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KBIT-2 practice questions

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Question 1Cell Biology and Biochemistry

In eukaryotic cells, which organelle is the primary site of ATP synthesis by oxidative phosphorylation?

  1. Peroxisome
  2. Mitochondrion Correct answer.
  3. Golgi apparatus
  4. Lysosome

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. Other organelles listed perform biosynthesis, degradation, or oxidative reactions but do not generate most cellular ATP.

Question 2Cell Biology and Biochemistry

Which structural feature distinguishes prokaryotic cells from eukaryotic cells?

  1. Presence of a plasma membrane
  2. Absence of a membrane-bound nucleus Correct answer.
  3. Presence of ribosomes
  4. Presence of DNA

Explanation

Prokaryotes lack a nuclear envelope, so their genetic material is not enclosed in a membrane-bound nucleus. Plasma membranes, ribosomes, and DNA are shared by both cell types and therefore do not distinguish them.

Question 3Cell Biology and Biochemistry

According to the fluid mosaic model, which component of the plasma membrane is primarily responsible for its selective permeability to small polar molecules?

  1. The phospholipid bilayer Correct answer.
  2. Cholesterol molecules
  3. Integral membrane proteins
  4. Peripheral membrane proteins

Explanation

The hydrophobic core of the phospholipid bilayer restricts the passage of polar and charged solutes, making the lipid bilayer the main permeability barrier. Proteins and cholesterol modulate membrane properties but do not form the continuous hydrophobic barrier itself.

Question 4Cell Biology and Biochemistry

Which enzyme catalyzes the rate-limiting step of glycolysis in most mammalian cells?

  1. Phosphofructokinase-1 Correct answer.
  2. Aldolase
  3. Hexokinase
  4. Pyruvate kinase

Explanation

Phosphofructokinase-1 catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, the committed and rate-limiting step of glycolysis. Hexokinase and pyruvate kinase also catalyze irreversible steps, but PFK-1 is the principal regulatory control point.

Question 5Cell Biology and Biochemistry

Which of the following correctly describes the direction of the central dogma of molecular biology?

  1. RNA to DNA to protein
  2. DNA to protein to RNA
  3. Protein to RNA to DNA
  4. DNA to RNA to protein Correct answer.

Explanation

The central dogma states that genetic information flows from DNA to RNA during transcription and from RNA to protein during translation. Reverse transcription and RNA replication are exceptions, but the standard pathway is DNA to RNA to protein.

Question 6Cell Biology and Biochemistry

Which stage of the cell cycle is characterized by the separation of sister chromatids?

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

Explanation

During anaphase, sister chromatids are pulled apart toward opposite poles of the cell after the spindle checkpoint is satisfied. Prophase, metaphase, and telophase involve chromosome condensation, alignment, and nuclear reformation, respectively.

Question 7Cell Biology and Biochemistry

In enzyme kinetics, what does the Michaelis constant (Km) represent?

  1. The inhibitor concentration that halves the reaction velocity
  2. The enzyme concentration at which the reaction is half-maximal
  3. The maximum reaction velocity
  4. The substrate concentration at which the reaction velocity is half of Vmax Correct answer.

Explanation

Km is defined as the substrate concentration at which the initial reaction velocity equals half of the maximum velocity, Vmax. It is an inverse indicator of enzyme affinity for substrate under the Michaelis-Menten model.

Question 8Cell Biology and Biochemistry

Which of the following amino acids is classified as an acidic amino acid at physiological pH?

  1. Arginine
  2. Histidine
  3. Lysine
  4. Glutamic acid Correct answer.

Explanation

Glutamic acid has a carboxyl side chain that is deprotonated and negatively charged at physiological pH, making it an acidic amino acid. Lysine, arginine, and histidine carry basic side chains that are protonated and positively charged or partially charged.

Question 9Cell Biology and Biochemistry

A cell is placed in a hypertonic solution. What is the expected net movement of water across the plasma membrane?

  1. Water moves into the cell, causing it to swell
  2. There is no net movement of water
  3. Water moves out of the cell, causing it to shrink Correct answer.
  4. Water moves equally in both directions with no change in cell volume

Explanation

In a hypertonic solution, the external solute concentration is higher than that inside the cell, creating an osmotic gradient that drives water out of the cell. The resulting loss of water causes the cell to shrink, a process sometimes called crenation in animal cells.

