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Question 1Cell and Molecular Biology

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

  1. Cytochrome c
  2. ATP Correct answer.
  3. NADH
  4. Glucose

Explanation

ATP is the universal short-term energy currency because hydrolysis of its terminal phosphoanhydride bond releases free energy that can be coupled to endergonic reactions. Other carriers such as NADH store reducing power, and glucose is a fuel that must first be catabolized.

Question 2Cell and Molecular Biology

During DNA replication, which enzyme synthesizes the new strand by adding nucleotides to a growing 3' end?

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

Explanation

DNA polymerases extend a primer by adding deoxynucleotides to the free 3'-hydroxyl group, reading the template strand in the 3'-to-5' direction. This template-directed extension is the defining catalytic activity of replication.

Question 3Cell and Molecular Biology

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

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

Explanation

The inner mitochondrial membrane houses the electron transport chain and ATP synthase, where chemiosmotic coupling generates most cellular ATP. The proton gradient across that membrane drives the phosphorylation of ADP.

Question 4Cell and Molecular Biology

Which process converts a primary transcript into a mature mRNA by removing noncoding sequences?

  1. Replication
  2. Translation
  3. Splicing Correct answer.
  4. Transcription

Explanation

Splicing removes introns and joins exons, producing a mature mRNA that can be exported and translated. The spliceosome catalyzes this reaction by recognizing conserved splice-site sequences.

Question 5Cell and Molecular Biology

Which type of bond links adjacent amino acids in a polypeptide chain?

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

Explanation

A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing water. This amide linkage defines the primary structure of a protein.

Question 6Cell and Molecular Biology

What is the fundamental unit of heredity that occupies a specific position on a chromosome?

  1. Gene Correct answer.
  2. Chromatid
  3. Allele
  4. Operon

Explanation

A gene is a segment of DNA at a defined chromosomal locus that encodes a functional product such as a protein or RNA. Alleles are alternative versions of that same gene.

Question 7Cell and Molecular Biology

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

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

Explanation

S phase is the synthesis phase in which the genome is duplicated so that each daughter cell receives a complete copy. G1 and G2 are gap phases that prepare for and follow replication.

Question 8Cell and Molecular Biology

Which cytoskeletal element is the principal component of the mitotic spindle?

  1. Keratin fibers
  2. Microtubules Correct answer.
  3. Actin filaments
  4. Intermediate filaments

Explanation

Microtubules, polymers of tubulin dimers, assemble into the spindle apparatus that captures chromosomes and pulls them apart. Their dynamic instability allows rapid reorganization during mitosis.

Question 9Cell and Molecular Biology

A mutation changes a single nucleotide in a codon but the encoded amino acid remains the same. What is this called?

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

Explanation

A silent mutation alters the nucleotide sequence without changing the amino acid, exploiting the degeneracy of the genetic code. Because the protein product is unchanged, such mutations are often phenotypically neutral.

Question 10Cell and Molecular Biology

In a reaction catalyzed by an enzyme, what is the effect of a competitive inhibitor?

  1. It raises the apparent Km without changing Vmax Correct answer.
  2. It increases Vmax at high substrate levels
  3. It lowers Vmax without changing Km
  4. It lowers both Km and Vmax

Explanation

A competitive inhibitor binds the active site reversibly, so higher substrate concentrations can outcompete it. This raises the apparent Michaelis constant while the maximum velocity remains attainable at saturating substrate.

Question 11Cell and Molecular Biology

A cell is placed in a hypertonic solution. What is the most likely immediate response?

  1. It swells and may lyse
  2. It actively pumps water inward
  3. It remains unchanged in volume
  4. It shrinks as water leaves Correct answer.

Explanation

In a hypertonic environment the external solute concentration exceeds that inside the cell, so water moves outward by osmosis. The resulting loss of water shrinks the cell and can impair its function.

Question 12Cell and Molecular Biology

Which feature distinguishes prokaryotic gene expression from eukaryotic gene expression?

  1. Prokaryotes splice introns before translation
  2. Prokaryotes couple transcription and translation in the same compartment Correct answer.
  3. Prokaryotes replicate DNA only during mitosis
  4. Prokaryotes export mRNA through a nuclear pore

Explanation

Lacking a nuclear envelope, prokaryotes can begin translation on an mRNA while transcription is still underway. Eukaryotes separate these processes spatially because transcription occurs in the nucleus.

Question 13Cell and Molecular Biology

A diploid organism with genotype AaBb undergoes meiosis. Assuming independent assortment, what fraction of gametes carries both recessive alleles?

