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Lebanese University Faculty of Medical Sciences Entrance Examination - Second Year, General Medicine study materials
Lebanese University Faculty of Medical Sciences Entrance Examination - Second Year, General Medicine study resources

Lebanese University Faculty of Medical Sciences Entrance Examination - Second Year, General Medicine practice questions

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Question 1Biology - Cell Biology and Molecular Biology

During DNA replication, which enzyme removes the RNA primer and replaces it with DNA nucleotides?

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

Explanation

DNA polymerase I possesses 5' to 3' exonuclease activity that excises the RNA primer, and its polymerase activity then fills the gap with deoxynucleotides. Ligase only seals the remaining nick, helicase unwinds duplex DNA, and primase synthesizes the primer rather than removing it.

Question 2Biology - Cell Biology and Molecular Biology

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

  1. A frameshift affecting all downstream codons
  2. Substitution of one amino acid by another
  3. No change because the genetic code is degenerate
  4. Premature termination of translation Correct answer.

Explanation

UAA is one of the three stop codons, so the ribosome releases the nascent chain at that point, producing a truncated protein. Degeneracy would only protect against changes that still specify the same amino acid, and a single base change without insertion or deletion cannot shift the reading frame.

Question 3Biology - Cell Biology and Molecular Biology

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, which together generate most cellular ATP through chemiosmosis. The other listed organelles serve biosynthetic processing, sorting, or hydrolytic degradation rather than bulk ATP production.

Question 4Biology - Cell Biology and Molecular Biology

In the fluid mosaic model, which property best describes the movement of phospholipids within a single leaflet of the plasma membrane?

  1. Lateral diffusion is rapid while transverse flip-flop is very slow Correct answer.
  2. Both lateral diffusion and flip-flop occur at similar high rates
  3. Neither lateral nor transverse movement occurs without ATP
  4. Lateral diffusion is slow while flip-flop is rapid

Explanation

Phospholipids diffuse laterally within a leaflet on a timescale of microseconds, but moving between leaflets requires the polar head group to cross the hydrophobic core, which is energetically costly. Protein catalysts called flippases, floppases, and scramblases are needed to accelerate transverse movement.

Question 5Biology - Cell Biology and Molecular Biology

Which phase of the cell cycle is characterized by DNA replication, producing two sister chromatids per chromosome?

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

Explanation

S phase is defined by semiconservative DNA synthesis, after which each chromosome consists of two identical sister chromatids held at the centromere. G1 and G2 are gap phases of growth and checkpoint control, and M phase is when the chromatids are segregated.

Question 6Biology - Cell Biology and Molecular Biology

A patient's cells show normal transcription but a protein is synthesized that is 40 amino acids shorter than expected. Which level of gene expression is most directly affected?

  1. mRNA capping
  2. DNA replication
  3. Transcription initiation
  4. Translation Correct answer.

Explanation

A shortened polypeptide with unchanged transcription indicates that the reading of the mRNA by ribosomes is altered, for example by a premature stop codon or a splicing change affecting the coding sequence. The defect therefore lies at the translational level rather than in the production of the transcript.

Question 7Biology - Cell Biology and Molecular Biology

Which structural feature distinguishes prokaryotic from eukaryotic chromosomes?

  1. Prokaryotic DNA is typically circular and not packaged with histones Correct answer.
  2. Prokaryotic DNA contains introns that are spliced after transcription
  3. Prokaryotic DNA is enclosed within a nuclear envelope
  4. Prokaryotic DNA is linear and wrapped around histones

Explanation

Bacterial chromosomes are usually circular and associate with histone-like proteins rather than true histones, forming a nucleoid without a membrane boundary. Eukaryotic DNA is linear, histone-packaged, and separated from the cytoplasm by the nuclear envelope.

Question 8Biology - Cell Biology and Molecular Biology

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

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

Explanation

Nucleotides in a strand are joined by phosphodiester bonds between the 3' hydroxyl of one sugar and the 5' phosphate of the next. Hydrogen bonds instead hold complementary bases of opposite strands together, while peptide and glycosidic linkages belong to proteins and sugars respectively.

