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ORT Biology study materials
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ORT Biology practice questions

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Question 1Cell Biology and Biochemistry

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

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

Explanation

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

Question 2Cell Biology and Biochemistry

A polypeptide is synthesized by a ribosome free in the cytosol. Which feature of the nascent protein would most directly redirect the ribosome to the rough endoplasmic reticulum?

  1. An internal disulfide bond
  2. A C-terminal polyhistidine tag
  3. A stretch of consecutive proline residues
  4. A hydrophobic N-terminal signal peptide Correct answer.

Explanation

A hydrophobic N-terminal signal peptide is recognized by the signal recognition particle, which pauses translation and docks the ribosome at the ER translocon. This co-translational targeting is what makes a protein enter the secretory pathway.

Question 3Cell Biology and Biochemistry

In the fluid mosaic model, which statement best describes the behavior of membrane phospholipids in a bilayer at physiological temperature?

  1. They diffuse laterally within the same leaflet Correct answer.
  2. They are locked in place by the underlying cytoskeleton
  3. They flip between leaflets many times per second
  4. They are covalently anchored to integral membrane proteins

Explanation

Lateral diffusion of phospholipids within a leaflet is rapid, whereas transverse flip-flop is thermodynamically unfavorable and rare without flippase enzymes. This asymmetry underlies the maintenance of distinct inner and outer leaflet compositions.

Question 4Cell Biology and Biochemistry

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

  1. 0.50
  2. 0.25
  3. 0.75 Correct answer.
  4. 0.33

Explanation

The Michaelis-Menten equation gives v/Vmax = [S]/(Km + [S]). Substituting 15/(5 + 15) yields 15/20, which equals 0.75.

Question 5Cell Biology and Biochemistry

Which interaction contributes most to the stability of a DNA double helix under physiological salt conditions?

  1. Base stacking between adjacent aromatic rings Correct answer.
  2. Hydrogen bonds between deoxyribose sugars
  3. Ionic bonds between phosphate groups
  4. Van der Waals contacts between the two backbone strands

Explanation

Base stacking interactions, driven largely by hydrophobic and van der Waals effects between adjacent aromatic bases, contribute more free energy to helix stability than the base-pair hydrogen bonds themselves. The hydrogen bonds provide specificity rather than the dominant stabilizing energy.

Question 6Cell Biology and Biochemistry

Which statement correctly distinguishes DNA replication from transcription in eukaryotes?

  1. Replication copies the entire genome, whereas transcription copies selected regions Correct answer.
  2. Replication requires a primer, whereas transcription requires a primer
  3. Replication uses ribonucleotide triphosphates, whereas transcription uses deoxyribonucleotides
  4. Replication occurs only in the cytoplasm, whereas transcription occurs in the nucleus

Explanation

Genome duplication during S phase copies all chromosomes, while transcription is selective and regulated by promoters and enhancers. Both processes occur in the nucleus and both use polymerases that extend a growing strand.

Question 7Cell Biology and Biochemistry

A mutation changes a codon from UAC to UAA in the middle of an open reading frame. What is the most likely consequence for the encoded protein?

  1. A frameshift affecting all downstream residues
  2. Premature termination of translation Correct answer.
  3. No change because the codon is synonymous
  4. Substitution of one amino acid for another

Explanation

UAA is a stop codon, so the ribosome will terminate translation at that point and release a truncated polypeptide. This is a nonsense mutation, which typically abolishes protein function.

Question 8Cell Biology and Biochemistry

During glycolysis, which step produces ATP by substrate-level phosphorylation?

  1. Conversion of glucose to glucose-6-phosphate
  2. Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate Correct answer.
  3. Conversion of fructose-6-phosphate to fructose-1,6-bisphosphate
  4. Reduction of pyruvate to lactate

Explanation

The phosphoglycerate kinase reaction transfers a phosphate from 1,3-bisphosphoglycerate directly to ADP, forming ATP without an electron transport chain. This substrate-level phosphorylation occurs twice per glucose molecule.

Question 9Cell Biology and Biochemistry

Which property of water allows it to absorb substantial heat with only a small rise in temperature, buffering cells against thermal shock?

  1. Its low viscosity
  2. Its high surface tension
  3. Its ability to act as a solvent
  4. Its high specific heat capacity Correct answer.

Explanation

Extensive hydrogen bonding means a large amount of energy is needed to raise water's temperature, giving it a high specific heat capacity. This stabilizes cellular temperature when metabolic heat fluctuates.