  1. 1/2
  2. 1/16
  3. 1/8
  4. 1/4 Correct answer.

Explanation

Each heterozygous locus produces half gametes carrying the recessive allele, and independent assortment multiplies these probabilities. The product of one-half and one-half is one-quarter of the gametes.

Question 14Cell and Molecular Biology

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

  1. Silent mutation
  2. Polyploidy
  3. Alternative splicing Correct answer.
  4. Codominance

Explanation

Alternative splicing joins different combinations of exons from the same transcript, yielding variant mRNAs that encode different proteins. This expands proteome diversity without requiring additional genes.

Question 15Cell and Molecular Biology

A protein is synthesized on ribosomes and must be secreted from the cell. Which pathway does it follow?

  1. Cytosol to nucleus to plasma membrane
  2. Smooth ER to mitochondria to cytosol
  3. Rough ER to Golgi to secretory vesicles Correct answer.
  4. Free ribosomes to peroxisomes to Golgi

Explanation

Secretory proteins carry a signal peptide that directs the ribosome to the rough endoplasmic reticulum, after which vesicles carry them through the Golgi for processing and sorting. Secretory vesicles then fuse with the plasma membrane to release the contents.

Question 16Cell and Molecular Biology

Which statement best describes the role of the tumor suppressor p53?

  1. It phosphorylates Rb to drive S phase
  2. It promotes cell cycle entry after DNA damage
  3. It halts the cycle or triggers apoptosis when DNA is damaged Correct answer.
  4. It activates telomerase in stem cells

Explanation

p53 acts as a transcription factor that, upon DNA damage, induces cell cycle arrest for repair or apoptosis if damage is irreparable. This guardian function prevents propagation of mutations.

Question 17Cell and Molecular Biology

Which property of the genetic code explains why many amino acids are specified by more than one codon?

  1. It is universal
  2. It is triplet-based
  3. It is degenerate Correct answer.
  4. It is non-overlapping

Explanation

Degeneracy means several codons can encode the same amino acid, which buffers against some point mutations. The redundancy is most pronounced at the third codon position.

Question 18Cell and Molecular Biology

During glycolysis, what is the net yield of ATP and NADH per molecule of glucose?

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

Explanation

Glycolysis consumes two ATP in the preparatory phase and produces four ATP in the payoff phase, giving a net of two ATP. Two molecules of NADH are also generated when glyceraldehyde-3-phosphate is oxidized.

Question 19Cell and Molecular Biology

A researcher observes that a gene is transcribed only when a specific activator protein binds an enhancer far upstream. What does this illustrate?

  1. Positive regulation of transcription Correct answer.
  2. Attenuation of translation
  3. RNA interference
  4. Post-translational control

Explanation

An activator that increases transcription by binding a distant enhancer exemplifies positive regulation, often mediated by DNA looping that brings the enhancer near the promoter. The default state in such cases is low expression.

Question 20Cell and Molecular Biology

Which observation would support the hypothesis that a trait is inherited in an autosomal recessive pattern?

  1. Males are affected far more often than females
  2. Affected individuals appear in every generation
  3. Two unaffected parents can have an affected child Correct answer.
  4. An affected parent transmits the trait to all offspring

Explanation

Recessive traits can skip generations because heterozygous carriers are phenotypically unaffected yet can transmit the allele. Two carriers mating have a one-in-four chance of producing an affected child.

Question 21Cell and Molecular Biology

Which enzyme activity is required to remove RNA primers and replace them with DNA during replication?

  1. Ligase
  2. Helicase
  3. Exonuclease activity of DNA polymerase Correct answer.
  4. Primase

Explanation

DNA polymerase uses its exonuclease activity to excise RNA primers, then fills the resulting gaps with DNA. The remaining nick is later sealed by ligase to complete the sugar-phosphate backbone.

Question 22Cell and Molecular Biology

A mutation in a proto-oncogene converts it into an oncogene. Which change in protein behavior is most consistent with this conversion?

  1. Gain of function that is dominant at the cellular level Correct answer.
  2. Loss of function that requires both alleles to be mutated
  3. Increased dependence on growth factors for activation
  4. Complete absence of the protein product

Explanation

Oncogenic activation typically produces a constitutively active or overexpressed protein that drives proliferation, and a single altered allele can suffice. This dominant gain of function contrasts with the recessive loss of function typical of tumor suppressors.

