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UEE Biology practice questions

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Question 1Cell Biology and Biochemistry

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

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

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, so oxidative phosphorylation occurs there. The other listed organelles perform processing, degradation, or lipid synthesis rather than bulk ATP production.

Question 2Cell Biology and Biochemistry

A polypeptide is synthesized and then covalently tagged with ubiquitin chains. Which fate does this tag most directly specify?

  1. Import into the mitochondrial matrix
  2. Delivery to the proteasome for degradation Correct answer.
  3. Addition of N-linked glycans in the ER lumen
  4. Export through the nuclear pore complex

Explanation

Polyubiquitin chains, typically linked through lysine 48, are recognized by the proteasome, which unfolds and degrades the tagged protein. Mitochondrial import, glycosylation and nuclear export depend on other signals such as presequences, sequons and nuclear export sequences.

Question 3Cell Biology and Biochemistry

Which statement best describes the fluidity of a typical plasma membrane at physiological temperature?

  1. It is a rigid crystalline lattice in which phospholipids are fixed in place
  2. It is a dynamic two-dimensional fluid in which lipids and many proteins diffuse laterally Correct answer.
  3. It is a static bilayer in which only integral proteins can move
  4. It is a fluid layer in which proteins freely flip between leaflets

Explanation

The fluid mosaic model describes the bilayer as a two-dimensional fluid: phospholipids and many membrane proteins undergo lateral diffusion. Transverse flip-flop of phospholipids is very slow without flippase enzymes, and the bilayer is not a rigid lattice.

Question 4Cell Biology and Biochemistry

In the reaction catalyzed by hexokinase, glucose and ATP are converted to glucose-6-phosphate and ADP. Which term correctly describes ATP in this reaction?

  1. The oxidizing agent
  2. The allosteric product
  3. The phosphorylated product
  4. The phosphate-group donor Correct answer.

Explanation

Hexokinase transfers the terminal phosphoryl group of ATP onto the C6 hydroxyl of glucose, so ATP acts as the phosphate donor and is converted to ADP. Glucose, not ATP, is the substrate that becomes phosphorylated.

Question 5Cell Biology and Biochemistry

A competitive inhibitor is added to an enzyme-catalyzed reaction. How does it change the kinetic parameters measured with a fixed enzyme concentration?

  1. Vmax decreases while Km is unchanged
  2. Both apparent Km and Vmax increase
  3. Both apparent Km and Vmax decrease
  4. The apparent Km increases while Vmax is unchanged Correct answer.

Explanation

A competitive inhibitor binds the free enzyme and can be outcompeted by high substrate concentration, so Vmax is unchanged but the apparent Km rises. Uncompetitive inhibition instead lowers both parameters, and noncompetitive inhibition lowers Vmax without changing Km.

Question 6Cell Biology and Biochemistry

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

  1. Phosphorylation of glucose by hexokinase
  2. Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate Correct answer.
  3. Oxidation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
  4. Cleavage of fructose-1,6-bisphosphate by aldolase

Explanation

Phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP directly from a high-energy substrate. The other listed steps consume ATP or perform cleavage and oxidation without ATP synthesis.

Question 7Cell Biology and Biochemistry

Which feature distinguishes archaeal membrane lipids from those of bacteria and eukaryotes?

  1. They contain cholesterol as the main sterol
  2. They contain ester linkages to fatty acids
  3. They contain a single monolayer of phospholipids
  4. They contain ether linkages to isoprenoid chains Correct answer.

Explanation

Archaeal lipids typically join glycerol to isoprenoid chains through ether bonds, which resist hydrolysis and heat. Bacterial and eukaryotic lipids use ester-linked fatty acids, and cholesterol is a eukaryotic membrane sterol.

Question 8Cell Biology and Biochemistry

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

  1. Water enters the cell and the cell swells
  2. No net water movement occurs
  3. Solutes move out until the cell lyses
  4. Water leaves the cell and the cell shrinks Correct answer.

Explanation

In a hypertonic solution the external solute concentration exceeds that inside, so water moves outward by osmosis and the cell shrinks. Water entry and swelling occur in hypotonic solutions, and net water movement stops only at isotonic equilibrium.

Question 9Cell Biology and Biochemistry

Which property of the genetic code explains why a single-base substitution in a coding region sometimes leaves the amino acid sequence unchanged?

