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MTLE Life Science (Grades 9-12) study materials
MTLE Life Science (Grades 9-12) study resources

MTLE Life Science (Grades 9-12) practice questions

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Question 1Cellular and Molecular Biology

A student views a cell and observes a rigid outer layer that provides structural support and is composed primarily of cellulose. Which cellular structure is being observed?

  1. Nuclear envelope
  2. Cell wall Correct answer.
  3. Plasma membrane
  4. Golgi apparatus

Explanation

The cell wall is a rigid extracellular layer that supports and protects plant cells, and its main structural component is cellulose. The plasma membrane lies just inside the wall and is a phospholipid bilayer, not a cellulose structure.

Question 2Cellular and Molecular Biology

Which organelle is the primary site of ATP production through oxidative phosphorylation in a eukaryotic cell?

  1. Smooth endoplasmic reticulum
  2. Ribosome
  3. Mitochondrion Correct answer.
  4. Lysosome

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, making the mitochondrion the main site of oxidative phosphorylation. Ribosomes synthesize proteins, lysosomes degrade material, and smooth ER synthesizes lipids.

Question 3Cellular and Molecular Biology

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

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

Explanation

DNA replication takes place during S phase, which lies between G1 and G2. G1 and G2 are gap phases of growth and preparation, and M phase is when chromosomes are segregated and the cell divides.

Question 4Cellular and Molecular Biology

Which molecule serves as the primary energy currency that couples exergonic reactions to endergonic cellular work?

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

Explanation

ATP hydrolysis releases free energy that drives endergonic processes, so ATP is the cell's direct energy currency. NADH and glucose store energy but must first be oxidized or catabolized to produce ATP.

Question 5Cellular and Molecular Biology

In a DNA molecule, adenine pairs with which nitrogenous base, and how many hydrogen bonds form between them?

  1. Thymine; two hydrogen bonds Correct answer.
  2. Uracil; two hydrogen bonds
  3. Guanine; three hydrogen bonds
  4. Cytosine; three hydrogen bonds

Explanation

By complementary base pairing, adenine pairs with thymine in DNA and the pair is held together by two hydrogen bonds. Guanine pairs with cytosine using three hydrogen bonds, and uracil occurs in RNA rather than DNA.

Question 6Cellular and Molecular Biology

A biologist treats a cell with a drug that halts transcription. Which process is directly inhibited?

  1. Synthesis of mRNA from a DNA template Correct answer.
  2. Movement of vesicles along microtubules
  3. Synthesis of a polypeptide at the ribosome
  4. Replication of the chromosome during S phase

Explanation

Transcription is the synthesis of an RNA molecule, such as mRNA, using DNA as a template. Translation, replication, and vesicle transport are separate processes that do not directly depend on the transcription step being active.

Question 7Cellular and Molecular Biology

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

  1. A solid barrier of cellulose enclosing the cytoplasm
  2. A single layer of phospholipids surrounding the cytoplasm
  3. A dynamic bilayer of phospholipids with embedded proteins Correct answer.
  4. A rigid, static layer of protein covering the cell

Explanation

The fluid mosaic model describes the membrane as a fluid phospholipid bilayer in which proteins are embedded and can move laterally. This accounts for membrane fluidity and the varied distribution of proteins.

Question 8Cellular and Molecular Biology

Which statement correctly distinguishes prokaryotic from eukaryotic cells?

  1. Prokaryotic cells contain a nuclear envelope, whereas eukaryotic cells contain a nucleoid.
  2. Prokaryotic cells lack a membrane-bound nucleus, whereas eukaryotic cells possess one. Correct answer.
  3. Prokaryotic cells contain a membrane-bound nucleus, whereas eukaryotic cells do not.
  4. Prokaryotic cells contain mitochondria, whereas eukaryotic cells lack them.

Explanation

Prokaryotes keep their genetic material in a nucleoid region without a membrane-bound nucleus, while eukaryotes enclose DNA within a nuclear envelope. This is the central structural distinction between the two cell types.

Question 9Cellular and Molecular Biology

An enzyme catalyzes a reaction in a cell. If the substrate concentration is increased while enzyme concentration remains constant, what is the expected effect on reaction rate at low substrate levels?

  1. The rate decreases because substrate competes for active sites.
  2. The rate becomes zero because enzymes are saturated.
  3. The rate increases because more substrate molecules bind to active sites. Correct answer.
  4. The rate stays constant because enzymes are not consumed.