Question 9Biology - Cell Biology and Molecular Biology

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

  1. Water enters the cell and it swells
  2. Water leaves the cell and it shrinks Correct answer.
  3. There is no net water movement
  4. The cell actively pumps water inward

Explanation

In a hypertonic medium the solute concentration outside exceeds that inside, so the osmotic gradient drives water outward across the membrane. The cell loses volume and shrinks, a process called crenation in cells with rigid walls absent.

Question 10Biology - Cell Biology and Molecular Biology

Which component of the electron transport chain directly reduces molecular oxygen in aerobic respiration?

  1. Cytochrome bc1 complex (complex III)
  2. Cytochrome c oxidase (complex IV) Correct answer.
  3. NADH dehydrogenase (complex I)
  4. ATP synthase

Explanation

Complex IV, cytochrome c oxidase, transfers electrons from cytochrome c to oxygen, producing water, and pumps protons across the inner membrane. Complexes I and III transfer electrons to downstream carriers, while ATP synthase uses the proton gradient rather than reducing oxygen.

Question 11Biology - Cell Biology and Molecular Biology

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

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

Explanation

In the payoff phase, phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, yielding ATP and 3-phosphoglycerate. This is substrate-level phosphorylation because the phosphate donor is a high-energy metabolic intermediate rather than ATP itself.

Question 12Biology - Cell Biology and Molecular Biology

Which feature of the genetic code explains why a silent mutation can occur without altering the protein sequence?

  1. Comma-less reading, because codons are read continuously
  2. Non-overlapping reading, because each base belongs to one codon
  3. Degeneracy, because several codons specify the same amino acid Correct answer.
  4. Universality, because the code is shared across organisms

Explanation

Degeneracy means that multiple synonymous codons encode the same amino acid, so a base change can leave the amino acid unchanged. The other properties concern how codons are arranged and read, not the existence of alternative codons for one residue.

Question 13Biology - Cell Biology and Molecular Biology

A restriction enzyme recognizes a palindromic six-base sequence and makes staggered cuts. What property of its cleavage products is most relevant for cloning a gene into a plasmid cut with the same enzyme?

  1. Both remove the phosphate groups needed for ligation
  2. Both produce incompatible overhangs requiring adaptors
  3. Both produce identical cohesive ends that can base-pair and be ligated Correct answer.
  4. Both produce blunt ends that cannot be ligated

Explanation

Cleaving the insert and the vector with the same enzyme generates complementary single-stranded overhangs, which anneal by base pairing and can then be sealed by DNA ligase. Compatibility of the ends, not the length of the recognition site, determines whether direct ligation is feasible.

Question 14Biology - Cell Biology and Molecular Biology

Which observation would best support the conclusion that a protein is targeted to the endoplasmic reticulum by a signal peptide?

  1. Removal of the signal sequence prevents association of the nascent chain with the ER membrane Correct answer.
  2. The protein contains more hydrophobic residues than a cytosolic protein
  3. The protein accumulates in the cytosol when transcription is inhibited
  4. The protein is synthesized more rapidly after adding a protease inhibitor

Explanation

The signal hypothesis predicts that the amino-terminal signal peptide directs the ribosome to the ER via the signal recognition particle. Deleting the sequence and observing loss of ER targeting is a direct functional test of the hypothesis.

Question 15Biology - Cell Biology and Molecular Biology

In a cell treated with a drug that blocks the G1/S cyclin-dependent kinase, what is the most immediate consequence?

  1. The cell completes mitosis but fails cytokinesis
  2. The cell arrests in metaphase with unseparated chromatids
  3. The cell undergoes apoptosis immediately
  4. The cell fails to enter S phase and DNA replication Correct answer.

Explanation

G1/S CDK activity phosphorylates the retinoblastoma protein and releases E2F, allowing transcription of S-phase genes. Blocking this kinase prevents the G1-to-S transition, so replication does not begin.

Question 16Biology - Cell Biology and Molecular Biology

Which mechanism allows a single gene to produce multiple distinct protein isoforms in eukaryotic cells?

