Both prokaryotic and eukaryotic chromosomes are circular and associated with histones.
  4. Prokaryotic chromosomes are enclosed in a nucleus, whereas eukaryotic chromosomes are free in the cytoplasm.

Explanation

Prokaryotes usually carry a single circular chromosome in the nucleoid region and use histone-like proteins rather than true nucleosomes. Eukaryotes package linear chromosomes with histones into chromatin inside a nucleus.

Question 31Molecules and Cells

Which of the following best describes the function of the sodium-potassium pump in maintaining the resting membrane potential of a neuron?

  1. It imports three Na+ ions and exports two K+ ions per ATP, contributing to a positive interior.
  2. It transports only K+ ions out of the cell while leaving Na+ distribution unchanged.
  3. It exports three Na+ ions and imports two K+ ions per ATP, contributing to a negative interior. Correct answer.
  4. It moves Na+ and K+ down their concentration gradients without using ATP.

Explanation

The Na+/K+ ATPase uses one ATP to pump three Na+ out and two K+ in, which leaves a net deficit of positive charge inside the cell. This electrogenic action, together with K+ leak channels, helps establish the negative resting potential.

Question 32Molecules and Cells

Which statement best describes the role of the centromere during mitosis?

  1. It is the region that degrades the nuclear envelope before prometaphase.
  2. It is the structure that synthesizes the spindle microtubules.
  3. It is the site where DNA replication begins during S phase.
  4. It is the region where sister chromatids are held together and where kinetochores assemble for spindle attachment. Correct answer.

Explanation

The centromere is the chromosomal region that joins sister chromatids and recruits kinetochore proteins, which bind spindle microtubules. Replication origins, centrosomes, and nuclear envelope breakdown are separate structures and processes.

Question 33Molecules and Cells

Which of the following correctly pairs a component of the cytoskeleton with its primary function?

  1. Intermediate filaments - tubulin-based chromosome segregation
  2. Microfilaments - tubulin-based movement of cilia
  3. Microtubules - intermediate filament-based anchoring of desmosomes
  4. Microfilaments - actin-based support of cell shape and contraction Correct answer.

Explanation

Microfilaments are polymers of actin that maintain cell shape, enable crawling, and participate in contraction. Microtubules are tubulin-based and handle chromosome segregation and cilia movement, while intermediate filaments provide mechanical strength.

Question 34Molecules and Cells

A cell is treated with a drug that inhibits the enzyme that adds the 5' cap to mRNA. Which step of gene expression is most directly affected?

  1. Attachment of the ribosome to the mature mRNA in the cytoplasm
  2. Splicing of introns by the spliceosome
  3. Replication of DNA during S phase
  4. Processing of the pre-mRNA transcript in the nucleus Correct answer.

Explanation

The 5' cap is added co-transcriptionally during pre-mRNA processing in the nucleus, so inhibiting the capping enzyme directly blocks that processing step. Capping also influences stability and translation, but the immediate molecular defect is in transcript processing.

Question 35Molecules and Cells

A mutation in a gene encoding a mitochondrial matrix enzyme removes its N-terminal targeting sequence. Where is the mutant protein most likely to accumulate?

  1. In the lysosome
  2. In the mitochondrial matrix
  3. In the nucleus
  4. In the cytosol Correct answer.

Explanation

Mitochondrial matrix proteins are synthesized on cytosolic ribosomes and directed to the organelle by an N-terminal presequence. Without that signal, the protein remains in the cytosol because it cannot be recognized by the mitochondrial import machinery.

Question 36Molecules and Cells

During the light-dependent reactions of photosynthesis, what is the immediate consequence of the photolysis of water at photosystem II?

  1. Reduction of NADP+ to NADPH at photosystem I
  2. Fixation of carbon dioxide into a three-carbon compound
  3. Synthesis of ATP by the Calvin cycle
  4. Replacement of electrons lost by the reaction center and release of oxygen Correct answer.

Explanation

When photosystem II loses electrons to the electron transport chain, water is split to supply replacement electrons, producing oxygen as a byproduct. NADPH formation and ATP synthesis are downstream events, and carbon fixation occurs in the Calvin cycle.

Question 37Molecules and Cells

A competitive inhibitor of an enzyme is added to a reaction. Which kinetic parameter is most likely to change?

