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BTL2 practice questions

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Question 1Cell Biology and Biochemistry

Which organelle is the principal site of ATP production by oxidative phosphorylation in a eukaryotic cell?

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together couple proton pumping to ATP synthesis. Other organelles perform specialized tasks such as modification, degradation, or lipid synthesis rather than bulk oxidative ATP generation.

Question 2Cell Biology and Biochemistry

Which type of bond links the two strands of a DNA double helix together?

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

Explanation

Complementary bases on opposing strands pair through hydrogen bonds: adenine with thymine forms two, and guanine with cytosine forms three. These relatively weak bonds allow strand separation during replication and transcription while collectively stabilizing the duplex.

Question 3Cell Biology and Biochemistry

A researcher treats cells with a drug that blocks the assembly of microtubules. Which process would be most directly impaired?

  1. Synthesis of phospholipids in the smooth ER
  2. Formation of the mitotic spindle Correct answer.
  3. Proton pumping across the inner mitochondrial membrane
  4. Translation of mRNA on ribosomes

Explanation

The mitotic spindle is built from microtubules, so preventing tubulin polymerization arrests chromosome segregation. Translation, lipid synthesis, and mitochondrial proton pumping depend on ribosomes, ER enzymes, and respiratory complexes rather than on the microtubule cytoskeleton.

Question 4Cell Biology and Biochemistry

Which amino acid is specified by the start codon AUG during translation initiation in eukaryotes?

  1. Valine
  2. Tryptophan
  3. Leucine
  4. Methionine Correct answer.

Explanation

AUG serves both as the initiation codon and as a codon for methionine, so the first residue incorporated is methionine. In bacteria this residue is formylated, but the amino acid identity remains methionine.

Question 5Cell Biology and Biochemistry

Which statement best describes the role of the sodium-potassium pump in maintaining the resting membrane potential?

  1. It passively allows both ions to diffuse down their concentration gradients.
  2. It moves equal numbers of sodium and potassium ions in opposite directions.
  3. It moves three sodium ions out and two potassium ions in per ATP hydrolyzed. Correct answer.
  4. It moves three potassium ions out and two sodium ions in per ATP hydrolyzed.

Explanation

The pump is an ATPase that exports three sodium ions and imports two potassium ions per cycle, leaving the interior with a net negative charge. This electrogenic stoichiometry, together with potassium leak channels, helps establish the resting potential.

Question 6Cell Biology and Biochemistry

Which molecule serves as the primary energy currency that directly drives most endergonic cellular reactions?

  1. ATP Correct answer.
  2. FADH2
  3. Glucose
  4. NADH

Explanation

Hydrolysis of the terminal phosphoanhydride bond of ATP releases free energy that is coupled to many endergonic processes. NADH and FADH2 are electron carriers that feed the respiratory chain rather than direct donors to most reactions.

Question 7Cell Biology and Biochemistry

In eukaryotic cells, which enzyme is responsible for transcribing messenger RNA from a protein-coding gene?

  1. RNA polymerase III
  2. RNA polymerase II Correct answer.
  3. DNA polymerase III
  4. RNA polymerase I

Explanation

RNA polymerase II transcribes protein-coding genes to produce mRNA precursors, which are then processed by capping, splicing, and polyadenylation. RNA polymerase I makes ribosomal RNA and RNA polymerase III makes transfer RNA and small RNAs.

Question 8Cell Biology and Biochemistry

Which property of the lipid bilayer allows small nonpolar molecules such as oxygen to cross membranes readily?

  1. The presence of integral membrane proteins
  2. The hydrophobic core formed by fatty acid tails Correct answer.
  3. The hydrophilic phosphate head groups
  4. The attached carbohydrate chains of glycolipids

Explanation

The interior of the bilayer is a hydrophobic zone of fatty acid tails, and nonpolar gases dissolve in and diffuse through it without a transporter. Polar and charged solutes are excluded by this same barrier and require channels or carriers.

Question 9Cell Biology and Biochemistry

During glycolysis, one molecule of glucose is converted to two molecules of pyruvate. What is the net yield of ATP and NADH per glucose?

