Cell and Molecular Biology Practice Exam Quiz

Cell and Molecular Biology Practice Exam Quiz

What is Molecular Biology?

Molecular biology is the study of life at its most fundamental level: the molecules and processes that form the basis of cells and living organisms. It focuses on DNA, RNA, proteins, and the molecular interactions that regulate cell function, growth, and survival. By examining these processes, scientists uncover how genetic information is stored, expressed, and passed on, and how disruptions at the molecular level can lead to disease.

At its core, molecular biology explores DNA replication, transcription, and translation—the flow of genetic information known as the central dogma of biology. It also examines mechanisms of gene regulation, chromatin structure, epigenetic modifications, RNA processing, protein folding, signaling pathways, and DNA repair. These insights are crucial for understanding health, medicine, biotechnology, and evolution.

Modern advances in molecular biology have transformed medicine and research. Techniques such as PCR, CRISPR genome editing, RNA interference, and DNA sequencing are now routine tools that arose directly from molecular discoveries. Applications extend from diagnosing genetic diseases and developing targeted therapies, to improving agriculture and designing novel biomaterials.

For students, mastering molecular biology is more than just passing an exam—it is about building a foundation for careers in medicine, pharmacy, genetics, biochemistry, or research. A strong grasp of molecular concepts ensures readiness for higher studies and competitive exams. This molecular biology practice exam helps learners apply theoretical knowledge to real questions, reinforcing concepts through carefully designed examples and case-based scenarios.

About This Exam

The Cell and Molecular Biology Practice Exam is a comprehensive test bank crafted to help students, researchers, and professionals strengthen their command of the subject. It brings together hundreds of molecular biology exam questions and answers covering both fundamental and advanced topics.

Unlike short review notes, this molecular biology practice test goes deep into every essential area: DNA replication enzymes, repair pathways like BER, NER, and MMR, transcription initiation, RNA processing, translation factors, cell cycle checkpoints, apoptosis regulators, signaling cascades, chromatin remodeling, histone modifications, and real clinical disorders linked to molecular defects.

The exam includes:

• Molecular biology midterm exam style questions that test foundational knowledge.
• Molecular biology final exam questions focusing on integrative understanding and problem-solving.
• Molecular genetics exam questions that connect genotype to phenotype.
• Molecular biology lab exam questions reflecting experiments, techniques, and applications used in research settings.
• Cell and molecular biology practice exam questions and answers with detailed explanations, so students learn why an answer is correct.

This practice exam is not simply about memorization. Each question is built to improve critical thinking, application of concepts, and exam readiness.

Our Coverage of Topics in this Exam:

The molecular biology practice exam comprehensively reflects the scope of modern syllabi:

• DNA Replication – enzymes like DNA polymerases, primase, ligase, topoisomerases, origin recognition complexes, Okazaki fragment processing.
• DNA Repair Mechanisms – base excision repair, nucleotide excision repair, mismatch repair, homologous recombination, non-homologous end joining, and disorders such as Xeroderma pigmentosum and Lynch syndrome.
• Transcription & RNA Biology – RNA polymerases I, II, III, transcription factors (TBP, TFIIB, Mediator, TFIIH), splicing via snRNAs, RNA modifications like m⁶A and pseudouridine, RNA editing (A-to-I), and regulation by miRNAs, siRNAs, piRNAs.
• Translation & Protein Synthesis – ribosomal subunits, initiation factors (eIFs), elongation factors (EF-Tu, EF-G), release factors, codon recognition, proofreading, peptide bond formation by rRNA.
• Epigenetics & Chromatin – histone modifications (H3K4me3, H3K27me3, H3K36me3), DNA methylation, chromatin remodeling by SWI/SNF, enhancer–promoter looping via CTCF and cohesin.
• Cell Cycle & Checkpoints – regulation by cyclins and CDKs, APC/C, geminin, Chk1/Chk2, ATM/ATR, cohesion and condensin dynamics.
• Apoptosis & Signaling – Bcl-2 family, Bax/Bak, caspases, intrinsic vs extrinsic apoptosis, pathways like Wnt/β-catenin, Ras–MAPK, PI3K–Akt, TGF-β–SMAD, NF-κB, cGAS–STING, and circadian regulators (PER/CRY).
• Metabolism Links – glycolysis regulation, PDH, pentose phosphate pathway, purine/pyrimidine synthesis enzymes (ADA, HGPRT, UMP synthase), anaplerotic reactions.
• Molecular Biology Examples in Disease – BRCA mutations in homologous recombination deficiency, BCR–ABL in CML, Fragile X syndrome from FMR1 silencing, Bloom and Werner syndromes from helicase defects.
• Biotechnology Applications – PCR, DNA sequencing, CRISPR Cas9, Cas12, Cas13, CRISPRi/a, RNAi, retroviral integration, diagnostic tools like SHERLOCK.