Question 10Cell Biology and Biochemistry

Which of the following statements about the sodium-potassium ATPase is correct?

  1. It moves three sodium ions into the cell and two potassium ions out per ATP hydrolyzed
  2. It moves sodium and potassium ions along their concentration gradients without consuming ATP
  3. It moves two sodium ions out of the cell and three potassium ions into the cell per ATP hydrolyzed
  4. It moves three sodium ions out of the cell and two potassium ions into the cell per ATP hydrolyzed Correct answer.

Explanation

The sodium-potassium ATPase is a P-type ATPase that exports three Na+ ions and imports two K+ ions per molecule of ATP hydrolyzed. This electrogenic transport maintains the resting membrane potential and the ionic gradients characteristic of animal cells.

Question 11Cell Biology and Biochemistry

During which phase of cellular respiration is the majority of ATP produced by oxidative phosphorylation?

  1. Pyruvate oxidation
  2. The electron transport chain and chemiosmosis Correct answer.
  3. Glycolysis
  4. The citric acid cycle

Explanation

The electron transport chain establishes a proton gradient that ATP synthase uses to produce ATP through chemiosmosis, yielding most of the ATP from glucose oxidation. Glycolysis and the citric acid cycle generate ATP mainly through substrate-level phosphorylation, which contributes a much smaller amount.

Question 12Cell Biology and Biochemistry

Which of the following best describes the role of the signal recognition particle (SRP) in protein targeting?

  1. It cleaves the signal peptide from the nascent polypeptide
  2. It binds to the signal peptide and directs the ribosome-nascent chain complex to the ER membrane Correct answer.
  3. It transports folded proteins through the nuclear pore complex
  4. It adds N-linked glycans to nascent polypeptides in the cytosol

Explanation

The SRP recognizes the N-terminal signal peptide on a nascent secretory protein and binds the ribosome-nascent chain complex, targeting it to the SRP receptor on the endoplasmic reticulum. This interaction allows the polypeptide to be translocated into the ER lumen or membrane through the translocon.

Question 13Cell Biology and Biochemistry

A competitive inhibitor is added to an enzyme-catalyzed reaction. Which kinetic parameter is affected?

  1. Vmax remains unchanged, and Km increases Correct answer.
  2. Both Vmax and Km increase
  3. Vmax decreases, and Km remains unchanged
  4. Both Vmax and Km decrease

Explanation

A competitive inhibitor binds reversibly to the free enzyme active site, raising the apparent Km because more substrate is needed to reach half-maximal velocity. Vmax is unchanged because the inhibitor can be outcompeted at sufficiently high substrate concentrations.

Question 14Cell Biology and Biochemistry

Which of the following correctly pairs a stage of cellular respiration with its subcellular location in a eukaryotic cell?

  1. Glycolysis: mitochondrial matrix
  2. Electron transport chain: inner mitochondrial membrane Correct answer.
  3. Citric acid cycle: cytosol
  4. Pyruvate oxidation: cytosol

Explanation

The electron transport chain is embedded in the inner mitochondrial membrane, where it generates the proton gradient used for ATP synthesis. Glycolysis occurs in the cytosol, whereas the citric acid cycle and pyruvate oxidation occur in the mitochondrial matrix.

Question 15Cell Biology and Biochemistry

During DNA replication, which enzyme is responsible for synthesizing RNA primers that provide a free 3'-OH group for DNA polymerase?

  1. DNA ligase
  2. Topoisomerase
  3. Primase Correct answer.
  4. Helicase

Explanation

Primase is an RNA polymerase that synthesizes short RNA primers complementary to the template strand, providing the 3'-OH required by DNA polymerase to begin synthesis. DNA ligase joins Okazaki fragments, helicase unwinds the double helix, and topoisomerase relieves supercoiling.

Question 16Cell Biology and Biochemistry

Which of the following statements about the genetic code is correct?

  1. The code is degenerate, meaning most amino acids are encoded by more than one codon Correct answer.
  2. The code is overlapping and read in groups of three bases
  3. The code is identical in the nucleus and mitochondria of all eukaryotes
  4. Each codon specifies more than one amino acid

Explanation

The genetic code is degenerate because multiple codons can encode the same amino acid, with the exception of methionine and tryptophan. It is non-overlapping and read in triplets, and mitochondrial genetic codes differ from the standard nuclear code in several organisms.