Question 10Cell Biology and Biochemistry

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

  1. The apparent Km increases while Vmax is unchanged Correct answer.
  2. Both Km and Vmax decrease
  3. Neither Km nor Vmax changes
  4. Vmax decreases while Km is unchanged

Explanation

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

Question 11Cell Biology and Biochemistry

Which feature distinguishes prokaryotic from eukaryotic chromosome organization?

  1. Prokaryotic DNA lacks base pairing between strands
  2. Prokaryotic DNA contains no phosphate backbone
  3. Prokaryotic DNA is always single-stranded
  4. Prokaryotic DNA is typically circular and not wrapped around histones Correct answer.

Explanation

Bacterial chromosomes are usually circular and associate with histone-like proteins rather than forming nucleosomes. Eukaryotic DNA is linear and packaged around histone octamers into chromatin.

Question 12Cell Biology and Biochemistry

In the electron transport chain, what is the primary role of oxygen?

  1. It transports protons across the inner membrane
  2. It phosphorylates ADP directly
  3. It donates electrons to complex I
  4. It is the terminal electron acceptor, forming water Correct answer.

Explanation

Complex IV transfers electrons from cytochrome c to molecular oxygen, which combines with protons to form water. This keeps electrons flowing through the chain and sustains the proton gradient.

Question 13Cell Biology and Biochemistry

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

  1. It moves Na+ and K+ down their gradients without using ATP
  2. It exports three Na+ ions and imports two K+ ions per ATP hydrolyzed Correct answer.
  3. It imports equal numbers of Na+ and K+ ions
  4. It imports three Na+ ions and exports two K+ ions per ATP hydrolyzed

Explanation

Each cycle of the pump hydrolyzes one ATP and moves three sodium ions out and two potassium ions in, both against their gradients. The unequal stoichiometry contributes a small net outward current that helps maintain the negative resting potential.

Question 14Cell Biology and Biochemistry

A cell is treated with a drug that blocks the G1/S cyclin-dependent kinase. What is the most direct expected outcome?

  1. Arrest before DNA replication begins Correct answer.
  2. Failure of sister chromatid separation
  3. Failure of cytokinesis
  4. Inability to assemble the mitotic spindle

Explanation

G1/S CDK activity drives passage through the restriction point and commits the cell to replicating its DNA. Blocking it prevents entry into S phase, so replication does not begin.

Question 15Cell Biology and Biochemistry

Which structural feature of the lipid bilayer creates the permeability barrier to most charged and polar solutes?

  1. The glycocalyx on the extracellular face
  2. The presence of integral membrane proteins
  3. The hydrophobic fatty acid tails in the bilayer interior Correct answer.
  4. The hydrophilic phosphate head groups at the surface

Explanation

The hydrophobic core formed by fatty acid tails excludes charged and polar molecules, forcing them to rely on transporters or channels. Small nonpolar gases can still diffuse through this core.

Question 16Cell Biology and Biochemistry

Which property of the genetic code allows a single-base substitution to leave the amino acid sequence unchanged?

  1. Non-overlapping reading of codons
  2. Degeneracy of the code Correct answer.
  3. Universality of the code
  4. Triplet nature of codons

Explanation

Degeneracy means several codons encode the same amino acid, so some substitutions, especially at the third position, are silent. This redundancy reduces the phenotypic impact of certain mutations.

Question 17Cell Biology and Biochemistry

In a reaction catalyzed by an enzyme, which effect explains why the reaction rate increases with temperature up to an optimum?

  1. The enzyme's active site becomes permanently more flexible
  2. More reactant molecules have sufficient kinetic energy to reach the transition state Correct answer.
  3. The substrate concentration increases spontaneously
  4. The equilibrium constant shifts to favor products

Explanation

Raising temperature increases molecular kinetic energy, so a larger fraction of collisions exceeds the activation barrier. Beyond the optimum, denaturation of the enzyme causes the rate to fall.

Question 18Cell Biology and Biochemistry

Which organelle is the primary site of ribosomal subunit assembly in a eukaryotic cell?

  1. Golgi apparatus
  2. Nucleolus Correct answer.
  3. Mitochondrial matrix
  4. Peroxisome

Explanation

The nucleolus is a specialized region of the nucleus where ribosomal RNA is transcribed and assembled with ribosomal proteins into subunits. These subunits are then exported to the cytoplasm.