  1. Wobble base pairing at the third codon position
  2. Semiconservative DNA replication
  3. Alternative splicing of the pre-mRNA Correct answer.
  4. Addition of the poly(A) tail

Explanation

Alternative splicing joins different combinations of exons, so one pre-mRNA can yield several mature mRNAs encoding different proteins. Polyadenylation and wobble pairing affect stability or decoding efficiency but do not recombine coding segments.

Question 17Biology - Cell Biology and Molecular Biology

A tRNA molecule has the anticodon 5'-GAC-3'. Which mRNA codon does it recognize by antiparallel base pairing?

  1. 5'-GAC-3'
  2. 5'-GUC-3' Correct answer.
  3. 5'-CAG-3'
  4. 5'-CUG-3'

Explanation

Antiparallel pairing requires the anticodon to align in reverse orientation with the codon, so 5'-GAC-3' pairs with 5'-GUC-3'. Writing the anticodon in the same orientation as the codon, or reversing the wrong strand, gives the incorrect choices.

Question 18Biology - Cell Biology and Molecular Biology

Which property of the sodium-potassium ATPase distinguishes it as an active transporter?

  1. It forms a pore that opens only in response to a ligand
  2. It hydrolyzes ATP to move ions against their electrochemical gradients Correct answer.
  3. It moves water molecules along an osmotic gradient
  4. It allows ions to diffuse down their gradients without energy input

Explanation

The pump couples ATP hydrolysis to the export of three sodium ions and import of two potassium ions, both against their gradients. This electrogenic exchange is the defining example of primary active transport in the plasma membrane.

Question 19Biology - Cell Biology and Molecular Biology

Which type of point mutation is most likely to produce a completely nonfunctional protein through loss of the normal start codon?

  1. A mutation in the 3' untranslated region
  2. A substitution in the initiation codon that abolishes translation initiation Correct answer.
  3. A missense mutation substituting a similar amino acid far from the active site
  4. A silent mutation in the third base of an internal codon

Explanation

If the AUG start codon is changed, the ribosome cannot initiate at the correct site, so no full-length protein is made from that transcript. The other changes either preserve the amino acid sequence, cause a mild functional alteration, or affect regulation rather than the coding frame.

Question 20Biology - Cell Biology and Molecular Biology

Which experimental result would indicate that a DNA sequence acts as an enhancer for a reporter gene?

  1. Reporter expression increases when the element is placed at a distance from the promoter, in either orientation Correct answer.
  2. Reporter mRNA becomes more stable when the element is inserted downstream of the poly(A) signal
  3. Reporter protein is synthesized faster when the element is fused to the coding sequence
  4. Reporter expression falls to zero when the element is deleted from the coding region

Explanation

Enhancers are defined operationally by their ability to stimulate transcription from a distance and independently of orientation relative to the promoter. Position independence and orientation independence distinguish enhancers from promoter-proximal elements.

Question 21Biology - Cell Biology and Molecular Biology

In an in vitro translation system, addition of a nonhydrolyzable GTP analog blocks polypeptide chain elongation. Which factor is most directly inhibited?

  1. Peptidyl transferase formation of the peptide bond
  2. Aminoacyl-tRNA synthetase charging of tRNA
  3. Translocation of mRNA through the ribosome by the small subunit alone
  4. EF-Tu delivery of aminoacyl-tRNA to the A site Correct answer.

Explanation

EF-Tu binds GTP and delivers aminoacyl-tRNA to the ribosomal A site, and hydrolysis of GTP is required for its release and for proofreading. Peptidyl transferase is a ribozyme-catalyzed reaction that does not consume GTP directly, and tRNA charging uses ATP.

Question 22Biology - Cell Biology and Molecular Biology

A frameshift mutation arises from insertion of two nucleotides near the 5' end of a coding sequence. Which outcome is most probable?