  1. The code is degenerate, with several codons specifying the same amino acid Correct answer.
  2. The code is overlapping, so one base encodes several amino acids
  3. The code is ambiguous, so a codon can specify several amino acids
  4. The code is non-universal, so codons vary among genes

Explanation

Degeneracy means more than one codon can specify the same amino acid, so many substitutions are silent. The code is non-overlapping, unambiguous within an organism, and nearly universal, so the other descriptions are incorrect.

Question 10Cell Biology and Biochemistry

A frameshift mutation arises in the second exon of a gene. Which outcome is most likely?

  1. A single amino acid substitution with no downstream change
  2. A completely different amino acid sequence downstream of the mutation Correct answer.
  3. A change confined to the promoter region
  4. A silent mutation with an unchanged protein product

Explanation

Insertion or deletion of a number of bases not divisible by three shifts the reading frame, so every codon downstream is read differently. A single substitution changes one codon, and promoter changes do not lie within an exon.

Question 11Cell Biology and Biochemistry

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

  1. Hydrogen bonds between complementary bases Correct answer.
  2. Glycosidic bonds between base and sugar
  3. Peptide bonds between amino acid residues
  4. Phosphodiester bonds between deoxyribose sugars

Explanation

Complementary base pairs are held together by hydrogen bonds, with two between A and T and three between G and C. Phosphodiester bonds instead link nucleotides within each strand, and glycosidic bonds attach bases to sugars.

Question 12Cell Biology and Biochemistry

In the citric acid cycle, which reaction produces NADH directly?

  1. Conversion of citrate to isocitrate
  2. Conversion of succinyl-CoA to succinate
  3. Conversion of fumarate to malate
  4. Conversion of isocitrate to alpha-ketoglutarate Correct answer.

Explanation

Isocitrate dehydrogenase oxidizes isocitrate to alpha-ketoglutarate and reduces NAD+ to NADH. Fumarate to malate uses water without NADH production, and the other two steps are a GTP/ATP-producing cleavage and an isomerization.

Question 13Cell Biology and Biochemistry

Which structural feature of a phospholipid makes it amphipathic?

  1. A charged head group attached to a rigid sterol ring
  2. A branched carbohydrate chain attached to a glycerol backbone
  3. A single hydrophobic head group with hydrophilic tails
  4. A hydrophilic phosphate head group and two hydrophobic hydrocarbon tails Correct answer.

Explanation

A phospholipid combines a polar, often charged head group with two nonpolar fatty acid tails, so it has both hydrophilic and hydrophobic regions. This amphipathic character drives bilayer self-assembly in water.

Question 14Cell Biology and Biochemistry

A drug blocks the Na+/K+-ATPase of a neuron. What immediate consequence is expected for the ion gradients?

  1. Intracellular Na+ rises and intracellular K+ falls over time Correct answer.
  2. Extracellular Na+ rises while intracellular K+ is unaffected
  3. Intracellular Na+ falls and intracellular K+ rises over time
  4. Both Na+ and K+ gradients remain unchanged indefinitely

Explanation

The pump normally exports three Na+ and imports two K+ per ATP hydrolyzed, maintaining low internal Na+ and high internal K+. Blocking it lets Na+ accumulate inside and K+ leak out, so the gradients gradually dissipate.

Question 15Cell Biology and Biochemistry

Which statement about the reaction catalyzed by DNA polymerase is correct?

  1. It unwinds the double helix ahead of the replication fork
  2. It adds nucleotides to the 3' hydroxyl end of a growing primer strand Correct answer.
  3. It joins two Okazaki fragments by forming phosphodiester bonds
  4. It adds nucleotides to the 5' phosphate end of a growing strand

Explanation

DNA polymerase extends a primer by forming a phosphodiester bond between the 3'-OH of the growing strand and the incoming deoxynucleoside triphosphate. Fragments are joined by DNA ligase, and helicase unwinds the duplex.

Question 16Cell Biology and Biochemistry

During transcription initiation in eukaryotes, which event most directly recruits RNA polymerase II to a protein-coding promoter?