Question 19Cell Biology and Biochemistry

Which bond links adjacent nucleotides within a single strand of DNA?

  1. Ionic bond between the base and the phosphate
  2. Hydrogen bond between complementary bases
  3. Phosphodiester bond between the 3' hydroxyl and 5' phosphate Correct answer.
  4. Peptide bond between the sugar and base

Explanation

Nucleotides in a strand are joined by phosphodiester linkages connecting the 3' carbon of one sugar to the 5' phosphate of the next. This backbone gives DNA its directionality.

Question 20Cell Biology and Biochemistry

A protein has a quaternary structure. Which statement must be true of it?

  1. It consists of two or more polypeptide chains Correct answer.
  2. It contains a covalently attached prosthetic group
  3. It is stabilized only by disulfide bonds
  4. It has no tertiary structure

Explanation

Quaternary structure refers to the arrangement of multiple folded polypeptide subunits into a single functional complex. Each subunit retains its own tertiary structure.

Question 21Cell Biology and Biochemistry

Which process directly converts the chemical energy of a proton gradient into the phosphate bond energy of ATP?

  1. Substrate-level phosphorylation by phosphofructokinase
  2. Reduction of NAD+ at complex I
  3. Hydrolysis of ATP by the sodium-potassium pump
  4. Chemiosmotic coupling through ATP synthase Correct answer.

Explanation

Protons flowing down their electrochemical gradient through ATP synthase drive rotation of the enzyme's c-ring, which powers ADP phosphorylation. This is the definition of chemiosmotic ATP synthesis.

Question 22Cell Biology and Biochemistry

Which mechanism best explains how a steroid hormone such as cortisol exerts its effects on a target cell?

  1. Binding a cell-surface G protein-coupled receptor
  2. Opening a ligand-gated ion channel
  3. Activating a receptor tyrosine kinase
  4. Binding an intracellular receptor and acting as a transcription factor Correct answer.

Explanation

Steroid hormones are lipophilic and diffuse across the plasma membrane to bind intracellular receptors, which then translocate to the nucleus and modulate transcription. This genomic mechanism distinguishes them from peptide hormones acting at the surface.

Question 23Cell Biology and Biochemistry

Which statement accurately describes the role of tRNA in translation?

  1. It forms the structural core of the ribosome
  2. It serves as the template for protein synthesis
  3. It catalyzes the formation of peptide bonds
  4. It carries a specific amino acid and pairs its anticodon with the mRNA codon Correct answer.

Explanation

Each tRNA is charged with its cognate amino acid by an aminoacyl-tRNA synthetase and delivers it to the ribosome by base pairing its anticodon with the complementary mRNA codon. This ensures the correct amino acid is added at each step.

Question 24Cell Biology and Biochemistry

In a cell, the concentration of a solute is higher outside than inside, and it enters the cell through a channel without ATP hydrolysis. What type of transport is this?

  1. Facilitated diffusion Correct answer.
  2. Primary active transport
  3. Endocytosis
  4. Secondary active transport

Explanation

Movement down an existing concentration gradient through a membrane protein without energy input is facilitated diffusion. The channel lowers the kinetic barrier but does not pump against the gradient.

Question 25Cell Biology and Biochemistry

Which change would most likely increase the fluidity of a biological membrane at low temperature?

  1. Adding more cholesterol above 37 degrees Celsius
  2. Increasing the proportion of unsaturated fatty acids in phospholipids Correct answer.
  3. Increasing the proportion of long saturated fatty acids
  4. Reducing the number of phospholipids per unit area

Explanation

Cis double bonds in unsaturated fatty acids create kinks that prevent tight packing, lowering the melting temperature and maintaining fluidity in the cold. Saturated chains pack more tightly and rigidify the bilayer.

Question 26Cell Biology and Biochemistry

Which statement describes the role of the Golgi apparatus in a eukaryotic cell?

  1. It contains hydrolytic enzymes that degrade worn-out organelles and macromolecules.
  2. It modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum. Correct answer.
  3. It synthesizes ATP by oxidative phosphorylation using the electron transport chain.
  4. It serves as the site of ribosomal RNA synthesis and ribosomal subunit assembly.

Explanation

The Golgi apparatus receives transport vesicles from the endoplasmic reticulum and performs post-translational modifications such as glycosylation, then sorts and packages the modified products into vesicles for delivery to their final destinations.