  1. All codons downstream of the insertion are read in an altered frame, usually producing a truncated or aberrant protein Correct answer.
  2. The mRNA is not transcribed because the mutation lies in the coding region
  3. Only the codon containing the insertion is altered, with the rest of the protein unchanged
  4. The reading frame is restored automatically because the code is degenerate

Explanation

Inserting two bases shifts the reading frame by two positions, so every subsequent codon is misread and a premature stop codon often appears. Degeneracy cannot restore the frame because it affects codon-anticodon matching, not the register of triplet reading.

Question 23Biology - Cell Biology and Molecular Biology

Which feature of telomerase explains why most somatic cells fail to maintain chromosome ends indefinitely?

  1. Telomerase is active only in the G2 phase and degrades DNA there
  2. Telomerase removes the terminal repeats during each S phase
  3. Telomerase is a reverse transcriptase whose catalytic subunit is repressed in most somatic cells Correct answer.
  4. Telomerase extends chromosomes by adding histone proteins to the ends

Explanation

Telomerase carries an RNA template and reverse-transcribes telomeric repeats onto the 3' overhang, but its catalytic protein subunit is not expressed in most differentiated somatic cells. Progressive shortening of chromosome ends then limits replicative capacity.

Question 24Biology - Cell Biology and Molecular Biology

In a cell, a missense mutation in the gene encoding a component of the signal recognition particle prevents it from binding the signal peptide. What is the predicted consequence?

  1. Lysosomal hydrolases are delivered to lysosomes but remain inactive
  2. Mitochondrial matrix proteins fail to be imported
  3. Proteins destined for the secretory pathway accumulate in the cytosol Correct answer.
  4. Proteins are inserted into the ER but are misfolded

Explanation

Without SRP recognition, the ribosome-nascent chain complex is not targeted to the ER translocon, so co-translational translocation cannot occur and the polypeptide is released in the cytosol. Mitochondrial import uses distinct Tom and Tim machinery and is unaffected.

Question 25Biology - Cell Biology and Molecular Biology

A plasmid carries an ampicillin-resistance gene and a lacZ gene with a multiple cloning site inside lacZ. After ligation and transformation into lacZ-positive, ampicillin-sensitive E. coli, which colonies indicate successful insertion of the foreign DNA?

  1. Colonies that grow on ampicillin but remain white on X-gal Correct answer.
  2. Colonies that grow on ampicillin and turn blue on X-gal
  3. Colonies that fail to grow on ampicillin and remain white on X-gal
  4. Colonies that fail to grow on ampicillin and turn blue on X-gal

Explanation

Insertion into the multiple cloning site disrupts lacZ, abolishing beta-galactosidase activity, so recombinants are white on X-gal. The plasmid confers ampicillin resistance, so only transformed cells grow on the antibiotic, and blue colonies carry an intact lacZ without an insert.

Question 26Biology - Cell Biology and Molecular Biology

A cell is treated with a drug that inhibits the enzyme responsible for adding the 5' cap to mRNA. Which step in gene expression is most directly affected?

  1. Binding of the mRNA to the ribosome for translation Correct answer.
  2. Export of the mRNA from the nucleus to the cytoplasm
  3. Addition of the poly(A) tail to the mRNA
  4. Splicing of introns from the pre-mRNA

Explanation

The 5' cap is required for efficient translation initiation because it is recognized by eukaryotic initiation factors that recruit the ribosome. Without the cap, translation is severely reduced. The cap also aids in export and splicing, but its primary and most direct role is in translation initiation.

Question 27Biology - Cell Biology and Molecular Biology

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

  1. It directly inserts the polypeptide into the ER membrane by forming a channel.
  2. It binds to the signal sequence of a nascent polypeptide and pauses translation until the ribosome docks at the ER. Correct answer.
  3. It degrades proteins that lack a signal sequence.
  4. It adds a signal sequence to proteins that are destined for the nucleus.

Explanation

The SRP recognizes the hydrophobic signal sequence on a nascent polypeptide as it emerges from the ribosome, causing a pause in translation. The SRP–ribosome complex then binds to the SRP receptor on the ER membrane, allowing translation to resume and the protein to be translocated into the ER.

Question 28Biology - Cell Biology and Molecular Biology

Which of the following is a key difference between mitosis and meiosis?