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

Explanation

Four ATP are produced by substrate-level phosphorylation, but two ATP are consumed in the preparatory phase, giving a net of two. Two NAD+ are reduced to NADH by glyceraldehyde-3-phosphate dehydrogenase, so two NADH are formed per glucose.

Question 10Cell Biology and Biochemistry

A mutation removes the signal recognition particle receptor from the endoplasmic reticulum membrane. Which outcome is most likely for a secreted protein?

  1. The protein is translated directly into the mitochondrial matrix.
  2. The protein is translated but fails to be targeted efficiently to the ER. Correct answer.
  3. The protein is degraded before translation begins.
  4. The protein is exported through the nuclear pore complex.

Explanation

The SRP receptor docks the SRP-ribosome complex onto the ER translocon; without it, the complex cannot engage the membrane efficiently and the protein is not delivered to the ER lumen. Secretory proteins would then accumulate in the cytosol rather than enter the secretory pathway.

Question 11Cell Biology and Biochemistry

Which feature of the genetic code explains why a single base substitution often does not change the encoded amino acid?

  1. The code is non-overlapping.
  2. The code contains a single start codon.
  3. The code is degenerate. Correct answer.
  4. The code is read in the 3' to 5' direction.

Explanation

Degeneracy means most amino acids are specified by more than one codon, so many third-position changes are silent. Non-overlapping reading and unidirectional reading are also features of the code but do not themselves buffer against substitutions.

Question 12Cell Biology and Biochemistry

In the citric acid cycle, which enzyme catalyzes the oxidative decarboxylation of isocitrate while reducing NAD+?

  1. Aconitase
  2. Citrate synthase
  3. Isocitrate dehydrogenase Correct answer.
  4. Malate dehydrogenase

Explanation

Isocitrate dehydrogenase converts isocitrate to alpha-ketoglutarate, releasing CO2 and reducing NAD+ to NADH. This step is one of the regulated, rate-limiting reactions of the cycle.

Question 13Cell Biology and Biochemistry

A cell is placed in a hypertonic solution. What immediate osmotic response is expected?

  1. Solutes move into the cell until isotonicity is reached.
  2. There is no net water movement across the membrane.
  3. Water enters the cell and it swells.
  4. Water leaves the cell and it shrinks. Correct answer.

Explanation

In a hypertonic environment the external solute concentration exceeds that inside, so water moves outward along its osmotic gradient. The cell loses volume and shrinks, a process called crenation in cells with rigid walls or shrinking in animal cells.

Question 14Cell Biology and Biochemistry

Which statement correctly distinguishes competitive from noncompetitive enzyme inhibition?

  1. A competitive inhibitor binds the active site and its effect can be overcome by raising substrate concentration. Correct answer.
  2. A noncompetitive inhibitor binds the active site and is outcompeted by excess substrate.
  3. A competitive inhibitor binds an allosteric site and lowers Vmax irreversibly.
  4. A noncompetitive inhibitor increases the apparent affinity of the enzyme for its substrate.

Explanation

Competitive inhibitors resemble the substrate and occupy the active site, so higher substrate concentrations outcompete them and restore maximal velocity. Noncompetitive inhibitors bind elsewhere and typically reduce Vmax without being reversed by added substrate.

Question 15Cell Biology and Biochemistry

Which process produces the greatest yield of ATP per molecule of glucose in an aerobic eukaryotic cell?

  1. Oxidative phosphorylation Correct answer.
  2. Anaerobic fermentation
  3. Substrate-level phosphorylation in glycolysis
  4. The pentose phosphate pathway

Explanation

Oxidative phosphorylation converts the NADH and FADH2 generated by glycolysis, pyruvate oxidation, and the citric acid cycle into a large ATP yield. Fermentation and glycolysis regenerate far less ATP per glucose, and the pentose phosphate pathway mainly produces reducing power and precursors.

Question 16Cell Biology and Biochemistry

During DNA replication, which activity of DNA polymerase removes the RNA primer and replaces it with DNA?

  1. 3' to 5' exonuclease activity
  2. Helicase activity
  3. Ligase activity
  4. 5' to 3' exonuclease activity Correct answer.