Question 23Cell and Molecular Biology

In the lac operon, what happens when lactose is present but glucose is also abundant?

  1. Constitutive high expression regardless of glucose
  2. Low expression because CAP cannot activate transcription Correct answer.
  3. Complete repression because the repressor stays bound
  4. Full induction because the repressor is removed

Explanation

Lactose removes the repressor, but high glucose lowers cAMP, so the catabolite activator protein cannot bind and stimulate transcription. The operon is therefore only weakly expressed despite derepression.

Question 24Cell and Molecular Biology

A cell accumulates misfolded proteins in the endoplasmic reticulum. Which response is most directly triggered?

  1. Immediate entry into mitosis
  2. Stimulation of glycolysis
  3. Increased ribosomal subunit export
  4. Activation of the unfolded protein response Correct answer.

Explanation

The unfolded protein response senses misfolded protein load in the ER and reduces translation while increasing chaperone production and folding capacity. If homeostasis cannot be restored, the response can trigger apoptosis.

Question 25Cell and Molecular Biology

A frameshift mutation occurs near the start of a coding sequence. Which outcome is most likely?

  1. A truncated or aberrant protein due to a changed reading frame Correct answer.
  2. A single amino acid substitution with normal function
  3. Duplication of the entire chromosome
  4. No change because the code is degenerate

Explanation

Inserting or deleting nucleotides that are not a multiple of three shifts the reading frame, so all downstream codons are misread. This usually yields a premature stop codon or a nonfunctional protein.

Question 26Cell and Molecular Biology

Which molecule serves as the immediate energy currency that powers most cellular work, such as active transport and biosynthesis?

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

Explanation

ATP is the cell's immediate energy currency: hydrolysis of its terminal phosphoanhydride bond releases free energy that can be coupled to endergonic processes such as pumping ions or building polymers. Glucose and NADH store energy but must first be oxidized or processed to regenerate ATP.

Question 27Cell and Molecular Biology

A DNA sample contains 30% adenine. According to base-pairing rules, what percentage of the bases is guanine?

  1. 70%
  2. 40%
  3. 30%
  4. 20% Correct answer.

Explanation

Chargaff's rules state that adenine pairs with thymine and guanine pairs with cytosine, so the A fraction equals the T fraction and G equals C. If adenine is 30%, thymine is also 30%, leaving 40% shared equally between guanine and cytosine, which gives 20% guanine.

Question 28Cell and Molecular Biology

A transmembrane receptor binds its ligand, triggering autophosphorylation of tyrosine residues and assembly of a signaling complex. Which class of receptor does this describe?

  1. G protein-coupled receptor
  2. Receptor tyrosine kinase Correct answer.
  3. Nuclear hormone receptor
  4. Ion channel-linked receptor

Explanation

Receptor tyrosine kinases dimerize upon ligand binding and cross-phosphorylate tyrosine residues, creating docking sites for downstream proteins. This intrinsic kinase activity distinguishes them from receptors that rely on separate effector proteins or second messengers.

Question 29Cell and Molecular Biology

In eukaryotes, which modification of histone tails is most directly associated with a more open, transcriptionally active chromatin state?

  1. Histone H3 lysine 9 methylation
  2. Histone H3 lysine 27 methylation
  3. Histone H3 lysine 4 methylation Correct answer.
  4. Histone H2A ubiquitination

Explanation

Methylation of histone H3 lysine 4 marks active promoters and euchromatic regions, helping recruit factors that keep chromatin accessible. In contrast, methylation at lysine 9 or lysine 27 recruits repressive complexes that compact chromatin and silence transcription.

Question 30Cell and Molecular Biology

A cell enters mitosis with unreplicated chromosomes and no sister chromatids because S phase was skipped. Which checkpoint failure most directly explains the entry into mitosis under these conditions?

  1. Failure of the anaphase-promoting complex
  2. Failure of the G2 DNA damage checkpoint Correct answer.
  3. Failure of the restriction point in G1
  4. Failure of the spindle assembly checkpoint

Explanation

The G2 checkpoint verifies that replication is complete and DNA is undamaged before allowing entry into mitosis. If this surveillance fails, a cell with unreplicated chromosomes can slip into mitosis, producing chromosomes without sister chromatids.

Question 31Genetics and Genomics

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

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

Explanation

Transfer RNA molecules have an anticodon loop that base-pairs with the messenger RNA codon and an acceptor end covalently linked to a specific amino acid, so they deliver amino acids to the growing polypeptide chain.