  1. Mitosis involves one round of DNA replication, whereas meiosis involves two.
  2. Mitosis produces genetically identical daughter cells, whereas meiosis produces genetically diverse gametes. Correct answer.
  3. Mitosis produces haploid cells, whereas meiosis produces diploid cells.
  4. Mitosis separates homologous chromosomes, whereas meiosis separates sister chromatids.

Explanation

Mitosis is an equational division that produces two daughter cells genetically identical to the parent cell. Meiosis consists of two divisions and includes crossing over and independent assortment, generating genetic diversity in the haploid gametes.

Question 29Biology - Cell Biology and Molecular Biology

Which component of the cytoskeleton is primarily responsible for the movement of chromosomes during mitosis?

  1. Intermediate filaments
  2. Microtubules Correct answer.
  3. Septins
  4. Actin microfilaments

Explanation

The mitotic spindle is composed of microtubules, which attach to kinetochores on chromosomes and move them to opposite poles during anaphase. Microfilaments and intermediate filaments have other roles in the cytoskeleton.

Question 30Biology - Cell Biology and Molecular Biology

In a cell, a mutation in a gene encoding a DNA repair enzyme causes an increased rate of mutations. Which type of repair is most likely defective if the cell cannot remove pyrimidine dimers?

  1. Nucleotide excision repair Correct answer.
  2. Base excision repair
  3. Homologous recombination
  4. Mismatch repair

Explanation

Pyrimidine dimers, such as those caused by UV light, are bulky DNA lesions that are repaired by nucleotide excision repair. This pathway removes a short single-stranded segment containing the damage and fills it in using the complementary strand as a template.

Question 31Biology - Cell Biology and Molecular Biology

Which of the following best explains why the lac operon is expressed at high levels only when lactose is present and glucose is absent?

  1. Glucose directly binds to the lac repressor and prevents it from binding to the operator.
  2. The lac operon is constitutively expressed and only affected by glucose levels.
  3. Lactose activates CAP, and glucose inhibits the lac repressor.
  4. Lactose directly binds to the lac repressor and inhibits its DNA binding, while glucose absence increases cAMP levels, which activates CAP. Correct answer.

Explanation

The lac operon is under both negative and positive control. Lactose (via allolactose) binds the repressor, reducing its affinity for the operator. When glucose is low, cAMP levels rise, and cAMP binds CAP, which then binds upstream of the promoter and activates transcription.

Question 32Biology - Cell Biology and Molecular Biology

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

  1. No change in the amino acid sequence
  2. A frameshift leading to a truncated protein
  3. Premature termination of translation Correct answer.
  4. A single amino acid substitution

Explanation

UAA is a stop codon (ochre). When a sense codon is mutated to a stop codon, translation terminates prematurely, producing a truncated protein. This is a nonsense mutation.

Question 33Biology - Cell Biology and Molecular Biology

Which statement about the role of GTP in translation is correct?

  1. GTP provides energy for the binding of the initiator tRNA to the small ribosomal subunit.
  2. GTP hydrolysis is required for the translocation of the ribosome along the mRNA. Correct answer.
  3. GTP is used to charge the tRNA with its amino acid.
  4. GTP is hydrolyzed during the formation of the peptide bond.

Explanation

During elongation, the translocation of the ribosome by one codon is driven by GTP hydrolysis by elongation factor G (EF-G in prokaryotes, eEF2 in eukaryotes). GTP is also used in initiation and in tRNA selection, but translocation is a key GTP-dependent step.

Question 34Biology - Cell Biology and Molecular Biology

Which of the following is a function of the 5' untranslated region (5' UTR) of an mRNA?

  1. It determines the polyadenylation site.
  2. It contains the Shine-Dalgarno sequence or Kozak sequence for ribosome binding. Correct answer.
  3. It encodes the stop codon.
  4. It encodes the signal peptide for protein targeting.

Explanation

The 5' UTR contains sequences that are important for ribosome binding and regulation of translation. In prokaryotes, the Shine-Dalgarno sequence pairs with the rRNA; in eukaryotes, the Kozak sequence surrounds the start codon and enhances initiation.