Question 17Cell Biology and Biochemistry

Which of the following best explains why the citric acid cycle is described as amphibolic?

  1. It produces only ATP and no reducing equivalents
  2. It is active only in the presence of oxygen
  3. It serves both catabolic and anabolic pathways Correct answer.
  4. It occurs in the cytosol of prokaryotes and mitochondria of eukaryotes

Explanation

The citric acid cycle is amphibolic because its intermediates are used both for oxidative catabolism to generate NADH and FADH2 and for anabolic biosynthesis of molecules such as amino acids and heme. This dual role distinguishes it from pathways that are exclusively catabolic or anabolic.

Question 18Cell Biology and Biochemistry

A mutation changes a codon from UAC to UAA. What is the most likely consequence for the encoded protein?

  1. No change in the amino acid sequence
  2. Premature termination of translation Correct answer.
  3. A single amino acid substitution
  4. A frameshift mutation

Explanation

UAA is a stop codon, so the change from UAC to UAA creates a nonsense mutation that terminates translation prematurely. This typically produces a truncated protein that may be nonfunctional.

Question 19Cell Biology and Biochemistry

Which of the following statements about the regulation of the cell cycle by cyclin-dependent kinases (CDKs) is correct?

  1. Cyclin levels oscillate, and cyclin binding activates CDKs at specific stages Correct answer.
  2. CDK activity is constant throughout the cell cycle
  3. CDKs are degraded by the proteasome at each cell cycle checkpoint
  4. CDK inhibitors promote cell cycle progression by activating CDKs

Explanation

Cyclin proteins accumulate and are degraded in a cell-cycle-dependent manner, and their binding to CDKs activates the kinases to drive progression through specific phases. CDK inhibitors, by contrast, block CDK activity and restrain cell cycle progression.

Question 20Cell Biology and Biochemistry

Which of the following best describes the role of the unfolded protein response (UPR) in the endoplasmic reticulum?

  1. It increases the folding capacity of the ER and reduces the load of unfolded proteins Correct answer.
  2. It transports proteins from the ER to the Golgi apparatus
  3. It degrades misfolded proteins in the cytosol
  4. It promotes apoptosis by activating caspases directly

Explanation

The UPR is a signaling network activated by the accumulation of unfolded or misfolded proteins in the ER. It increases the expression of chaperones and folding enzymes, enhances ER-associated degradation, and reduces protein synthesis to restore ER homeostasis.

Question 21Cell Biology and Biochemistry

Which of the following is a correct statement about the role of ubiquitin in protein degradation?

  1. Ubiquitin is added to proteins in the nucleus only
  2. Polyubiquitination targets proteins for degradation by the proteasome Correct answer.
  3. Ubiquitin is a protease that directly cleaves target proteins
  4. Monoubiquitination always targets proteins for proteasomal degradation

Explanation

Polyubiquitin chains, typically linked through lysine 48 of ubiquitin, mark proteins for recognition and degradation by the 26S proteasome. Monoubiquitination and other chain linkages often serve regulatory or sorting functions rather than proteasomal degradation.

Question 22Cell Biology and Biochemistry

Which of the following best explains how the sodium-glucose cotransporter (SGLT) couples glucose transport to the sodium gradient?

  1. It facilitates glucose diffusion down its concentration gradient without coupling to sodium
  2. It uses ATP hydrolysis directly to pump glucose against its gradient
  3. It uses the electrochemical sodium gradient to drive secondary active transport of glucose Correct answer.
  4. It moves glucose and sodium in opposite directions across the membrane

Explanation

SGLT proteins are secondary active transporters that use the inward sodium electrochemical gradient, maintained by the sodium-potassium ATPase, to drive glucose uptake against its concentration gradient. This coupling allows intestinal and renal epithelial cells to absorb glucose efficiently.

Question 23Cell Biology and Biochemistry

A patient with a defect in complex I of the mitochondrial electron transport chain would be expected to show which of the following metabolic consequences?