Explanation

At low substrate concentrations, the rate rises as substrate concentration increases because more enzyme active sites become occupied. The rate eventually plateaus when all active sites are saturated, but not at low substrate levels.

Question 10Cellular and Molecular Biology

A human cell is placed in a hypertonic solution. What is the most likely immediate consequence for the cell?

  1. Water leaves the cell and it shrinks. Correct answer.
  2. Solutes move into the cell until isotonic.
  3. No net movement of water occurs.
  4. Water enters the cell and it swells.

Explanation

In a hypertonic solution, the surrounding fluid has a higher solute concentration than the cytoplasm, so water leaves the cell by osmosis and the cell shrinks. Solute movement is limited, and net water flow is outward rather than zero.

Question 11Cellular and Molecular Biology

During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. Which net yield of ATP and NADH is produced per glucose in glycolysis?

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

Explanation

Glycolysis has a net yield of 2 ATP and 2 NADH per glucose molecule after accounting for the ATP consumed in the investment phase. Four ATP are produced in total, but two are consumed, leaving a net of two.

Question 12Cellular and Molecular Biology

In the light-dependent reactions of photosynthesis, what is the primary role of water?

  1. It donates electrons to photosystem II and releases oxygen. Correct answer.
  2. It accepts electrons at photosystem I to form NADPH.
  3. It serves as the final electron acceptor in the electron transport chain.
  4. It provides carbon for the Calvin cycle.

Explanation

Water is split at photosystem II, supplying electrons to replace those lost by chlorophyll and releasing oxygen as a byproduct. NADP+ accepts electrons at the end of the chain to form NADPH, and carbon fixation occurs separately in the Calvin cycle.

Question 13Cellular and Molecular Biology

A mutation changes a single nucleotide in a coding region but the resulting polypeptide has the same amino acid sequence. What type of mutation is this?

  1. Missense mutation
  2. Silent mutation Correct answer.
  3. Frameshift mutation
  4. Nonsense mutation

Explanation

A silent mutation alters a codon but, because of the redundancy of the genetic code, does not change the encoded amino acid. Nonsense and missense mutations change the protein outcome, and frameshifts alter downstream reading.

Question 14Cellular and Molecular Biology

Which feature of the genetic code explains why a single amino acid can be specified by more than one codon?

  1. The code is non-universal.
  2. The code is overlapping.
  3. The code is degenerate. Correct answer.
  4. The code is read in pairs.

Explanation

Degeneracy means that multiple codons can encode the same amino acid, providing some protection against silent mutations. The code is read in non-overlapping triplets and is nearly universal across organisms.

Question 15Cellular and Molecular Biology

A cell is treated with a drug that inhibits the function of the sodium-potassium pump. What immediate effect would this have on the ion distribution across the membrane?

  1. Intracellular sodium would decrease and intracellular potassium would increase.
  2. Both intracellular sodium and potassium would increase equally.
  3. Ion distribution would remain unchanged because diffusion does not depend on the pump.
  4. Intracellular sodium would increase and intracellular potassium would decrease. Correct answer.

Explanation

The sodium-potassium pump normally exports three sodium ions and imports two potassium ions per ATP, maintaining low intracellular sodium and high intracellular potassium. Inhibiting it allows these gradients to run down, so sodium accumulates inside and potassium decreases inside.

Question 16Cellular and Molecular Biology

During meiosis, crossing over between homologous chromosomes occurs at which stage?

  1. Prophase I Correct answer.
  2. Anaphase II
  3. Metaphase I
  4. Telophase II

Explanation

Crossing over occurs during prophase I, when homologous chromosomes pair as tetrads and exchange segments at chiasmata. This recombination generates new combinations of alleles on the resulting chromosomes.

Question 17Cellular and Molecular Biology

Which molecule carries amino acids to the ribosome during translation and contains an anticodon that pairs with the mRNA codon?

  1. Transfer RNA Correct answer.
  2. Ribosomal RNA
  3. Small nuclear RNA
  4. Messenger RNA

Explanation

Transfer RNA molecules are charged with specific amino acids and use their anticodon to base-pair with the complementary codon on mRNA. This ensures the correct amino acid is added to the growing polypeptide.

Question 18Cellular and Molecular Biology

A researcher observes that a particular enzyme's activity drops sharply when the pH moves from 7 to 3. Which explanation best accounts for this observation?