  1. Binding of the TATA-binding protein and general transcription factors Correct answer.
  2. Cleavage of the poly(A) signal in the 3' untranslated region
  3. Phosphorylation of the RNA polymerase II C-terminal domain
  4. Addition of the 5' cap to the nascent transcript

Explanation

TATA-binding protein, part of TFIID, recognizes the TATA box and nucleates assembly of the preinitiation complex with other general transcription factors and RNA polymerase II. Capping, CTD phosphorylation and polyadenylation occur later, during elongation and termination.

Question 17Cell Biology and Biochemistry

Which feature of the genetic code allows a single amino acid to be specified by several different codons?

  1. Overlapping reading frames
  2. Degeneracy of the code Correct answer.
  3. Ambiguity of the code
  4. Non-universality of the code

Explanation

Degeneracy refers to the fact that most amino acids correspond to more than one codon, often differing in the third position. The code is non-overlapping and unambiguous, and it is nearly universal, so those alternatives are incorrect.

Question 18Cell Biology and Biochemistry

A cell is treated with a drug that prevents the addition of the 5' cap to mRNA. Which process is most directly impaired?

  1. Transcription of the mRNA by RNA polymerase II
  2. Recognition of the mRNA by the ribosome during translation initiation Correct answer.
  3. Addition of the poly(A) tail at the 3' end
  4. Splicing of introns from the pre-mRNA

Explanation

The 5' cap is bound by cap-binding proteins such as eIF4E and helps recruit the small ribosomal subunit, so translation initiation is impaired. Splicing and polyadenylation depend on other signals, and the cap is added after transcription has begun.

Question 19Cell Biology and Biochemistry

Which statement best describes the role of the signal recognition particle in the secretory pathway?

  1. It binds a hydrophobic signal peptide and targets the ribosome to the ER membrane Correct answer.
  2. It adds N-linked oligosaccharides to asparagine residues
  3. It transports folded proteins from the Golgi to the lysosome
  4. It cleaves the signal peptide after the polypeptide enters the ER lumen

Explanation

The signal recognition particle recognizes the hydrophobic signal sequence on the nascent chain, pauses translation and docks the ribosome at the ER translocon. Signal peptidase cleaves the peptide, and glycosyltransferases add glycans.

Question 20Cell Biology and Biochemistry

A red blood cell is placed in distilled water. Which outcome is expected?

  1. Water enters the cell by osmosis and the cell swells and may lyse Correct answer.
  2. Water leaves the cell and the cell shrinks
  3. No change occurs because the cell wall prevents swelling
  4. Solutes enter the cell until it becomes isotonic

Explanation

Distilled water is hypotonic relative to the cytosol, so water moves inward and the cell swells, potentially bursting because it lacks a cell wall. Animal cells do not have a wall to resist the osmotic pressure.

Question 21Cell Biology and Biochemistry

In an enzyme-catalyzed reaction, a noncompetitive inhibitor binds at a site distinct from the active site. How does it affect the kinetic parameters?

  1. Km increases while Vmax is unchanged
  2. Vmax decreases while Km is unchanged Correct answer.
  3. Km decreases while Vmax is unchanged
  4. Both Km and Vmax increase

Explanation

A noncompetitive inhibitor reduces the effective concentration of active enzyme, lowering Vmax, while substrate binding at the active site is unaffected, so Km stays the same. A rise in Km with unchanged Vmax instead indicates competitive inhibition.

Question 22Cell Biology and Biochemistry

Which molecule is the final electron acceptor in the mitochondrial electron transport chain under aerobic conditions?

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

Explanation

Complex IV transfers electrons to oxygen, which is reduced to water, making oxygen the terminal acceptor. NAD+, cytochrome c and ubiquinone are electron carriers within the chain, not the final acceptor.

Question 23Cell 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?

  1. A shift in the reading frame
  2. Silent mutation with no effect on the protein
  3. Premature termination of translation Correct answer.
  4. A single amino acid substitution

Explanation

UAA is a stop codon, so the ribosome terminates translation early, producing a truncated polypeptide. A missense change would require the new codon to specify another amino acid, and no frameshift occurs because the change is a substitution.

Question 24Cell Biology and Biochemistry

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

  1. It can proceed in either direction depending on oxygen availability
  2. It operates in both the cytosol and the mitochondrial matrix
  3. Its intermediates serve as precursors for biosynthesis as well as being oxidized for energy Correct answer.
  4. It produces both ATP and NADPH in equal amounts

Explanation

Cycle intermediates such as oxaloacetate and alpha-ketoglutarate are withdrawn for amino acid and other biosynthetic pathways, so the cycle serves both catabolism and anabolism. The cycle is confined to the matrix and operates in one direction.