  1. Impaired oxidation of NADH and reduced proton pumping at complex I Correct answer.
  2. Increased ATP synthesis because electrons bypass complex I
  3. Accumulation of FADH2 with no effect on NADH oxidation
  4. Enhanced transfer of electrons from NADH to oxygen directly at complex I

Explanation

Complex I (NADH dehydrogenase) oxidizes NADH and transfers electrons to ubiquinone while pumping protons across the inner mitochondrial membrane. A defect in complex I therefore impairs NADH oxidation and reduces the proton gradient driving ATP synthesis.

Question 24Cell Biology and Biochemistry

In the context of the lac operon, which condition leads to maximal transcription of the lac genes?

  1. High glucose and low lactose
  2. Low glucose and high lactose Correct answer.
  3. High glucose and high lactose
  4. Low glucose and low lactose

Explanation

Maximal lac operon transcription requires both relief of repression by the lac repressor and activation by CRP-cAMP. High lactose relieves repression by binding the repressor, and low glucose raises cAMP levels, allowing CRP to activate transcription.

Question 25Cell Biology and Biochemistry

Which of the following best explains why the oxidation of one FADH2 yields fewer ATP molecules than the oxidation of one NADH?

  1. FADH2 contains fewer high-energy phosphate bonds than NADH
  2. FADH2 is transported across the inner mitochondrial membrane by an antiporter
  3. FADH2 transfers electrons to complex II, bypassing the proton-pumping sites of complex I Correct answer.
  4. FADH2 is oxidized directly by ATP synthase in the mitochondrial matrix

Explanation

FADH2 donates electrons to complex II (succinate dehydrogenase), which does not pump protons, so fewer protons are translocated per electron pair than when electrons enter at complex I via NADH. The reduced proton gradient results in a lower ATP yield for FADH2.

Question 26Cell Biology and Biochemistry

Which structural feature distinguishes a eukaryotic mRNA cap from a prokaryotic mRNA 5' end?

  1. An N-acetylglucosamine residue transferred from dolichol phosphate
  2. A polyadenine tail added to the 5' terminus by poly(A) polymerase
  3. A 7-methylguanosine joined to the transcript through an unusual 5'-5' triphosphate linkage Correct answer.
  4. A formyl group attached to the initiator methionine of the nascent polypeptide

Explanation

Eukaryotic mRNAs receive a 7-methylguanosine cap attached to the first transcribed nucleotide by a 5'-5' triphosphate bond, which is not a standard phosphodiester linkage. This cap is bound by cap-binding proteins such as eIF4E and is required for efficient translation initiation and mRNA stability. Prokaryotic transcripts are not capped in this way.

Question 27Cell Biology and Biochemistry

A single-point mutation in the gene encoding a cytosolic glycolytic enzyme changes a surface lysine to arginine. Which consequence is most consistent with the resulting change in net charge at pH 7.4?

  1. The enzyme becomes covalently attached to the inner mitochondrial membrane
  2. The protein's isoelectric point shifts to a more alkaline value, altering its behavior on ion-exchange chromatography Correct answer.
  3. The protein's isoelectric point shifts to a more acidic value, altering its behavior on ion-exchange chromatography
  4. The enzyme's catalytic rate constant necessarily doubles because arginine is a stronger nucleophile

Explanation

Lysine carries a positive charge at physiological pH, whereas arginine is also basic but has a higher side-chain pKa, so replacing lysine with arginine removes one titratable positive group that would otherwise be lost at higher pH; the net effect is a shift of the pI toward more alkaline values. This changes the protein's charge profile and therefore its retention on cation- and anion-exchange resins. The catalytic consequences are not predictable from charge alone.

Question 28Cell Biology and Biochemistry

In a cell treated with a drug that blocks the adenine nucleotide translocase of the inner mitochondrial membrane, which process is most directly impaired?

  1. Transfer of fatty acyl groups from carnitine to coenzyme A in the matrix
  2. Reduction of oxygen to water at cytochrome c oxidase
  3. Export of ATP from the matrix in exchange for cytosolic ADP Correct answer.
  4. Transfer of electrons from complex III to cytochrome c

Explanation

Adenine nucleotide translocase exchanges matrix ATP for cytosolic ADP across the inner mitochondrial membrane, so inhibiting it directly blocks ATP export and ADP import. Oxidative phosphorylation may continue briefly but becomes limited by substrate availability. The other listed steps use different carriers or complexes.

Question 29Cell Biology and Biochemistry

Which feature of the genetic code allows a single aminoacyl-tRNA synthetase to charge several tRNAs that bear different anticodons with the same amino acid?