  1. Vmax decreases, while Km remains unchanged.
  2. The apparent Km increases, while Vmax remains unchanged. Correct answer.
  3. Both Km and Vmax decrease equally.
  4. The apparent Km decreases, while Vmax increases.

Explanation

A competitive inhibitor competes with substrate for the active site, so more substrate is needed to reach half-maximal velocity, raising the apparent Km. Because the inhibition can be overcome at high substrate concentration, Vmax is unaffected.

Question 38Molecules and Cells

In the lac operon of E. coli, what is the effect of high glucose and high lactose simultaneously?

  1. High cAMP increases CAP binding, so transcription is maximal.
  2. Low cAMP reduces CAP binding, so transcription remains low despite lactose-mediated derepression. Correct answer.
  3. The repressor binds the operator and transcription is fully repressed.
  4. The operon is transcribed constitutively because lactose is present.

Explanation

High glucose lowers cAMP levels, which prevents CAP from activating the lac promoter. Lactose inactivates the repressor, but without CAP activation the transcription rate stays low, illustrating dual positive and negative control.

Question 39Molecules and Cells

In a eukaryote, which modification of histone tails is most directly associated with transcriptional activation?

  1. Ubiquitination of histone H2A at lysine 119
  2. Methylation of lysine 9 on histone H3
  3. Phosphorylation of histone H2AX at serine 139
  4. Acetylation of lysine residues Correct answer.

Explanation

Histone acetylation neutralizes positive charges on lysine, loosening chromatin and promoting access for transcription. H3K9 methylation and H2A ubiquitination are linked to repression, while H2AX phosphorylation marks DNA damage.

Question 40Molecules and Cells

A cell is exposed to a drug that depolymerizes microtubules. Which process would be most immediately disrupted?

  1. Addition of the poly(A) tail to mRNA
  2. Synthesis of the leading strand during DNA replication
  3. Separation of sister chromatids during anaphase Correct answer.
  4. Formation of the nuclear envelope during telophase

Explanation

The mitotic spindle is built from microtubules, and chromosome separation during anaphase depends on microtubule dynamics and motor proteins. Nuclear envelope reformation, DNA synthesis, and polyadenylation do not require an intact spindle.

Question 41Molecules and Cells

A mutation in a proto-oncogene converts it into an oncogene. Which change best explains the dominant effect of this mutation?

  1. A change that only affects cells after both alleles are deleted
  2. A gain-of-function change that makes one mutant allele sufficient to promote proliferation Correct answer.
  3. A loss-of-function change that requires inactivation of both alleles
  4. A change that prevents the protein from being synthesized at all

Explanation

Oncogenic mutations are typically gain-of-function and dominant, so a single altered allele can drive excessive proliferation. Tumor suppressor genes, by contrast, usually require loss of both alleles to contribute to cancer.

Question 42Molecules and Cells

A cell is shifted from a medium containing 15N to a medium containing 14N and allowed to replicate its DNA for one generation. After density-gradient centrifugation, where would the DNA band appear?

  1. At an intermediate density corresponding to hybrid DNA Correct answer.
  2. At the heavy density corresponding to 15N/15N DNA
  3. At two distinct bands, one heavy and one light
  4. At the light density corresponding to 14N/14N DNA

Explanation

Semiconservative replication produces molecules in which each double helix contains one old 15N strand and one new 14N strand. After one generation, all DNA is hybrid and bands at intermediate density.

Question 43Molecules and Cells

Which of the following best explains why a single amino acid substitution can abolish an enzyme's activity even when the substituted residue is far from the active site?

  1. The substitution directly replaces a catalytic residue in the active site.
  2. The substitution changes the reading frame of the mRNA.
  3. The substitution prevents transcription of the gene.
  4. The substitution can disrupt the protein's overall folding or conformational change required for catalysis. Correct answer.

Explanation

Residues distant from the active site can contribute to tertiary structure, subunit contacts, or dynamic motions needed for catalysis. A single substitution there can destabilize the fold or block a required conformational transition.

Question 44Molecules and Cells

In a cell, which condition would most directly trigger the unfolded protein response in the endoplasmic reticulum?