Question 27Cell Biology and Biochemistry

A mutation in a gene encoding a cytosolic enzyme changes a surface lysine residue to a glutamate residue at physiological pH. What is the most likely effect on the protein's interaction with a negatively charged substrate?

  1. The interaction will be strengthened because the enzyme gains a negative charge.
  2. The interaction will be unchanged because lysine and glutamate have similar sizes.
  3. The interaction will be weakened or lost because the enzyme's surface charge changes from positive to negative. Correct answer.
  4. The interaction will be strengthened because glutamate can form disulfide bonds with the substrate.

Explanation

At physiological pH, lysine is positively charged and glutamate is negatively charged. Replacing a positive surface residue with a negative one repels a negatively charged substrate, reducing electrostatic attraction at the binding site.

Question 28Cell Biology and Biochemistry

Which statement best explains why a high concentration of a noncompetitive inhibitor cannot be overcome by adding excess substrate?

  1. The inhibitor binds to an allosteric site and changes the enzyme's conformation, reducing its catalytic efficiency regardless of substrate concentration. Correct answer.
  2. The inhibitor competes with the substrate for the active site, but with a lower affinity than the substrate.
  3. The inhibitor binds irreversibly to the active site, permanently inactivating the enzyme.
  4. The inhibitor denatures the enzyme by disrupting peptide bonds in the active site.

Explanation

A noncompetitive inhibitor binds at a site distinct from the active site and induces a conformational change that lowers the enzyme's turnover number or substrate affinity. Because it does not compete for the active site, increasing substrate concentration cannot displace it or restore maximal velocity.

Question 29Cell Biology and Biochemistry

During the electron transport chain, which statement accurately describes the role of oxygen?

  1. It is phosphorylated by ATP synthase to produce ATP.
  2. It is the final electron acceptor, combining with electrons and protons to form water at complex IV. Correct answer.
  3. It donates electrons to complex I, initiating the flow of electrons through the chain.
  4. It acts as a mobile electron carrier that shuttles electrons from complex III to complex IV.

Explanation

Oxygen serves as the terminal electron acceptor at complex IV (cytochrome c oxidase), where it accepts four electrons and four protons to form two molecules of water. This step maintains the proton gradient by removing electrons from the chain.

Question 30Cell Biology and Biochemistry

A cell is treated with a drug that increases the activity of the sodium-potassium pump (Na+/K+-ATPase) in neurons. What is the most likely immediate effect on the resting membrane potential?

  1. The membrane potential will hyperpolarize because the pump extrudes three sodium ions for every two potassium ions taken in, increasing the negative charge inside. Correct answer.
  2. The membrane potential will depolarize because the pump brings potassium ions into the cell, reducing the negative charge inside.
  3. The membrane potential will depolarize because more sodium ions enter the cell.
  4. The membrane potential will remain unchanged because the pump is electrically neutral.

Explanation

The Na+/K+-ATPase moves three Na+ out and two K+ in per ATP hydrolyzed, producing a net outward movement of one positive charge. Increased pump activity therefore makes the inside of the cell more negative, hyperpolarizing the membrane potential.

Question 31Genetics and Molecular Biology

In a DNA double helix, which nitrogenous base pairs with guanine, and how many hydrogen bonds typically form between them?

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

Explanation

In DNA, guanine forms a specific base pair with cytosine, and this G–C pair is stabilized by three hydrogen bonds. The G–C pair is therefore slightly more thermally stable than the A–T pair, which has only two hydrogen bonds. Uracil is found in RNA rather than DNA.

Question 32Genetics and Molecular Biology

Which enzyme is primarily responsible for adding nucleotides to the growing DNA strand during replication?

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

Explanation

DNA polymerase catalyzes the addition of deoxynucleotides to the 3' end of a growing primer, extending the new strand in the 5' to 3' direction. Ligase joins fragments rather than synthesizing the bulk of the strand, and helicase unwinds the duplex.

Question 33Genetics and Molecular Biology

During transcription, which strand of DNA serves as the template for the RNA molecule?

  1. The leading strand only
  2. The coding strand, read in the 5' to 3' direction
  3. The template strand, read in the 3' to 5' direction Correct answer.
  4. Either strand, chosen randomly at each gene

Explanation

RNA polymerase reads the template strand in the 3' to 5' direction, producing RNA that is complementary and antiparallel to it. The resulting RNA therefore matches the coding (nontemplate) strand except that uracil replaces thymine. The choice of strand is fixed by promoter orientation, not random.