Explanation

The 5' to 3' exonuclease activity allows DNA polymerase I to degrade the RNA primer ahead of synthesis while extending the new DNA strand. The 3' to 5' exonuclease activity instead acts as a proofreading function that removes misincorporated nucleotides.

Question 17Cell Biology and Biochemistry

A frameshift mutation inserts a single nucleotide near the beginning of a coding sequence. What is the most likely consequence for the protein product?

  1. The reading frame shifts, producing a different and often truncated protein. Correct answer.
  2. A single amino acid is substituted with no downstream effect.
  3. The mRNA fails to be transcribed from the gene.
  4. The mutation is always silent because the code is degenerate.

Explanation

Inserting one nucleotide changes the grouping of all downstream codons, so the amino acid sequence is altered from that point onward. Such shifts often introduce premature stop codons and yield nonfunctional or truncated proteins.

Question 18Cell Biology and Biochemistry

Which component of the cytoskeleton is principally responsible for producing the contractile force during cytokinesis in animal cells?

  1. Intermediate filaments at desmosomes
  2. Microtubules forming the spindle midzone
  3. Actin filaments forming the contractile ring Correct answer.
  4. Lamin filaments lining the nuclear envelope

Explanation

A ring of actin filaments and myosin II assembles at the cell equator and constricts to divide the cytoplasm. Microtubules position the furrow, but the contractile force itself comes from the actomyosin ring.

Question 19Cell Biology and Biochemistry

Which feature of the mitochondrial inner membrane directly enables chemiosmotic ATP synthesis?

  1. Its content of hydrolytic enzymes that degrade damaged proteins
  2. Its ability to synthesize lipids independently of the ER
  3. Its impermeability to protons, which allows a gradient to be stored Correct answer.
  4. Its high permeability to protons in the absence of ATP synthase

Explanation

The inner membrane is largely impermeable to protons, so respiratory complexes can build an electrochemical gradient across it. Protons then return through ATP synthase, and the free energy of that flow drives ATP production.

Question 20Cell Biology and Biochemistry

Which post-translational modification typically targets cytosolic proteins for degradation by the proteasome?

  1. Addition of a long O-linked glycan
  2. Phosphorylation of a single serine residue
  3. Cleavage of the initiator methionine
  4. Attachment of a polyubiquitin chain Correct answer.

Explanation

Lysine-linked polyubiquitin chains are recognized by the proteasome, which unfolds and degrades the tagged protein. Glycosylation, phosphorylation, and methionine removal regulate other processes and do not by themselves mark proteins for proteasomal destruction.

Question 21Cell Biology and Biochemistry

A culture of cells is shifted from 37 degrees Celsius to 20 degrees Celsius. How does this change most directly affect membrane fluidity?

  1. Fluidity increases because acyl chains become more ordered.
  2. Fluidity is unaffected because temperature does not influence lipid motion.
  3. Fluidity decreases because phospholipid acyl chains pack more tightly. Correct answer.
  4. Fluidity decreases because cholesterol is fully removed from the bilayer.

Explanation

Lower temperature reduces thermal motion of acyl chains, which pack more closely and make the bilayer less fluid. Cells often compensate over time by increasing unsaturated fatty acid content, but the immediate physical effect is decreased fluidity.

Question 22Cell Biology and Biochemistry

Which molecular event converts a proto-oncogene into an oncogene in a way that produces a constitutively active protein?

  1. Methylation of CpG islands in the gene promoter
  2. Deletion of both alleles of the gene
  3. A point mutation that changes a regulatory phosphorylation site in the encoded protein Correct answer.
  4. Insertion of a premature stop codon in the coding region

Explanation

Gain-of-function point mutations can lock a signaling protein in its active state, as seen with certain RAS alleles. Oncogenic activation is dominant and increases or deregulates activity, whereas deletions, promoter methylation, and nonsense codons generally reduce or eliminate function.

Question 23Cell Biology and Biochemistry

An enzyme follows Michaelis-Menten kinetics with Km of 5 mM. At a substrate concentration of 15 mM, what fraction of Vmax is observed?