  1. The reaction became endergonic at low pH.
  2. The enzyme became more efficient because acidic conditions speed reactions.
  3. The substrate concentration decreased at low pH.
  4. The enzyme denatured or its active-site ionization changed at low pH. Correct answer.

Explanation

Each enzyme has an optimal pH at which its active-site residues are properly ionized and its tertiary structure is stable. Extreme pH can alter these charges or denature the enzyme, reducing activity.

Question 19Cellular and Molecular Biology

Which statement best describes the role of the promoter in gene expression?

  1. It is the sequence that signals the ribosome to terminate translation.
  2. It is the region where RNA polymerase and transcription factors bind to initiate transcription. Correct answer.
  3. It is the site where introns are removed from the transcript.
  4. It is the sequence that encodes the start codon of the polypeptide.

Explanation

A promoter is a DNA sequence upstream of a gene where RNA polymerase and associated transcription factors assemble to begin transcription. It determines where and how often transcription initiates.

Question 20Cellular and Molecular Biology

A cell in G2 phase contains 16 chromosomes. Assuming normal mitosis and cytokinesis, how many chromosomes will each daughter cell contain after division?

  1. 16 Correct answer.
  2. 4
  3. 8
  4. 32

Explanation

DNA replication in S phase produces sister chromatids but does not change chromosome number, so a G2 cell with 16 chromosomes has 16 chromosomes. Mitosis distributes one chromatid of each pair to each daughter, so each daughter also receives 16 chromosomes.

Question 21Cellular and Molecular Biology

A competitive inhibitor is added to an enzyme-catalyzed reaction. Which change in kinetic parameters is expected when substrate concentration is increased sufficiently?

  1. Vmax is unchanged and apparent Km increases. Correct answer.
  2. Vmax increases and Km decreases.
  3. Both Vmax and Km decrease.
  4. Maximum velocity (Vmax) decreases and Km is unchanged.

Explanation

A competitive inhibitor binds the active site, so higher substrate concentrations can outcompete it, leaving Vmax unchanged. However, more substrate is needed to reach half-maximal velocity, so the apparent Km increases.

Question 22Cellular and Molecular Biology

Consider a segment of a DNA template strand with the sequence 3'-TAC GGA TTC-5'. What mRNA sequence is transcribed from this template?

  1. 5'-AUG CCU AAG-3' Correct answer.
  2. 5'-UAC GGA UUC-3'
  3. 5'-ATG CCT AAG-3'
  4. 5'-AUG GGU AAC-3'

Explanation

RNA polymerase builds mRNA complementary and antiparallel to the template strand, substituting uracil for thymine. Complementing TAC GGA TTC gives AUG CCU AAG, read 5' to 3'.

Question 23Cellular and Molecular Biology

In a population of cells, a mutation inactivates a tumor suppressor gene. Which outcome is most consistent with the function of such genes?

  1. Oncogene activity is suppressed by increased protein function.
  2. Apoptosis is triggered more readily in damaged cells.
  3. DNA repair is enhanced, preventing mutation accumulation.
  4. Cell division becomes unchecked because a brake on the cell cycle is lost. Correct answer.

Explanation

Tumor suppressor genes normally restrain cell division, repair DNA, or promote apoptosis when damage is severe. Losing function removes a brake on proliferation, allowing unchecked division.

Question 24Cellular and Molecular Biology

In the electron transport chain, cyanide binds to cytochrome c oxidase and blocks electron transfer to oxygen. What is the immediate consequence for oxidative phosphorylation?

  1. Glycolysis stops immediately because it requires oxygen directly.
  2. NADH production increases because the chain is blocked.
  3. The proton gradient collapses and ATP synthesis decreases. Correct answer.
  4. ATP synthase continues to produce ATP at the normal rate.

Explanation

Blocking electron flow to oxygen stops proton pumping, so the proton gradient that drives ATP synthase dissipates and ATP synthesis falls. Glycolysis does not use oxygen directly and can continue via fermentation.

Question 25Cellular and Molecular Biology

A segment of DNA undergoes a deletion of two nucleotides within a coding region. Which consequence is most likely?

  1. A single amino acid substitution only
  2. A silent mutation with no change to the polypeptide
  3. A frameshift that alters all downstream codons Correct answer.
  4. A lengthening of the chromosome by two nucleotides

Explanation

Deleting two nucleotides is not a multiple of three, so it shifts the reading frame from that point onward. All downstream codons are read differently, usually producing a truncated or nonfunctional protein.