Question 25Cell Biology and Biochemistry

Which type of membrane protein can be removed from the bilayer by treatment with a high-salt solution without disrupting the membrane?

  1. A lipid-anchored protein
  2. A peripheral membrane protein Correct answer.
  3. A transmembrane protein
  4. An integral monotopic protein

Explanation

Peripheral proteins associate with the membrane through electrostatic and hydrogen-bond interactions with head groups or other proteins, so high salt can dissociate them. Integral proteins, including transmembrane and lipid-anchored proteins, require detergents or lipases to release them.

Question 26Cell Biology and Biochemistry

A student examines a cell and finds a rigid structure outside the plasma membrane that provides shape and prevents osmotic lysis. In a plant cell, which molecule is the principal component of this structure?

  1. Phospholipid
  2. Cellulose Correct answer.
  3. Chitin
  4. Peptidoglycan

Explanation

The plant cell wall is composed mainly of cellulose microfibrils, which give tensile strength and resist turgor pressure. Chitin forms fungal walls and arthropod exoskeletons, while peptidoglycan is bacterial. Phospholipids form membranes, not the rigid outer wall.

Question 27Cell Biology and Biochemistry

A protein is synthesized on the rough endoplasmic reticulum and then travels through the Golgi apparatus before being secreted. Which sequence correctly describes the route of this protein?

  1. Rough ER → Golgi → secretory vesicle → plasma membrane Correct answer.
  2. Cytosol → mitochondrion → plasma membrane
  3. Golgi → rough ER → lysosome → plasma membrane
  4. Nucleus → smooth ER → Golgi → cytosol

Explanation

Secretory proteins enter the rough ER lumen, are transported in vesicles to the Golgi for modification and sorting, then leave in secretory vesicles that fuse with the plasma membrane. This is the standard biosynthetic-secretory pathway.

Question 28Cell Biology and Biochemistry

An enzyme follows Michaelis-Menten kinetics. A competitive inhibitor is added at a concentration that does not saturate the enzyme. Which combination of kinetic changes is expected?

  1. Apparent Km decreases; Vmax unchanged
  2. Apparent Km unchanged; Vmax decreases
  3. Apparent Km increases; Vmax unchanged Correct answer.
  4. Apparent Km unchanged; Vmax increases

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 inhibition can be overcome by high substrate concentration, Vmax is unchanged.

Question 29Cell Biology and Biochemistry

A eukaryotic mRNA is transcribed, capped, and polyadenylated, then exported to the cytoplasm. Which feature most directly allows the small ribosomal subunit to recognize the start of translation?

  1. The TATA box in the promoter
  2. The 5' cap bound by initiation factors Correct answer.
  3. The introns retained in the transcript
  4. The poly(A) tail at the 3' end

Explanation

In eukaryotes, initiation factors such as eIF4E bind the 5' cap and recruit the small ribosomal subunit, which then scans to the first AUG. The poly(A) tail and promoter elements influence stability or transcription but do not directly position the ribosome at the start codon.

Question 30Cell Biology and Biochemistry

A mutation in a mitochondrial gene removes the proton-pumping activity of complex III of the electron transport chain while leaving electron transfer to cytochrome c partly intact. Which consequence is most consistent with chemiosmotic theory?

  1. The proton gradient decreases and ATP synthesis falls Correct answer.
  2. Glycolysis stops immediately because NADH cannot be oxidized
  3. ATP synthesis rises because electrons flow faster to oxygen
  4. Oxygen consumption ceases completely and the cell dies at once

Explanation

Chemiosmotic theory holds that ATP synthase is driven by the proton electrochemical gradient built by complexes I, III, and IV. Losing proton pumping at complex III weakens that gradient, so ATP synthesis declines even if some electron transfer continues.

Question 31Genetics and Molecular Biology

In a DNA double helix, which base pair is held together by three hydrogen bonds?