Question 35Biology - Cell Biology and Molecular Biology

A cell is treated with a drug that prevents the assembly of the mitotic spindle. Which phase of the cell cycle will the cell most likely arrest in?

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

Explanation

The mitotic spindle is required for chromosome segregation during M phase. Without a functional spindle, the cell cannot progress through mitosis and will arrest in M phase, often triggering apoptosis or a prolonged block.

Question 36Biology - Cell Biology and Molecular Biology

Which of the following best describes the function of a molecular chaperone?

  1. It covalently modifies proteins to target them for degradation.
  2. It synthesizes the polypeptide chain during translation.
  3. It transports proteins across the nuclear pore.
  4. It assists in the folding of newly synthesized proteins and prevents aggregation. Correct answer.

Explanation

Molecular chaperones are proteins that assist in the non-covalent folding or unfolding and the assembly or disassembly of other macromolecular structures. They help nascent polypeptides fold correctly and prevent misfolding and aggregation.

Question 37Biology - Cell Biology and Molecular Biology

In a cell, a protein that normally resides in the cytosol is found in the nucleus. Which change in the protein could explain this?

  1. Mutation that prevents phosphorylation
  2. Addition of a nuclear export signal
  3. Loss of a signal peptide
  4. Addition of a nuclear localization signal Correct answer.

Explanation

Proteins are directed to the nucleus by a nuclear localization signal (NLS) that is recognized by importins. Adding an NLS to a cytosolic protein would cause it to be imported into the nucleus.

Question 38Biology - Cell Biology and Molecular Biology

Which of the following is a characteristic of eukaryotic DNA replication?

  1. It involves a single origin of replication per chromosome.
  2. It requires a primer made of RNA. Correct answer.
  3. It occurs in the cytoplasm.
  4. It is catalyzed by DNA polymerase without the need for a template.

Explanation

DNA polymerases cannot initiate synthesis de novo; they require a primer with a free 3'-OH group. In eukaryotic replication, primase synthesizes a short RNA primer that is extended by DNA polymerase.

Question 39Biology - Cell Biology and Molecular Biology

A mutation in the gene encoding a cyclin-dependent kinase (CDK) inhibitor leads to uncontrolled cell division. Which of the following is the most likely explanation?

  1. The CDK inhibitor enhances the activity of the anaphase-promoting complex.
  2. The CDK inhibitor is overactive, causing excessive inhibition of CDKs.
  3. The CDK inhibitor promotes the degradation of cyclins.
  4. The CDK inhibitor is unable to bind to and inhibit CDK-cyclin complexes. Correct answer.

Explanation

CDK inhibitors normally bind to and inhibit CDK-cyclin complexes, preventing progression through the cell cycle. If a mutation causes loss of function of a CDK inhibitor, CDKs may become overactive, leading to uncontrolled proliferation.

Question 40Biology - Cell Biology and Molecular Biology

Which of the following mechanisms can lead to the production of multiple protein isoforms from a single gene by joining different combinations of exons?

  1. DNA methylation
  2. Alternative splicing of pre-mRNA Correct answer.
  3. RNA editing
  4. Histone acetylation

Explanation

Alternative splicing allows different combinations of exons to be joined, producing multiple mRNA variants from the same gene. Each mRNA can be translated into a distinct protein isoform. RNA editing instead changes the RNA sequence without joining different combinations of exons.

Question 41Biology - Genetics

In a pedigree, a trait appears in every generation, affects about half of the offspring of an affected parent, and is transmitted by affected fathers to all their daughters but to no sons. Which mode of inheritance best fits this pattern?

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

Explanation

X-linked dominant traits appear in each generation because one copy of the allele on the X chromosome is sufficient, and an affected father passes his only X to every daughter while giving sons his Y. This produces the characteristic father-to-all-daughters, father-to-no-sons transmission. Autosomal recessive traits typically skip generations, and Y-linked traits pass only from father to son.

Question 42Biology - Genetics

A DNA segment has the template strand sequence 3'-TAC GGA TTC-5'. Using the standard genetic code, what amino acid sequence does it encode?