Question 32Genetics and Genomics

A child inherits one copy of a mutated allele and develops the disease, while the parent carrying the same allele shows no symptoms. This pattern is most consistent with which inheritance mode?

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

Explanation

In autosomal dominant inheritance a single mutant allele is sufficient to cause disease, but penetrance may be incomplete so a carrier parent can remain unaffected. The child's disease from one inherited allele fits this model better than recessive or mitochondrial patterns.

Question 33Genetics and Genomics

What is the consequence of a nonsense mutation in a coding region?

  1. A premature stop codon is created Correct answer.
  2. A gene is duplicated
  3. A codon is deleted without frameshift
  4. A single amino acid is substituted

Explanation

A nonsense mutation changes a sense codon into a stop codon, which truncates the protein and often triggers nonsense-mediated decay of the messenger RNA.

Question 34Genetics and Genomics

During meiosis, crossing over between homologous chromosomes occurs at which stage?

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

Explanation

Homologous recombination occurs during prophase I of meiosis, when paired homologs form synaptonemal complexes and exchange segments at chiasmata. This exchange generates new combinations of alleles on the resulting chromosomes.

Question 35Genetics and Genomics

A couple are both carriers of an autosomal recessive condition. What is the probability that their next child is affected?

  1. 1 in 2
  2. 1 in 4 Correct answer.
  3. 3 in 4
  4. 1 in 8

Explanation

Two carrier parents each transmit the mutant allele with probability one half, so the chance of an affected child is one half times one half, which equals one quarter. This risk is independent of the outcomes of previous pregnancies.

Question 36Genetics and Genomics

Which technique is most appropriate for detecting a small deletion of three nucleotides in a specific gene?

  1. Karyotyping
  2. Sanger sequencing of the gene Correct answer.
  3. Fluorescence in situ hybridization of the whole chromosome
  4. Standard chromosome banding

Explanation

Sanger sequencing reads the nucleotide sequence of a targeted region and can resolve changes of a few base pairs, including a three-nucleotide deletion. Cytogenetic methods lack the resolution to detect such small lesions.

Question 37Genetics and Genomics

In a pedigree, an affected father transmits the condition to all of his daughters and none of his sons. Which inheritance pattern does this suggest?

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

Explanation

In X-linked dominant inheritance, an affected father passes his single X chromosome to all daughters, who become affected, and passes his Y chromosome to sons, who are unaffected. This sex-specific transmission pattern is characteristic of the X-linked dominant model.

Question 38Genetics and Genomics

Which type of genetic variation describes a single nucleotide substitution present in at least one percent of a population?

  1. Copy number variant
  2. Structural variant
  3. Tandem repeat
  4. Single nucleotide polymorphism Correct answer.

Explanation

A single nucleotide polymorphism is a one-base substitution that reaches appreciable frequency in a population, conventionally at least one percent. Rarer single-base changes are generally termed mutations or variants rather than polymorphisms.

Question 39Genetics and Genomics

A laboratory reports a variant of uncertain significance in a cancer predisposition gene. What is the most appropriate interpretation?

  1. It can be dismissed as a benign polymorphism
  2. It confirms the patient has the hereditary cancer syndrome
  3. The evidence is insufficient to classify the variant as disease-causing Correct answer.
  4. It should be treated as pathogenic for family screening

Explanation

A variant of uncertain significance has insufficient evidence to be classified as pathogenic or benign, so it should not be used to make clinical management decisions. Further evidence or segregation data may allow reclassification over time.

Question 40Genetics and Genomics

Which process produces a mature messenger RNA in eukaryotes?

  1. Addition of a poly-A tail to the gene
  2. Reverse transcription of the transcript
  3. Intron removal and exon joining Correct answer.
  4. Replication of the coding strand

Explanation

Eukaryotic pre-messenger RNA undergoes splicing, in which introns are removed and exons are joined, along with capping and polyadenylation, to yield the mature transcript.

Question 41Genetics and Genomics

A patient's tumor shows loss of both alleles of a DNA mismatch repair gene. This finding is most consistent with which concept?

  1. Gain of function mutation
  2. Chromosomal translocation
  3. Oncogene amplification
  4. Two-hit tumor suppressor inactivation Correct answer.

Explanation

Tumor suppressor genes generally require inactivation of both alleles before the cell loses the protective function, which is the two-hit model. Loss of both mismatch repair alleles fits this mechanism and can produce microsatellite instability.

Question 42Genetics and Genomics

What does the term penetrance describe?