  1. Thymine paired with guanine
  2. Adenine paired with uracil
  3. Guanine paired with cytosine Correct answer.
  4. Adenine paired with thymine

Explanation

Guanine and cytosine form three hydrogen bonds because the G–C pair has three complementary hydrogen-bonding positions. Adenine and thymine form only two hydrogen bonds. This is why GC-rich regions of DNA require more energy to separate than AT-rich regions.

Question 32Genetics and Molecular Biology

Which enzyme is responsible for synthesising the RNA primer needed to begin DNA replication on the lagging strand?

  1. Helicase
  2. DNA polymerase III
  3. Ligase
  4. Primase Correct answer.

Explanation

Primase is an RNA polymerase that creates a short RNA primer complementary to the template strand, giving DNA polymerase a free 3'-OH to extend. DNA polymerase cannot initiate synthesis de novo, so a primer is essential on both leading and lagging strands.

Question 33Genetics and Molecular Biology

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

  1. Duplication of the entire codon
  2. Premature termination of translation Correct answer.
  3. A single amino acid substitution with no effect on length
  4. A frameshift affecting all downstream codons

Explanation

UAA is one of the three stop codons (UAA, UAG, UGA), so translation terminates early and produces a truncated polypeptide. Because only one base changed, the reading frame downstream is unaffected until the new stop is reached.

Question 34Genetics and Molecular Biology

Which statement correctly describes the difference between mitosis and meiosis I?

  1. Mitosis separates homologous chromosomes; meiosis I separates sister chromatids.
  2. Mitosis involves crossing over; meiosis I does not.
  3. Mitosis separates sister chromatids; meiosis I separates homologous chromosomes. Correct answer.
  4. Mitosis produces haploid cells; meiosis I produces diploid cells.

Explanation

In mitosis, sister chromatids separate at anaphase, maintaining chromosome number. In meiosis I, homologous chromosomes separate after pairing and crossing over, reducing the chromosome number by half. This distinction is the basis of the reductional nature of meiosis I.

Question 35Genetics and Molecular Biology

In a dihybrid cross between two heterozygotes (AaBb × AaBb) with independent assortment, what fraction of offspring is expected to show both recessive phenotypes?

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

Explanation

Each recessive phenotype appears with probability 1/4 in a monohybrid cross of heterozygotes. By the product rule for independent events, the joint probability is 1/4 × 1/4 = 1/16.

Question 36Genetics and Molecular Biology

Which molecule carries the amino acid to the ribosome during translation?

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

Explanation

Transfer RNA (tRNA) has an anticodon loop that base-pairs with the mRNA codon and an acceptor stem covalently linked to a specific amino acid. It delivers the amino acid to the growing polypeptide chain.

Question 37Genetics and Molecular Biology

Which of the following is a characteristic of the genetic code?

  1. It is overlapping, so each base codes for more than one amino acid.
  2. It contains no stop signals, so translation continues indefinitely.
  3. It is ambiguous, so one codon can specify different amino acids.
  4. It is degenerate, meaning several codons can specify the same amino acid. Correct answer.

Explanation

Degeneracy means multiple codons encode the same amino acid, which buffers against some point mutations. The code is also non-overlapping, unambiguous, and includes three stop codons.

Question 38Genetics and Molecular Biology

Which type of RNA is the primary component of the ribosome's catalytic site that forms peptide bonds?

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

Explanation

Ribosomal RNA (rRNA) forms the core of the ribosome and, in the large subunit, catalyses peptide bond formation as a ribozyme. Proteins in the ribosome mainly provide structural support.

Question 39Genetics and Molecular Biology

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

  1. 1/4
  2. 1/2 Correct answer.
  3. All sons
  4. 0

Explanation

Sons inherit their single X chromosome from the mother. Half of the mother's X chromosomes carry the recessive allele, so half of the sons receive X^a and are affected. Daughters receive X^A from the father and are unaffected (half are carriers).

Question 40Genetics and Molecular Biology

In a population at Hardy–Weinberg equilibrium, the frequency of the recessive allele is 0.2. What proportion of the population is expected to be heterozygous?

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

Explanation

Heterozygote frequency is 2pq. With q = 0.2, p = 0.8, so 2pq = 2 × 0.8 × 0.2 = 0.32. This is the expected fraction of carriers under equilibrium conditions.

Question 41Genetics and Molecular Biology

Which process produces genetic variation by exchanging segments between non-sister chromatids of homologous chromosomes?