  1. One half
  2. One third
  3. Three quarters Correct answer.
  4. One quarter

Explanation

Substituting into the Michaelis-Menten rate law gives v/Vmax equal to 15 divided by the sum of 15 and 5, which is 0.75, or three quarters of Vmax. Km equals the substrate concentration giving half-maximal velocity, so tripling Km raises the fraction to three quarters.

Question 24Cell Biology and Biochemistry

A cell lacks functional telomerase yet continues dividing. Which consequence is expected for its chromosomes after many divisions?

  1. Telomeres remain unchanged because replication is fully end-replicative.
  2. Chromosomes accumulate extra telomeric repeats and become longer.
  3. Chromosomes progressively shorten at their ends, eventually triggering a growth arrest. Correct answer.
  4. Chromosome ends fuse permanently without any loss of sequence.

Explanation

DNA polymerase cannot fully replicate chromosome ends, so without telomerase the telomeres erode with each cycle. When telomeres become critically short, a DNA damage response is triggered that leads to senescence or crisis.

Question 25Cell Biology and Biochemistry

A drug raises the Km of an enzyme for its substrate without changing Vmax. What effect does this have on enzyme behavior?

  1. The catalytic rate constant of the enzyme increases proportionally.
  2. The apparent affinity for substrate decreases, so more substrate is needed to reach half-maximal velocity. Correct answer.
  3. The apparent affinity for substrate increases, so less substrate is needed for half-maximal velocity.
  4. The enzyme becomes irreversibly inactivated after one catalytic cycle.

Explanation

Km is inversely related to apparent substrate affinity, so a higher Km means weaker binding and a higher substrate concentration required for half-maximal rate. Unchanged Vmax indicates that the turnover capacity at saturating substrate is preserved.

Question 26Cell Biology and Biochemistry

Which bond links two adjacent amino acids in a polypeptide chain?

  1. Hydrogen bond between the carboxyl carbon and the amino nitrogen
  2. Peptide bond between the carboxyl carbon and the amino nitrogen Correct answer.
  3. Phosphodiester bond between the carboxyl carbon and the amino nitrogen
  4. Glycosidic bond between the carboxyl carbon and the amino nitrogen

Explanation

A peptide bond forms by condensation between the alpha-carboxyl group of one amino acid and the alpha-amino group of the next, releasing water. The resulting amide linkage is planar and partially double-bonded, which restricts rotation. Phosphodiester, glycosidic, and hydrogen bonds serve other roles and do not covalently join residues in a chain.

Question 27Cell Biology and Biochemistry

A DNA sample contains 30% adenine. What percentage of its bases is guanine?

  1. 40%
  2. 20% Correct answer.
  3. 30%
  4. 10%

Explanation

By complementary base pairing, adenine equals thymine, so thymine is also 30%, accounting for 60% of bases. The remaining 40% is split equally between guanine and cytosine, giving 20% guanine. This follows directly from Chargaff's rules for double-stranded DNA.

Question 28Cell Biology and Biochemistry

Which organelle is the principal site of lipid synthesis and detoxification of drugs in eukaryotic cells?

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

Explanation

The smooth endoplasmic reticulum lacks ribosomes and houses enzymes for phospholipid and steroid synthesis as well as cytochrome P450 systems that modify xenobiotics. The Golgi apparatus mainly sorts and modifies proteins, lysosomes degrade material, and the nucleolus assembles ribosomal subunits.

Question 29Cell Biology and Biochemistry

Which statement describes the role of the sodium-potassium pump in maintaining the resting membrane potential?

  1. It imports three sodium ions and exports two potassium ions per ATP hydrolyzed
  2. It moves sodium and potassium down their gradients without consuming ATP
  3. It exports three sodium ions and imports two potassium ions per ATP hydrolyzed Correct answer.
  4. It exports equal numbers of sodium and potassium ions per ATP hydrolyzed

Explanation

The Na+/K+-ATPase uses energy from ATP hydrolysis to move three Na+ out and two K+ in against their gradients. This unequal exchange leaves the cytosol with a net deficit of positive charge and contributes to the negative resting potential. Because the transport is electrogenic, it adds to the charge separation set up by ion leak channels.