  1. The rate at which a mutation arises de novo
  2. The number of different phenotypes caused by one gene
  3. The severity of disease in affected individuals
  4. The proportion of individuals with a genotype who show the associated phenotype Correct answer.

Explanation

Penetrance is the fraction of people carrying a given genotype who express the corresponding phenotype. It is distinct from expressivity, which describes the range or severity of the phenotype among those who do express it.

Question 43Genetics and Genomics

A woman carries a pathogenic variant in BRCA1. Which statement best reflects the associated risk profile?

  1. Risk is limited to colorectal cancer
  2. Risk of breast and ovarian cancer is substantially increased Correct answer.
  3. Risk is increased only in male relatives
  4. Risk is negligible because the gene is recessive

Explanation

Pathogenic BRCA1 variants are inherited in an autosomal dominant manner and markedly increase lifetime risk of breast and ovarian cancer. The tumor suppressor effect is recessive at the cellular level through loss of the second allele, but susceptibility is dominant at the family level.

Question 44Genetics and Genomics

Which statement correctly describes the difference between genotype and phenotype?

  1. Both terms refer only to alleles at a single locus
  2. Genotype is the allele combination and phenotype is the observable trait Correct answer.
  3. Genotype is the observable trait and phenotype is the allele combination
  4. Phenotype is fixed by genotype with no environmental input

Explanation

Genotype refers to the particular alleles an individual carries, whereas phenotype is the observable or measurable characteristic that results from genotype interacting with environment.

Question 45Genetics and Genomics

A reciprocal translocation is detected in a phenotypically normal parent whose child has multiple congenital anomalies. What is the most likely explanation for the child's findings?

  1. The child has a de novo point mutation
  2. The child has a mitochondrial disorder
  3. The child inherited the balanced translocation unchanged
  4. The child inherited an unbalanced chromosomal complement Correct answer.

Explanation

A balanced reciprocal translocation carrier can produce gametes with unbalanced chromosome complements after segregation during meiosis. Fertilization with such a gamete yields a child with partial trisomy and monosomy, causing congenital anomalies.

Question 46Genetics and Genomics

In Hardy-Weinberg equilibrium, if the frequency of a recessive disease allele is 0.1, what is the expected carrier frequency?

  1. 0.09
  2. 0.01
  3. 0.18 Correct answer.
  4. 0.81

Explanation

With allele frequency q equal to 0.1, the dominant allele frequency p is 0.9, and the carrier frequency 2pq equals 2 times 0.9 times 0.1, which is 0.18. The affected homozygote frequency q squared is 0.01.

Question 47Genetics and Genomics

Which type of mutation most often alters the reading frame of a protein-coding gene?

  1. A substitution in an intron far from a splice site
  2. A synonymous substitution
  3. A single base insertion in an exon Correct answer.
  4. A silent change in the third codon position

Explanation

Inserting or deleting a number of bases that is not a multiple of three shifts the reading frame downstream, changing every subsequent codon. Synonymous and silent substitutions preserve the amino acid sequence.

Question 48Genetics and Genomics

What is the primary purpose of genetic counseling before predictive testing for an adult-onset disorder?

  1. To discuss implications, risks, and options so the person can make an informed choice Correct answer.
  2. To guarantee that the test will be negative
  3. To decide the result of the test in advance
  4. To replace the need for informed consent

Explanation

Pretest counseling provides information about the condition, the meaning of possible results, and the psychological and family implications so that the individual can give informed consent and decide whether to proceed. It supports autonomous decision-making rather than predicting outcomes.

Question 49Genetics and Genomics

Which mechanism best explains anticipation in certain neurodegenerative disorders?

  1. Deletion of an entire chromosome
  2. Somatic loss of the other allele
  3. Expansion of a trinucleotide repeat across generations Correct answer.
  4. Silencing of a gene by methylation

Explanation

Anticipation is the tendency for a disorder to appear earlier or more severely in successive generations, typically because a trinucleotide repeat expands when transmitted. Longer repeats generally produce earlier onset and greater severity.

Question 50Genetics and Genomics

A male has a pathogenic variant on his X chromosome causing an X-linked recessive disorder. Which statement is correct?

  1. All of his sons will inherit the variant
  2. Half of his daughters will be carriers
  3. All of his daughters will be carriers Correct answer.
  4. None of his children can inherit the variant

Explanation

A father transmits his X chromosome to every daughter and his Y chromosome to every son, so all daughters of an affected male are obligate carriers of an X-linked recessive variant while sons are unaffected.