Question 34Genetics and Molecular Biology

A human somatic cell in G1 phase contains how many chromosomes and how many DNA molecules per cell?

  1. 23 chromosomes and 23 DNA molecules
  2. 46 chromosomes and 92 DNA molecules
  3. 46 chromosomes and 46 DNA molecules Correct answer.
  4. 92 chromosomes and 92 DNA molecules

Explanation

Before S phase, each chromosome consists of a single DNA molecule, so a diploid human G1 cell has 46 chromosomes and 46 DNA molecules. DNA replication in S phase doubles the DNA content to 92 molecules while the chromosome count stays at 46.

Question 35Genetics and Molecular Biology

Which of the following describes the phenotype of an individual with genotype Aa for a completely dominant trait?

  1. It shows the dominant phenotype. Correct answer.
  2. It shows the recessive phenotype.
  3. It cannot be determined without knowing the gametes produced.
  4. It shows an intermediate blended phenotype.

Explanation

With complete dominance, a single dominant allele is sufficient to produce the dominant phenotype, so the heterozygote Aa resembles the AA homozygote. The recessive phenotype appears only in the homozygous aa condition. Blending would indicate incomplete dominance instead.

Question 36Genetics and Molecular Biology

Which process generates new combinations of alleles on the same chromosome?

  1. Crossing over during prophase I of meiosis Correct answer.
  2. Mitotic division
  3. Semiconservative replication
  4. DNA methylation

Explanation

Crossing over between homologous chromatids during prophase I of meiosis exchanges segments, creating recombinant chromosomes that carry new allele combinations. This recombination is a major source of genetic variation in gametes. Mitosis and replication copy DNA without producing such recombination.

Question 37Genetics and Molecular Biology

In the genetic code, what does the term 'degenerate' mean?

  1. The code differs substantially between species.
  2. Some codons specify no amino acid at all.
  3. Codons are read in overlapping triplets.
  4. Most amino acids are specified by more than one codon. Correct answer.

Explanation

Degeneracy means that several different codons can encode the same amino acid, which buffers some mutations against changing the protein. The code is nonetheless nearly universal and is read in non-overlapping triplets. Degeneracy does not mean that codons are meaningless.

Question 38Genetics and Molecular Biology

Which statement describes the central dogma of molecular biology as originally formulated?

  1. DNA is translated into protein by ribosomes.
  2. RNA is reverse-transcribed into DNA in all cells.
  3. Genetic information flows from DNA to RNA to protein. Correct answer.
  4. Proteins are translated directly from DNA without an RNA intermediate.

Explanation

The central dogma describes the flow of sequence information from DNA through RNA (transcription) to protein (translation). Reverse transcription is a special case found in retroviruses and some other elements, not the general rule. Translation always uses an mRNA intermediate in cellular organisms.

Question 39Genetics and Molecular Biology

Two heterozygous individuals (Aa × Aa) are crossed for a trait showing complete dominance. What is the expected phenotypic ratio among the offspring?

  1. All offspring show the dominant phenotype
  2. 3 dominant : 1 recessive Correct answer.
  3. 1 dominant : 1 recessive
  4. 1 dominant : 2 intermediate : 1 recessive

Explanation

An Aa × Aa cross yields genotypes in a 1 AA : 2 Aa : 1 aa ratio. Because AA and Aa both show the dominant phenotype under complete dominance, the phenotypic ratio is 3 dominant to 1 recessive. The 1:2:1 pattern describes genotypes, not this phenotype ratio.

Question 40Genetics 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. 50% Correct answer.
  2. 0%
  3. 100%
  4. 25%

Explanation

Sons inherit their single X chromosome from the mother, and a carrier mother transmits the affected allele to half of her sons. Thus about 50% of sons are expected to be affected, while daughters are unaffected because they receive a normal X from the father.

Question 41Genetics and Molecular Biology

A mutation changes a codon from GAA to GAG. Both codons specify glutamic acid. What type of mutation is this?

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

Explanation

A silent mutation changes the nucleotide sequence but not the encoded amino acid, which is possible because of codon degeneracy. The protein product is therefore unchanged in sequence. Missense and nonsense mutations alter the amino acid sequence or introduce a stop codon.