Question 26Cellular and Molecular Biology

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

  1. The Golgi apparatus
  2. The smooth endoplasmic reticulum
  3. The mitochondrion Correct answer.
  4. The lysosome

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Other organelles listed perform sorting, digestion, or lipid synthesis rather than bulk ATP production.

Question 27Cellular and Molecular Biology

A ribosome is best described as a structure that performs which function?

  1. Transcribing RNA from a DNA template
  2. Synthesizing polypeptides using mRNA as a template Correct answer.
  3. Replicating DNA during S phase
  4. Packaging proteins into secretory vesicles

Explanation

Ribosomes translate the codon sequence of mRNA into an amino acid chain, forming peptide bonds between successive amino acids. DNA replication and transcription are carried out by polymerases, and vesicle packaging occurs at the endoplasmic reticulum and Golgi.

Question 28Cellular and Molecular Biology

Which molecule serves as the immediate energy currency that powers most endergonic cellular reactions?

  1. Glucose
  2. Cholesterol
  3. ATP Correct answer.
  4. NADPH

Explanation

Hydrolysis of ATP to ADP and inorganic phosphate releases free energy that can be coupled to endergonic reactions. Glucose and NADPH store or carry energy but must first be processed to drive most reactions directly.

Question 29Cellular and Molecular Biology

A student examines a cell and finds a nucleus, mitochondria, and a cell wall but no chloroplasts. Which cell type is most consistent with these observations?

  1. An animal liver cell
  2. A cyanobacterium
  3. A plant mesophyll cell
  4. A fungal cell Correct answer.

Explanation

Fungi are eukaryotes with a nucleus and mitochondria and possess a chitin cell wall, but they lack chloroplasts. Animal cells lack a cell wall, cyanobacteria are prokaryotes, and mesophyll cells contain chloroplasts.

Question 30Cellular and Molecular Biology

Which nitrogenous base is found in RNA but not in DNA?

  1. Guanine
  2. Adenine
  3. Uracil Correct answer.
  4. Thymine

Explanation

RNA contains uracil in place of the thymine found in DNA, while adenine and guanine occur in both nucleic acids. The uracil-thymine difference reflects the distinct chemical roles of the two nucleic acids.

Question 31Cellular and Molecular Biology

Which type of bond links adjacent amino acids within a polypeptide chain?

  1. A glycosidic bond
  2. A phosphodiester bond
  3. A hydrogen bond
  4. A peptide bond Correct answer.

Explanation

A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water. Phosphodiester bonds join nucleotides, and glycosidic bonds join sugars to other groups.

Question 32Cellular and Molecular Biology

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

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

Explanation

Osmosis is the diffusion of water down its own concentration gradient across a selectively permeable membrane. It requires no ATP and is driven by differences in solute concentration between compartments.

Question 33Cellular and Molecular Biology

A DNA molecule contains 30% adenine. According to complementary base-pairing rules, what percentage of its bases is guanine?

  1. 70%
  2. 20% Correct answer.
  3. 30%
  4. 40%

Explanation

Adenine pairs with thymine, so thymine is also 30%, leaving 40% of the bases to be shared equally by guanine and cytosine. Each of those therefore accounts for 20% of the total.

Question 34Cellular and Molecular Biology

A cell is treated with a drug that blocks the function of ribosomes. Which process is most directly affected?

  1. Transcription of DNA into mRNA
  2. Replication of DNA before mitosis
  3. Translation of mRNA into protein Correct answer.
  4. Formation of the mitotic spindle

Explanation

Ribosomes are the machines of translation, so blocking them halts polypeptide synthesis. Transcription, replication, and spindle assembly depend on RNA polymerase, DNA polymerase, and microtubules, respectively.

Question 35Cellular and Molecular Biology

An enzyme is studied at increasing substrate concentrations until the reaction rate levels off. Which statement explains why the plateau occurs?

  1. All active sites are occupied, so the enzyme is saturated Correct answer.
  2. The substrate has been completely consumed
  3. The enzyme has been denatured by the substrate
  4. The reaction has reversed direction

Explanation

At saturation every enzyme active site is bound to substrate, so adding more substrate cannot increase the rate further. The maximum velocity reflects enzyme amount and catalytic efficiency rather than substrate depletion.