  1. Accumulation of misfolded proteins in the ER lumen Correct answer.
  2. A decrease in cytosolic ATP concentration
  3. Failure of the Golgi to glycosylate proteins
  4. Loss of the mitochondrial membrane potential

Explanation

The unfolded protein response is activated by the buildup of misfolded or unfolded proteins in the ER lumen, sensed by transmembrane kinases such as Ire1 and PERK. Cytosolic ATP depletion and mitochondrial depolarization trigger different stress pathways.

Question 45Molecules and Cells

A mutation in a gene encoding a DNA mismatch repair protein leads to a mutator phenotype. Which outcome is most likely?

  1. An increased rate of mutations, especially in repetitive sequences Correct answer.
  2. A block in transcription elongation
  3. A decreased rate of mutations because repair is more efficient
  4. A block in DNA replication initiation

Explanation

Mismatch repair corrects errors that escape polymerase proofreading, particularly at microsatellites. Loss of this repair activity elevates mutation rates and can cause microsatellite instability, a hallmark of certain cancers.

Question 46Molecules and Cells

A researcher treats cells with a drug that inhibits the proteasome. Which process would be most directly affected?

  1. Transport of mRNA out of the nucleus
  2. Degradation of ubiquitin-tagged proteins Correct answer.
  3. Translation of mRNA into protein
  4. Transcription of rRNA by RNA polymerase I

Explanation

The proteasome recognizes polyubiquitin chains and degrades tagged proteins, so proteasome inhibition directly impairs this degradation pathway. Translation, rRNA transcription, and mRNA export are separate processes.

Question 47Molecules and Cells

In a signal transduction cascade, a ligand binds a receptor tyrosine kinase and activates Ras. Which immediate molecular event is required for Ras activation?

  1. Exchange of GDP for GTP on Ras, promoted by a guanine nucleotide exchange factor Correct answer.
  2. Hydrolysis of GTP to GDP by Ras
  3. Phosphorylation of Ras by the receptor tyrosine kinase
  4. Dephosphorylation of Ras by a phosphatase

Explanation

Ras is active when bound to GTP, and activation requires a guanine nucleotide exchange factor such as SOS to promote GDP release and GTP binding. Hydrolysis of GTP to GDP by Ras returns it to the inactive state.

Question 48Molecules and Cells

A cell is treated with an inhibitor of the enzyme that adds the poly(A) tail to mRNA. Which consequence is most likely?

  1. Reduced stability and impaired nuclear export of the mRNA Correct answer.
  2. Failure of the 5' cap to be added to the transcript
  3. Inability of the ribosome to bind the mRNA
  4. A block in splicing of introns

Explanation

The poly(A) tail protects mRNA from degradation and contributes to nuclear export and translation initiation. Inhibiting polyadenylation therefore reduces stability and export, while capping, splicing, and ribosome binding are distinct steps.

Question 49Molecules and Cells

In the electron transport chain, cyanide binds to cytochrome c oxidase and blocks electron transfer to oxygen. Which immediate consequence is most likely?

  1. The citric acid cycle is stimulated because NADH accumulates.
  2. The proton gradient collapses, and ATP synthesis by oxidative phosphorylation stops. Correct answer.
  3. ATP synthase reverses direction and hydrolyzes ATP to maintain the gradient.
  4. Glycolysis stops because glucose cannot be imported into the cell.

Explanation

Blocking cytochrome c oxidase prevents the terminal electron transfer to oxygen, so proton pumping and the proton gradient cannot be maintained. Without the gradient, ATP synthase stops producing ATP, and the cell relies on glycolysis for ATP.

Question 50Molecules and Cells

A frameshift mutation introduces a premature stop codon early in a gene, yet a truncated but partially functional protein is produced. Which explanation is most consistent with this observation?

  1. The mutation is a silent substitution that does not change the amino acid sequence.
  2. The mutation occurs in an intron and is removed by splicing.
  3. A nonsense-mediated decay pathway is incomplete, allowing translation of the truncated mRNA. Correct answer.
  4. The ribosome reads through the stop codon and continues translation.

Explanation

A premature stop codon usually triggers nonsense-mediated decay, but if that pathway is incomplete or the stop is in a position that escapes decay, the mRNA can be translated into a truncated protein. A silent substitution or intronic mutation would not produce a truncated protein.