  1. Methionine–Glycine–Lysine
  2. Tyrosine–Glycine–Phenylalanine
  3. Methionine–Glycine–Glutamic acid
  4. Methionine–Proline–Lysine Correct answer.

Explanation

The mRNA is complementary and antiparallel to the template, giving 5'-AUG CCU AAG-3'. AUG codes for methionine, CCU for proline, and AAG for lysine. Reading the template directly as if it were the coding strand is the usual error.

Question 43Biology - Genetics

Which statement describes the behavior of homologous chromosomes during meiosis I?

  1. They exchange entire chromatids with non-homologous chromosomes.
  2. They replicate during prophase I to double their DNA content.
  3. They pair as bivalents and separate from each other at anaphase I. Correct answer.
  4. They separate at anaphase II after aligning on the metaphase plate.

Explanation

Homologous chromosomes synapse during prophase I, forming bivalents, and are pulled to opposite poles at anaphase I. This reductional division halves the chromosome number. DNA replication occurs before meiosis, in S phase, not during prophase I.

Question 44Biology - Genetics

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What is the probability that a son of this couple is affected?

  1. 1/8
  2. 1/2 Correct answer.
  3. 1/4
  4. 0

Explanation

A son inherits his X chromosome from his mother, and a carrier mother transmits the mutant allele to half of her sons. Because sons cannot receive an X from their father, the father's status does not alter this probability. Thus each son has a 50% risk regardless of birth order.

Question 45Biology - Genetics

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

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

Explanation

With q = 0.2, p = 0.8, and the carrier frequency is 2pq = 2 × 0.8 × 0.2 = 0.32. The homozygous affected frequency is q² = 0.04. The calculation assumes random mating and no selection, mutation, or migration.

Question 46Biology - Genetics

A point mutation changes a codon from GAG to GUG in the coding region of a gene. Which classification best describes this change?

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

Explanation

GAG encodes glutamic acid and GUG encodes valine, so a single base substitution replaces one amino acid with another, which is a missense mutation. A nonsense mutation would create a stop codon, and a silent mutation would leave the amino acid unchanged. Frameshifts require insertion or deletion of bases not a multiple of three.

Question 47Biology - Genetics

Which enzyme is responsible for joining Okazaki fragments by forming phosphodiester bonds between adjacent nucleotides on the lagging strand?

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

Explanation

DNA ligase seals the nicks between Okazaki fragments by catalyzing phosphodiester bonds. DNA polymerase extends chains but cannot join two pre-existing fragments without a free 3'-OH adjacent to a primer. Helicase unwinds the duplex and primase synthesizes RNA primers.

Question 48Biology - Genetics

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

Explanation

Each gene independently shows a 1/4 probability of the recessive phenotype in a monohybrid cross of heterozygotes. Multiplying the independent probabilities gives 1/4 × 1/4 = 1/16 for the double recessive phenotype. The 9:3:3:1 ratio reflects the combined phenotypic classes.

Question 49Biology - Genetics

A chromosomal translocation in which a segment moves between two non-homologous chromosomes without any loss of genetic material is best described as which type of rearrangement?

  1. Reciprocal translocation Correct answer.
  2. Robertsonian translocation
  3. Deletion
  4. Inversion

Explanation

A reciprocal translocation involves exchange of segments between two non-homologous chromosomes with no net loss of DNA. It differs from a Robertsonian translocation, which involves fusion of two acrocentric chromosomes. Inversions and deletions do not involve exchange between non-homologues.

Question 50Biology - Genetics

Which of the following best describes the role of the anticodon in protein synthesis?

  1. It carries the amino acid to the ribosome.
  2. It forms the peptide bond between amino acids.
  3. It signals the end of transcription.
  4. It base-pairs with the mRNA codon during translation. Correct answer.

Explanation

The anticodon is a three-base sequence on tRNA that pairs with a complementary codon on mRNA through antiparallel base pairing. This ensures the correct amino acid is positioned in the growing polypeptide. The amino acid is attached at the acceptor stem, and peptide bonds are formed by the ribosome's peptidyl transferase activity.