Question 30Cell Biology and Biochemistry

A competitive inhibitor is added to an enzyme reaction. Which kinetic change is observed when substrate concentration is increased sufficiently?

  1. Both Km and Vmax fall proportionally
  2. Vmax falls and Km is unchanged
  3. Km increases and Vmax can be reached at high substrate Correct answer.
  4. Vmax increases and Km decreases

Explanation

A competitive inhibitor binds the free enzyme at the active site, so more substrate is needed to reach half-maximal velocity, raising the apparent Km. Because the inhibition is reversible and surmountable, sufficiently high substrate displaces the inhibitor and the same Vmax is attainable. Noncompetitive inhibition instead lowers Vmax without altering Km.

Question 31Cell Biology and Biochemistry

Which molecule is the final electron acceptor of the electron transport chain in aerobic respiration?

  1. NAD+
  2. Molecular oxygen Correct answer.
  3. Cytochrome c
  4. FAD

Explanation

Complex IV transfers electrons to molecular oxygen, which is reduced to water, making oxygen the terminal acceptor of the chain. NAD+ and FAD are electron carriers that deliver reducing equivalents earlier in the pathway. Cytochrome c is a mobile electron shuttle between complexes, not the terminal acceptor.

Question 32Cell Biology and Biochemistry

Which phase of the cell cycle is characterized by DNA replication?

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

Explanation

DNA synthesis occurs during S phase, when each chromosome is duplicated into two sister chromatids. G1 and G2 are gap phases that prepare for and follow replication, while M phase carries out chromosome segregation and division. The order is G1, S, G2, then M.

Question 33Cell Biology and Biochemistry

Which statement best describes the function of a ribosome during translation?

  1. It synthesizes the tRNA molecules used in translation
  2. It catalyzes formation of peptide bonds between amino acids held on tRNAs Correct answer.
  3. It transports folded proteins into the endoplasmic reticulum
  4. It covalently attaches amino acids directly to mRNA codons

Explanation

The ribosome is a ribozyme whose rRNA catalyzes peptidyl transfer, joining the amino acid on the A-site tRNA to the growing chain on the P-site tRNA. tRNAs, not the ribosome, pair anticodons with mRNA codons. Protein import into the endoplasmic reticulum is handled by the signal recognition particle and translocon.

Question 34Cell Biology and Biochemistry

An enzyme has a Km of 2 mM. At a substrate concentration of 6 mM, what fraction of Vmax is achieved, assuming Michaelis-Menten behavior?

  1. 0.75 Correct answer.
  2. 0.67
  3. 0.90
  4. 0.50

Explanation

The Michaelis-Menten equation gives v/Vmax equal to [S] divided by Km plus [S]. Substituting 6 mM over 2 plus 6 mM yields 6/8, which is 0.75. The reaction therefore runs at three-quarters of maximal velocity at this substrate level.

Question 35Cell Biology and Biochemistry

A mutation changes a single base in a codon so that it now specifies a different amino acid. What type of mutation is this?

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

Explanation

A missense mutation is a base substitution that alters a codon so that a different amino acid is incorporated. Silent mutations change the codon but not the amino acid, and nonsense mutations create a stop codon. Frameshift mutations arise from insertions or deletions that shift the reading frame.

Question 36Cell Biology and Biochemistry

Which property of water allows it to absorb considerable heat with little change in temperature, buffering cells against thermal fluctuations?

  1. Its nonpolar character
  2. Its high specific heat capacity Correct answer.
  3. Its low molecular weight
  4. Its low boiling point

Explanation

Water's extensive hydrogen-bonding network must be disrupted for temperature to rise, giving it a high specific heat capacity. This lets cells absorb or release heat with modest temperature change. Water is polar and has an unusually high, not low, boiling point for its size.

Question 37Cell Biology and Biochemistry

A protein is normally secreted from the cell. Which sequence feature directs its entry into the secretory pathway?