Question 36Cellular and Molecular Biology

A mutation changes a codon for glutamate into a codon for valine in the active site of an enzyme. Which effect is most likely?

  1. The enzyme will acquire a new active site identical to valine's structure
  2. The enzyme's three-dimensional shape and activity may be altered Correct answer.
  3. The enzyme will be completely degraded by the ribosome
  4. The enzyme will synthesize DNA instead of catalyzing its normal reaction

Explanation

Substituting a charged amino acid with a hydrophobic one can change folding and interactions in the active site, potentially altering catalysis. The primary sequence determines the tertiary structure on which function depends.

Question 37Cellular and Molecular Biology

A drug increases the permeability of the inner mitochondrial membrane to protons. What is the most likely consequence for ATP production?

  1. ATP synthesis increases because oxygen consumption doubles
  2. ATP synthesis increases because more protons reach ATP synthase
  3. ATP synthesis decreases because the proton gradient is dissipated Correct answer.
  4. ATP synthesis is unchanged because substrate-level phosphorylation compensates

Explanation

Oxidative phosphorylation depends on a proton gradient across the inner membrane. Uncoupling agents let protons leak back without passing through ATP synthase, so the gradient shrinks and ATP output falls even though electron transport may accelerate.

Question 38Cellular and Molecular Biology

During DNA replication, which enzyme joins Okazaki fragments by forming phosphodiester bonds between them?

  1. DNA helicase
  2. Topoisomerase
  3. RNA primase
  4. DNA ligase Correct answer.

Explanation

DNA ligase seals the nicks between adjacent Okazaki fragments on the lagging strand. Helicase unwinds the duplex, primase lays down RNA primers, and topoisomerase relieves supercoiling.

Question 39Cellular and Molecular Biology

A segment of mRNA has the sequence 5'-AUG UUU UAA-3'. How many amino acids does the resulting polypeptide contain?

  1. Three
  2. Zero
  3. Two Correct answer.
  4. One

Explanation

AUG is the start codon and also codes for methionine, UUU codes for phenylalanine, and UAA is a stop codon that terminates translation without adding an amino acid. The polypeptide therefore contains two amino acids.

Question 40Cellular and Molecular Biology

Which statement best explains why the sodium-potassium pump is classified as an electrogenic pump?

  1. It moves equal numbers of sodium and potassium ions across the membrane
  2. It moves three sodium ions out and two potassium ions in, creating a net charge difference Correct answer.
  3. It uses the sodium gradient to drive glucose transport
  4. It transports sodium and potassium down their concentration gradients

Explanation

Because the pump exports three positive charges for every two it imports, it contributes to the membrane potential, making it electrogenic. It also maintains the ion gradients used in secondary active transport.

Question 41Cellular and Molecular Biology

A cell that normally divides by mitosis is treated with a drug that prevents spindle microtubule assembly. At which point will division most likely arrest?

  1. Prophase, because chromosomes cannot condense
  2. Telophase, because nuclei cannot reform
  3. S phase, because DNA cannot replicate
  4. Metaphase, because chromosomes cannot align at the equator Correct answer.

Explanation

Spindle microtubules attach to kinetochores and align chromosomes at the metaphase plate, so their absence triggers a spindle checkpoint arrest. Chromosome condensation and DNA replication do not depend on spindle assembly.

Question 42Cellular and Molecular Biology

In a eukaryotic cell, a protein destined for secretion is synthesized on ribosomes attached to which structure?

  1. The rough endoplasmic reticulum Correct answer.
  2. The smooth endoplasmic reticulum
  3. Mitochondrial ribosomes
  4. Free ribosomes in the cytosol

Explanation

A signal peptide directs the ribosome to the rough endoplasmic reticulum, where the polypeptide is threaded into the lumen for folding and later transport through the Golgi. Free ribosomes make cytosolic proteins, and mitochondrial ribosomes make mitochondrial proteins.

Question 43Cellular and Molecular Biology

A cell in a hypotonic solution swells. Which feature of the plasma membrane best explains why the cell eventually bursts rather than shrinking?

  1. Water enters faster than it can be expelled because the membrane cannot withstand the pressure Correct answer.
  2. The membrane becomes freely permeable to large solutes
  3. The membrane actively pumps water inward
  4. The membrane begins to synthesize new lipids rapidly

Explanation

In a hypotonic environment water moves inward by osmosis, increasing turgor pressure. Without a rigid wall to oppose that pressure, the membrane stretches and eventually ruptures.