  1. The degeneracy of the code combined with recognition of identity elements outside the anticodon Correct answer.
  2. The proofreading of the ribosome that removes incorrectly charged tRNAs before peptide bond formation
  3. The presence of a single tRNA species that reads all codons for that amino acid
  4. The wobble pairing between the third codon base and the first anticodon base

Explanation

Aminoacyl-tRNA synthetases recognize identity elements distributed throughout the tRNA, including the acceptor stem and anticodon loop, so multiple isoacceptor tRNAs can be charged by one enzyme. Degeneracy means several codons specify the same amino acid, and different tRNAs with distinct anticodons can carry it. The ribosome does not repair mischarged tRNAs; it can only reject some errors during selection.

Question 30Cell Biology and Biochemistry

A cultured hepatocyte is shifted to a medium containing an uncoupler of oxidative phosphorylation plus oligomycin, which blocks the ATP synthase proton channel. Which combination of effects on the mitochondrial proton gradient and on oxygen consumption is expected?

  1. The proton gradient collapses and oxygen consumption falls to near zero
  2. The proton gradient is maintained and oxygen consumption falls to near zero
  3. The proton gradient collapses and oxygen consumption increases Correct answer.
  4. The proton gradient is maintained and oxygen consumption increases

Explanation

An uncoupler carries protons across the inner membrane independently of ATP synthase, so the gradient dissipates and the proton-motive force falls. Because the gradient no longer restrains electron transport, oxygen consumption rises even though oligomycin has blocked ATP synthesis. The two agents thus act on different steps and their combined effect is a collapsed gradient with elevated respiration.

Question 31Genetics and Molecular Biology

In a DNA double helix, adenine on one strand pairs with which base on the opposite strand, and how many hydrogen bonds typically join them?

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

Explanation

Adenine forms a complementary pair with thymine in DNA, and this pair is stabilized by two hydrogen bonds. Guanine and cytosine pair with three hydrogen bonds, which is why GC-rich regions need more heat to separate. Uracil replaces thymine only in RNA, so it is not the standard DNA partner.

Question 32Genetics and Molecular Biology

Which enzyme is responsible for adding nucleotides to the growing DNA strand during replication by extending an existing primer?

  1. DNA polymerase Correct answer.
  2. Helicase
  3. DNA ligase
  4. Topoisomerase

Explanation

DNA polymerase catalyzes the addition of deoxynucleotides to the 3' end of a primer, building the new strand in the 5' to 3' direction. Ligase instead seals nicks between fragments, helicase unwinds the duplex, and topoisomerase relieves supercoiling ahead of the fork.

Question 33Genetics and Molecular Biology

A codon in messenger RNA consists of how many nucleotides, and how many amino acids does one codon specify?

  1. Four nucleotides specifying two amino acids
  2. Three nucleotides specifying three amino acids
  3. Three nucleotides specifying one amino acid Correct answer.
  4. Two nucleotides specifying one amino acid

Explanation

The genetic code is a triplet code: each group of three mRNA nucleotides is read as one codon that specifies a single amino acid or a stop signal. Because there are four bases, triplets yield 64 possible codons, enough to encode the 20 standard amino acids.

Question 34Genetics and Molecular Biology

Which type of RNA carries amino acids to the ribosome and pairs its anticodon with the mRNA codon?

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

Explanation

Transfer RNA folds into a cloverleaf-like structure with an anticodon loop that base-pairs with the mRNA codon and an acceptor end covalently linked to a specific amino acid. Ribosomal RNA forms the catalytic core of the ribosome, mRNA carries the coding sequence, and small nuclear RNA participates in splicing.

Question 35Genetics and Molecular Biology

Which of the following describes a frameshift mutation and its typical consequence?

  1. Insertion or deletion of bases not in multiples of three, altering downstream reading frame Correct answer.
  2. A single base substitution that changes one codon
  3. Inversion of a chromosome segment without loss of genetic material
  4. Duplication of an entire chromosome set

Explanation

Insertions or deletions whose length is not a multiple of three shift the reading frame, so every codon downstream is reinterpreted and the protein is usually truncated or nonfunctional. A single substitution is a point mutation that typically alters only one codon, while inversions and whole-set duplications are chromosomal or genomic changes.