  1. A peroxisomal targeting signal
  2. An N-terminal signal peptide Correct answer.
  3. A mitochondrial targeting presequence
  4. A nuclear localization signal

Explanation

Secretory and membrane proteins carry an N-terminal signal peptide that is recognized by the signal recognition particle, which delivers the ribosome to the ER membrane. The protein enters the ER lumen and travels through the Golgi to the plasma membrane. Nuclear, mitochondrial, and peroxisomal signals route proteins to entirely different compartments.

Question 38Cell Biology and Biochemistry

Which type of RNA carries amino acids to the ribosome during protein synthesis?

  1. rRNA
  2. snRNA
  3. tRNA Correct answer.
  4. mRNA

Explanation

Transfer RNA molecules have an anticodon loop that pairs with mRNA codons and an acceptor end that is charged with the corresponding amino acid. mRNA carries the coding information, rRNA forms the catalytic core of the ribosome, and snRNA participates in splicing.

Question 39Cell Biology and Biochemistry

A cell is treated with a drug that blocks the proton gradient across the inner mitochondrial membrane. Which outcome is most direct?

  1. The citric acid cycle cannot oxidize acetyl-CoA
  2. Fatty acids cannot be activated to acyl-CoA
  3. Glycolysis stops immediately
  4. ATP synthase can no longer synthesize ATP Correct answer.

Explanation

ATP synthase uses the proton-motive force to drive rotation and ATP release, so collapsing the gradient uncouples oxidation from phosphorylation. Electron transport may continue but energy is released as heat instead of ATP. Glycolysis, the citric acid cycle, and fatty acid activation do not directly depend on the gradient.

Question 40Cell Biology and Biochemistry

Which structural feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Prokaryotes lack a plasma membrane
  2. Prokaryotes lack nucleic acids
  3. Prokaryotes lack ribosomes
  4. Prokaryotes lack a membrane-bound nucleus Correct answer.

Explanation

Prokaryotes keep their chromosome in a nucleoid region without a surrounding nuclear envelope, whereas eukaryotes enclose DNA in a nucleus. Prokaryotes still possess ribosomes, a plasma membrane, and DNA or RNA. The nuclear envelope is the defining structural difference.

Question 41Cell Biology and Biochemistry

Which mechanism allows a cell to move a substance against its concentration gradient without direct ATP hydrolysis at the transport protein?

  1. Facilitated diffusion
  2. Osmosis
  3. Secondary active transport Correct answer.
  4. Simple diffusion

Explanation

Secondary active transport couples uphill movement of one solute to the downhill movement of another ion whose gradient was established by an ATP-driven pump. The transporter itself does not hydrolyze ATP, but energy is ultimately supplied by the primary pump. Simple and facilitated diffusion move solutes down their gradients, and osmosis refers specifically to water movement.

Question 42Cell Biology and Biochemistry

Which feature of DNA replication ensures that each daughter molecule contains one parental and one newly synthesized strand?

  1. Dispersive replication
  2. Conservative replication
  3. Semiconservative replication Correct answer.
  4. Rolling-circle replication

Explanation

In semiconservative replication, the parental duplex unwinds and each strand serves as a template for a new complementary strand. Each daughter duplex therefore retains one original strand and one new strand. Conservative and dispersive models make different predictions about strand distribution, and rolling-circle replication is a specialized mechanism for certain plasmids and viruses.

Question 43Cell Biology and Biochemistry

Which enzyme removes introns and joins exons during processing of eukaryotic pre-mRNA?

  1. Spliceosome Correct answer.
  2. RNA polymerase II
  3. Ribonuclease P
  4. Reverse transcriptase

Explanation

The spliceosome is a large ribonucleoprotein complex that recognizes splice sites and catalyzes intron removal and exon ligation. RNA polymerase II synthesizes the pre-mRNA but does not splice it. Ribonuclease P processes tRNA precursors, and reverse transcriptase synthesizes DNA from an RNA template.

Question 44Cell Biology and Biochemistry

A protein has a pI of 5.0. At pH 7.0, what is the predominant net charge of the protein?