Question 44Cellular and Molecular Biology

Which change would most likely increase the rate of an enzyme-catalyzed reaction within its optimal temperature range?

  1. Lowering the temperature slightly, reducing molecular motion
  2. Removing the cofactor required for catalysis
  3. Adding a competitive inhibitor that binds the active site
  4. Raising the temperature slightly, increasing molecular collisions Correct answer.

Explanation

Within the optimal range, a modest temperature increase raises kinetic energy and the frequency of effective enzyme-substrate collisions. Beyond the optimum, however, denaturation reduces activity.

Question 45Cellular and Molecular Biology

A mutation destroys the function of a spliceosome in a eukaryotic cell. Which outcome is most directly expected?

  1. Introns remain in the mature mRNA transcript Correct answer.
  2. tRNA molecules fail to bind amino acids
  3. The ribosome cannot assemble its two subunits
  4. DNA replication stalls at the replication fork

Explanation

Spliceosomes remove introns and join exons during RNA processing, so their loss leaves introns in the transcript. Replication, aminoacylation of tRNA, and ribosomal assembly depend on other machinery.

Question 46Cellular and Molecular Biology

In a eukaryotic cell, a mutation in a gene encoding a DNA repair enzyme leads to accumulation of point mutations. Which process is most directly impaired by this defect?

  1. Synthesis of the ribosomal subunits
  2. Correction of mismatched or damaged bases in DNA Correct answer.
  3. Formation of the mitotic spindle
  4. Movement of vesicles along microtubules

Explanation

DNA repair enzymes detect and correct mismatched or chemically damaged bases, preventing mutations from becoming fixed. Spindle formation, ribosome synthesis, and vesicle transport do not depend on these repair enzymes.

Question 47Cellular and Molecular Biology

A biochemist finds that an enzyme's reaction rate is unchanged when substrate concentration increases but falls when a competitive inhibitor is added. Which interpretation is most consistent with these results?

  1. The enzyme has no active site and the inhibitor binds DNA
  2. The enzyme has reached its maximum velocity and the inhibitor is noncompetitive
  3. The enzyme is denatured and the inhibitor stabilizes it
  4. The enzyme is already saturated and the inhibitor competes for the active site Correct answer.

Explanation

A rate that is insensitive to added substrate indicates saturation of active sites. A competitive inhibitor binds the same active site, and its effect can be overcome only by raising substrate enough to outcompete it, consistent with the observed pattern.

Question 48Cellular and Molecular Biology

A cell is exposed to a toxin that blocks the Krebs cycle enzyme that converts pyruvate to acetyl-CoA. Which consequence is most likely?

  1. Loss of the proton gradient across the inner mitochondrial membrane
  2. Immediate failure of the electron transport chain to use oxygen
  3. Reduced NADH production and impaired entry of carbon into the Krebs cycle Correct answer.
  4. Increased ATP yield from glycolysis because pyruvate accumulates

Explanation

The pyruvate dehydrogenase reaction links glycolysis to the Krebs cycle and generates NADH. Blocking it restricts acetyl-CoA supply to the cycle, lowering NADH and FADH2 output for oxidative phosphorylation.

Question 49Cellular and Molecular Biology

A frameshift mutation inserts one nucleotide near the start of a coding sequence. Which outcome is most likely for the resulting polypeptide?

  1. The mRNA is degraded before translation begins
  2. Translation terminates immediately at the insertion site
  3. The reading frame shifts, producing a different amino acid sequence downstream Correct answer.
  4. A single amino acid is substituted with no downstream changes

Explanation

Inserting one nucleotide shifts the triplet reading frame, so all downstream codons are read differently and the protein is usually nonfunctional. A single substitution would alter only one codon, and the mRNA is not necessarily degraded.

Question 50Cellular and Molecular Biology

A cell is treated with a drug that inhibits the enzyme responsible for adding the poly-A tail to mRNA. Which outcome is most directly expected?

  1. Failure of the ribosome to bind the mRNA
  2. Loss of the 5' cap structure
  3. Inability of RNA polymerase to initiate transcription
  4. Reduced stability and altered export of the mRNA transcript Correct answer.

Explanation

The poly-A tail protects the transcript from degradation and assists nuclear export and translation. Its loss reduces mRNA stability and can impair export, while capping and transcription initiation are separate steps.