Question 36Genetics and Molecular Biology

In a dihybrid cross between two individuals heterozygous at both loci (AaBb x AaBb), assuming independent assortment and complete dominance, what fraction of offspring is expected to show both recessive phenotypes?

  1. 3/16
  2. 1/4
  3. 9/16
  4. 1/16 Correct answer.

Explanation

Each locus in a monohybrid cross of heterozygotes yields a 3:1 phenotypic ratio, so the recessive phenotype appears with probability 1/4 per locus. Multiplying independent probabilities gives 1/4 x 1/4 = 1/16 for the double-recessive class. The 9:3:3:1 ratio places the two dominant phenotypes at 9/16 and each single-dominant class at 3/16.

Question 37Genetics and Molecular Biology

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What proportion of their sons is expected to be affected?

  1. None of the sons
  2. All of the sons
  3. Half of the sons Correct answer.
  4. One quarter of the sons

Explanation

Sons inherit their single X chromosome from the mother and a Y from the father, so each son has a 50% chance of receiving the mother's mutant allele and being affected. Daughters instead receive the father's normal X, so they are carriers at most and are unaffected. The 1/2 figure applies to sons specifically, not to all children.

Question 38Genetics and Molecular Biology

Which statement correctly distinguishes the roles of the promoter and the enhancer in eukaryotic transcription?

  1. The promoter silences transcription, while the enhancer terminates it
  2. The promoter is a distal regulatory element, while the enhancer binds RNA polymerase directly
  3. The promoter is a proximal site where the preinitiation complex assembles, while the enhancer is a distal element that raises transcription when bound by activators Correct answer.
  4. Both the promoter and the enhancer are transcribed into mRNA

Explanation

The core promoter lies near the transcription start site and recruits RNA polymerase and general transcription factors, whereas enhancers act at a distance, often looping to contact the promoter and boosting initiation. Neither element is normally included in the mature mRNA. Silencing and termination are carried out by other elements and factors, not by the promoter in the sense described.

Question 39Genetics and Molecular Biology

During which phase of the cell cycle does DNA replication occur, and what is the resulting chromosome state before mitosis?

  1. M phase; chromosomes are replicated during anaphase
  2. S phase; each chromosome consists of two sister chromatids Correct answer.
  3. G2 phase; chromosomes are haploid
  4. G1 phase; chromosomes are single chromatids

Explanation

DNA synthesis takes place in S phase, after which each chromosome has been duplicated into two identical sister chromatids held at the centromere. G1 precedes replication, G2 follows it, and M phase segregates the already-replicated chromatids. Replication does not occur during anaphase.

Question 40Genetics and Molecular Biology

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

  1. Independent assortment
  2. Cytokinesis
  3. Sister chromatid cohesion
  4. Crossing over Correct answer.

Explanation

Crossing over occurs during prophase I, when homologous chromosomes physically exchange corresponding segments at chiasmata, creating new allele combinations on a chromosome. Independent assortment shuffles whole chromosomes but does not exchange segments, cohesion holds chromatids together, and cytokinesis divides the cytoplasm.

Question 41Genetics and Molecular Biology

A restriction enzyme recognizes the palindromic sequence GAATTC and cuts between G and A on both strands. What type of ends does this produce?

  1. Blunt ends
  2. Covalently closed hairpin ends
  3. Sticky ends with 5' overhangs Correct answer.
  4. Sticky ends with 3' overhangs

Explanation

Cutting between G and A within the palindromic EcoRI site leaves a single-stranded 5'-AATT extension on each fragment, so the ends are cohesive with 5' overhangs. Blunt ends arise when cuts fall at the same position on both strands, and 3' overhangs would require cutting on the other side of the recognition sequence.

Question 42Genetics and Molecular Biology

In the polymerase chain reaction, what is the function of the denaturation step typically performed near 95 degrees Celsius?

  1. It allows primers to anneal to the template
  2. It separates the double-stranded DNA template into single strands Correct answer.
  3. It activates DNA polymerase to extend primers
  4. It degrades contaminating RNA

Explanation

High temperature breaks the hydrogen bonds between complementary strands, yielding single-stranded templates for primer binding. Annealing occurs at a lower temperature, and extension by a thermostable polymerase occurs near 72 degrees. RNA removal depends on RNase treatment, not on the thermal cycle.