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

Explanation

Crossing over occurs during prophase I when homologous chromosomes synapse and non-sister chromatids exchange reciprocal segments at chiasmata. It creates new combinations of alleles on a single chromosome.

Question 42Genetics and Molecular Biology

A frameshift mutation is most likely caused by which change in DNA?

  1. Inversion of a single base pair
  2. Substitution of one purine for another purine
  3. Deletion of two nucleotides Correct answer.
  4. Deletion of three nucleotides

Explanation

Insertions or deletions whose length is not a multiple of three shift the reading frame, altering every codon downstream. A two-nucleotide deletion therefore changes the entire downstream amino acid sequence.

Question 43Genetics and Molecular Biology

Which enzyme removes RNA primers and replaces them with DNA during replication?

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

Explanation

DNA polymerase I has 5'→3' exonuclease activity that removes RNA primers and polymerase activity that fills the resulting gaps with DNA. Ligase then seals the remaining nick.

Question 44Genetics and Molecular Biology

In the lac operon of E. coli, what happens when lactose is present and glucose is absent?

  1. Transcription occurs at a low basal rate because CAP is inactive.
  2. The repressor binds the operator and transcription is blocked.
  3. CAP binds and the repressor is inactivated, allowing high transcription. Correct answer.
  4. The repressor binds the promoter and transcription is enhanced.

Explanation

Lactose (as allolactose) binds the repressor, preventing it from binding the operator. With glucose absent, cAMP levels rise, CAP binds the promoter and stimulates transcription, giving high expression.

Question 45Genetics and Molecular Biology

Which of the following best describes the role of the promoter in transcription?

  1. It encodes the sequence of the polypeptide chain.
  2. It is the site where introns are removed from pre-mRNA.
  3. It is the sequence that signals termination of translation.
  4. It is the DNA region where RNA polymerase and transcription factors assemble. Correct answer.

Explanation

A promoter is a DNA sequence upstream of a gene that binds RNA polymerase and transcription factors to initiate transcription. Its position and orientation determine where and in which direction transcription begins.

Question 46Genetics and Molecular Biology

A test cross is performed to determine the genotype of a tall pea plant. Which result indicates the plant was heterozygous?

  1. All offspring are tall.
  2. All offspring are short.
  3. Half the offspring are short. Correct answer.
  4. Offspring show a 3:1 tall-to-short ratio.

Explanation

A test cross uses a homozygous recessive partner. A heterozygote crossed with a recessive homozygote yields roughly equal numbers of dominant and recessive phenotypes, so short offspring reveal the hidden recessive allele.

Question 47Genetics and Molecular Biology

Which of the following is a post-translational modification that can regulate protein activity?

  1. Phosphorylation of serine residues Correct answer.
  2. Removal of introns from pre-mRNA
  3. Attachment of an amino acid to tRNA
  4. Addition of a 5' cap to mRNA

Explanation

Phosphorylation of serine, threonine or tyrosine residues by kinases can switch protein activity on or off and is a classic post-translational modification. The other options occur during transcription or translation.

Question 48Genetics and Molecular Biology

A human somatic cell in G1 phase contains 46 chromosomes. How many DNA molecules are present in the same cell after S phase but before mitosis?

  1. 23
  2. 184
  3. 46
  4. 92 Correct answer.

Explanation

During S phase each chromosome is replicated into two sister chromatids, so the chromosome number stays at 46 but the DNA molecule count doubles to 92. These chromatids separate during mitosis.

Question 49Genetics and Molecular Biology

Which type of mutation is most likely to be neutral in its effect on protein function?

  1. A missense mutation in the active site
  2. A frameshift deletion of one base
  3. A nonsense mutation near the start codon
  4. A silent mutation in the third codon position Correct answer.

Explanation

A silent (synonymous) mutation changes a codon but, because of degeneracy, still specifies the same amino acid, leaving the protein sequence unchanged. Such mutations are often neutral.

Question 50Genetics and Molecular Biology

In a pedigree, an affected father and unaffected mother have an affected son and an unaffected daughter. Which inheritance pattern is most consistent with this observation?

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

Explanation

Y-linked traits pass from father to son only, which fits an affected father with an affected son and an unaffected daughter. The trait appears in every male descendant of an affected male.