Question 42Genetics and Molecular Biology

In a dihybrid cross between two individuals heterozygous for both genes (AaBb × AaBb), assuming independent assortment, what fraction of offspring is expected to show both recessive phenotypes?

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

Explanation

Each gene independently gives a 1/4 probability of the recessive phenotype in an Aa × Aa cross. Multiplying the independent probabilities gives 1/4 × 1/4 = 1/16 for the double recessive class. The 9:3:3:1 ratio reflects these independent combinations.

Question 43Genetics and Molecular Biology

Which feature distinguishes the lagging strand from the leading strand during DNA replication?

  1. The lagging strand is not copied by DNA polymerase.
  2. The lagging strand is synthesized continuously in the 5' to 3' direction.
  3. The lagging strand is synthesized discontinuously as Okazaki fragments. Correct answer.
  4. The lagging strand is made of RNA rather than DNA.

Explanation

Because DNA polymerase only extends 5' to 3', the strand whose template runs in the opposite direction is synthesized in short Okazaki fragments that are later joined by ligase. The leading strand is synthesized continuously toward the replication fork. Both strands are ultimately made of DNA.

Question 44Genetics and Molecular Biology

A gene is transcribed and the resulting mRNA is alternatively spliced in two different tissues. What is the most likely consequence?

  1. The mRNA is translated into DNA in one tissue.
  2. The two tissues produce identical proteins from the gene.
  3. The gene is replicated differently in each tissue.
  4. The two tissues produce different protein isoforms from the same gene. Correct answer.

Explanation

Alternative splicing joins different combinations of exons, so a single gene can yield multiple mRNA variants and thus different protein isoforms in different tissues. The genomic DNA sequence is unchanged and replication is not tissue-specific in this way. This mechanism greatly expands proteome diversity.

Question 45Genetics and Molecular Biology

Which molecule carries the amino acid to the ribosome and recognizes the mRNA codon during translation?

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

Explanation

Transfer RNA has an anticodon that base-pairs with the mRNA codon and carries the corresponding amino acid at its acceptor end. This coupling of codon recognition to amino acid identity ensures accurate translation. Ribosomal RNA forms the core of the ribosome but does not deliver amino acids.

Question 46Genetics and Molecular Biology

A pedigree shows an affected father and an unaffected mother producing only unaffected children, with the trait appearing in both sexes across generations. Which mode of inheritance is most consistent with this pattern?

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

Explanation

An affected father with an unaffected partner can have unaffected carrier children under autosomal recessive inheritance, and the trait can reappear in later generations when two carriers mate. The appearance in both sexes also fits autosomal rather than Y-linked inheritance. Autosomal dominant would more typically pass directly from the affected father to about half his children.

Question 47Genetics and Molecular Biology

Which type of RNA is the primary structural and catalytic component of the ribosome?

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

Explanation

Ribosomal RNA forms the scaffold and catalytic core of the ribosome, including the peptidyl transferase activity that links amino acids. Proteins are present but largely support the RNA framework. This RNA-driven catalysis reflects the likely RNA-world origin of translation.

Question 48Genetics and Molecular Biology

A frameshift mutation occurs early in a coding sequence. Why is its effect often more severe than that of a single missense substitution?

  1. It alters every downstream codon and often introduces a premature stop. Correct answer.
  2. It changes only one codon and leaves the rest of the protein intact.
  3. It prevents transcription of the gene entirely.
  4. It always creates a silent mutation.

Explanation

An insertion or deletion that is not a multiple of three shifts the reading frame, so all codons downstream are translated differently and a premature stop codon frequently appears. The resulting protein is usually truncated and nonfunctional. A missense substitution changes just one amino acid.

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 homozygous recessive individuals?

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

Explanation

Under Hardy–Weinberg, the frequency of homozygous recessive individuals is q², where q is the recessive allele frequency. With q = 0.2, q² = 0.04. The value 2pq would give the heterozygote frequency.

Question 50Genetics and Molecular Biology

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

  1. It marks the end of the transcript for termination.
  2. It is the site where RNA polymerase and transcription factors assemble to initiate transcription. Correct answer.
  3. It encodes the amino acid sequence of the protein.
  4. It removes introns from the pre-mRNA.

Explanation

A promoter is a DNA region upstream of a gene where RNA polymerase and associated transcription factors bind to initiate transcription. Its sequence and orientation determine which strand is used and where transcription starts. Termination signals and splice sites serve different functions.