Question 43Genetics and Molecular Biology

A couple in which both partners are carriers of the same autosomal recessive allele (Aa x Aa) has one affected child. What is the probability that their next child will be affected?

  1. One half, because the first child was affected
  2. Three quarters, because the parents are carriers
  3. One quarter, because each conception is an independent event Correct answer.
  4. Zero, because one affected child already occurred

Explanation

Each conception is an independent event, so the probability of an affected child (aa) remains 1/4 regardless of previous outcomes. Prior children do not alter the parental genotypes or the segregation of alleles in future gametes. The 3/4 figure is the chance of an unaffected child, not an affected one.

Question 44Genetics and Molecular Biology

Which feature of the genetic code describes the fact that different codons can specify the same amino acid?

  1. Degeneracy Correct answer.
  2. Non-overlapping reading
  3. Universality
  4. Directionality

Explanation

Degeneracy means more than one codon can encode the same amino acid, which buffers some mutations from changing the protein. Universality refers to the code being nearly the same across organisms, non-overlapping means each nucleotide belongs to one codon, and directionality refers to reading in the 5' to 3' direction.

Question 45Genetics and Molecular Biology

In a pedigree, a trait appears in every generation, affects about half of the offspring of an affected parent, and appears in both sexes. Which mode of inheritance best fits these observations?

  1. Autosomal recessive
  2. X-linked recessive
  3. Mitochondrial
  4. Autosomal dominant Correct answer.

Explanation

Appearance in every generation with roughly half of an affected person's children affected points to a dominant allele, and equal involvement of both sexes is consistent with an autosomal location. Autosomal recessive traits often skip generations, X-linked recessive traits affect males far more often, and mitochondrial traits pass only through mothers.

Question 46Genetics and Molecular Biology

Which chromatin modification is generally associated with transcriptional activation when it occurs in promoter regions?

  1. Histone deacetylation
  2. Histone acetylation Correct answer.
  3. DNA methylation at CpG islands
  4. Histone H3 lysine 9 methylation

Explanation

Acetylation of lysine residues on histone tails neutralizes their positive charge and loosens histone-DNA contacts, making promoters more accessible to transcription factors. CpG methylation and H3K9 methylation are linked to repression, and deacetylation restores compact chromatin.

Question 47Genetics and Molecular Biology

A template DNA strand reads 3'-TAC GGA-5'. Assuming standard Watson-Crick pairing, what mRNA sequence is transcribed from it?

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

Explanation

RNA polymerase builds mRNA complementary and antiparallel to the template, substituting uracil for thymine. Pairing T with A, A with U, C with G, and G with C against 3'-TAC GGA-5' gives 5'-AUG CCU-3'. Options containing T are DNA sequences, and 5'-UAC GGA-3' keeps the same bases rather than complementing them.

Question 48Genetics and Molecular Biology

Which type of point mutation changes a codon into a different amino acid codon?

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

Explanation

A missense mutation substitutes one base so that the codon now specifies a different amino acid, potentially altering protein function. Silent mutations leave the amino acid unchanged because of degeneracy, nonsense mutations create a premature stop codon, and frameshifts arise from insertions or deletions rather than a single substitution.

Question 49Genetics and Molecular Biology

In a population at Hardy-Weinberg equilibrium, the frequency of the recessive allele is 0.2. What is the expected frequency of heterozygous individuals?

  1. 0.04
  2. 0.64
  3. 0.32 Correct answer.
  4. 0.16

Explanation

With q = 0.2, p = 0.8, and the heterozygote frequency is 2pq = 2 x 0.8 x 0.2 = 0.32. The homozygous recessive frequency is q squared, or 0.04, and the homozygous dominant frequency is p squared, or 0.64. These three classes sum to 1.00, confirming the calculation.

Question 50Genetics and Molecular Biology

Which enzyme joins Okazaki fragments on the lagging strand by forming phosphodiester bonds between adjacent nucleotides?

  1. Exonuclease
  2. Primase
  3. Helicase
  4. DNA ligase Correct answer.

Explanation

After RNA primers are removed and replaced with DNA, ligase seals the remaining nick between fragments, completing the sugar-phosphate backbone. Primase synthesizes the RNA primer, exonucleases remove primers or correct errors, and helicase unwinds the duplex.