  1. Strongly negative Correct answer.
  2. Cannot be inferred from pI
  3. Strongly positive
  4. Exactly neutral

Explanation

When the pH is above a protein's isoelectric point, the surrounding solution is more basic than the protein's neutral state, so acidic groups deprotonate and the net charge becomes negative. At pH 7.0, well above a pI of 5.0, the protein carries a net negative charge. Neutrality occurs only near the pI itself.

Question 45Cell Biology and Biochemistry

Which process generates genetic diversity by exchanging segments between homologous chromosomes during meiosis?

  1. Crossing over Correct answer.
  2. Cytokinesis
  3. Mitotic recombination
  4. Nondisjunction

Explanation

Crossing over occurs during prophase I, when homologous chromatids exchange corresponding segments at chiasmata. This recombination produces new combinations of alleles on a chromosome. Nondisjunction is a segregation error that changes chromosome number, and cytokinesis divides the cytoplasm.

Question 46Cell Biology and Biochemistry

A cell with a constitutively active Ras protein is studied. Which consequence is expected?

  1. Apoptosis is triggered immediately after receptor activation
  2. DNA repair enzymes are recruited to double-strand breaks
  3. The cell cycle arrests permanently in G1
  4. Proliferation continues even in the absence of growth factor signaling Correct answer.

Explanation

Ras is a small GTPase that relays signals from receptor tyrosine kinases to downstream pathways such as MAPK. A constitutively active mutant remains GTP-bound and signals without upstream stimulation, driving continued proliferation. This is a classic oncogenic gain-of-function mechanism.

Question 47Cell Biology and Biochemistry

Which structural difference between DNA and RNA affects their stability in alkaline conditions?

  1. DNA lacks a phosphate backbone
  2. DNA contains uracil instead of thymine
  3. RNA has a 2'-hydroxyl group that promotes strand cleavage Correct answer.
  4. RNA is always double-stranded

Explanation

The ribose of RNA carries a 2'-hydroxyl that can attack the adjacent phosphodiester bond under alkaline conditions, leading to strand cleavage. DNA contains deoxyribose without this group, making it more resistant to base-catalyzed hydrolysis. RNA also uses uracil in place of thymine, but that substitution is not what drives alkaline instability.

Question 48Cell Biology and Biochemistry

A mitochondrial uncoupler such as 2,4-dinitrophenol is added to cells. Which metabolic change is expected?

  1. Both ATP synthesis and oxygen consumption fall
  2. ATP synthesis rises while oxygen consumption falls
  3. Electron transport speeds up while ATP synthesis falls Correct answer.
  4. Electron transport slows and oxygen consumption falls

Explanation

Uncouplers carry protons across the inner membrane, dissipating the gradient that ATP synthase requires. Because the proton-motive force falls, electron transport is released from back-pressure and accelerates, raising oxygen consumption while ATP output declines. Energy is dissipated as heat.

Question 49Cell Biology and Biochemistry

Which structural feature of chromatin most directly prevents transcription factors from accessing a gene's promoter?

  1. Tight wrapping of DNA around histone octamers in condensed nucleosomes Correct answer.
  2. Charging of tRNA with amino acids
  3. Presence of a 5' cap on the mRNA
  4. Attachment of the ribosome to the transcript

Explanation

Nucleosomes package DNA by wrapping it around histone octamers, and higher-order compaction restricts access of transcription factors and RNA polymerase to promoters. Chromatin remodeling and histone acetylation loosen this packing to permit transcription. The other options concern post-transcriptional or translational events.

Question 50Cell Biology and Biochemistry

A cell lacks functional lysosomes but has normal proteasome activity. Which process is most impaired?

  1. Translation of cytosolic proteins
  2. Synthesis of ATP in mitochondria
  3. Degradation of long-lived organelles and endocytosed material Correct answer.
  4. Ubiquitin-dependent degradation of short-lived cytosolic proteins

Explanation

Lysosomes contain acid hydrolases that degrade material delivered by endocytosis, phagocytosis, or autophagy, including worn organelles. The proteasome handles ubiquitin-tagged cytosolic proteins and remains functional here. ATP synthesis and translation are unrelated to lysosomal degradation.