This extensive range makes the resource ideal whether preparing for a molecular biology test in class, a lab exam, or professional-level molecular biology final exam questions.

Who Can Take This Exam?

The molecular biology practice exam is designed for:

• Undergraduate students in biology, biochemistry, biotechnology, or pre-medical programs preparing for molecular biology midterm exam or finals.
• Graduate students and researchers needing a refresher before qualifying exams or thesis defenses.
• Medical and allied health students (medicine, pharmacy, nursing) where molecular biology underpins genetics, pathology, and pharmacology.
• Competitive exam candidates preparing for MCAT, GRE Subject Test, GATE, NET, or similar global examinations.
• Laboratory trainees facing molecular biology lab exam questions and practical applications of DNA/RNA techniques.

🎯 Useful For

• Exam readiness: Realistic molecular biology exam questions and answers improve confidence.
• Self-assessment: Identifies weak areas and reinforces concepts.
• Revision aid: Compact coverage of topics allows quick review before molecular genetics exam questions or finals.
• Conceptual clarity: Explanations provide context beyond simple fact recall.
• Application practice: Cases and molecular biology examples link theory with real-world science.

Study Tips to Pass the Cell and Molecular Biology Exam

1. Master the Central Dogma – Know DNA replication, transcription, and translation step by step. Practice with the cell and molecular biology practice exam to reinforce enzyme functions and regulation.
2. Practice with Real Exam Questions – Use molecular biology practice tests and molecular biology final exam questions repeatedly. Active recall improves retention.
3. Focus on DNA Repair and Cell Cycle – Many molecular biology exam questions emphasize checkpoint proteins, repair pathways, and disease links.
4. Integrate Clinical Connections – Understand disorders like SCID (ADA deficiency), Lynch syndrome, XP, and BRCA cancers. Clinical cases often appear in molecular genetics exam questions.
5. Don’t Neglect Lab Techniques – Be familiar with PCR, sequencing, CRISPR, gel electrophoresis, and cloning. These are common molecular biology lab exam questions.
6. Use Diagrams and Pathways – Visualize enzyme complexes, chromatin states, and signaling cascades to simplify recall during the molecular biology test.
7. Simulate Timed Exams – Attempt full-length molecular biology practice exams under timed conditions to build speed and accuracy.
8. Group Discussions and Flashcards – Explaining molecular biology examples to peers and testing with flashcards solidifies memory.
9. Regular Revision – Break topics into daily sections. Review key histone marks, polymerases, checkpoints, and repair pathways before exams.
10. Balance Theory and Application – Understand not just “what” but “why.” Exams test application through molecular biology midterm and final exam questions, not rote recall alone.

This Cell and Molecular Biology Practice Exam is more than a test bank—it is a complete learning resource. By covering molecular biology exam questions and answers across replication, repair, transcription, translation, chromatin, metabolism, signaling, biotechnology, and clinical relevance, it ensures deep mastery of the subject.

Whether you are sitting for a molecular biology midterm exam, preparing for molecular biology final exam questions, reviewing molecular biology lab exam questions, or tackling advanced molecular genetics exam questions, this practice resource equips you with the knowledge, confidence, and strategies needed to succeed.

Invest time in practice, apply study tips, and you will be ready to ace your molecular biology practice exam with confidence.

Sample Questions and Answers

Q1.

Which organelle is primarily responsible for protein glycosylation in eukaryotic cells?
A) Endoplasmic reticulum
B) Lysosome
C) Golgi apparatus
D) Peroxisome

Answer: C) Golgi apparatus
Explanation: The Golgi apparatus acts as the central hub for post-translational modifications such as glycosylation, sulfation, and sorting of proteins. While the rough ER initiates N-linked glycosylation during protein synthesis, the Golgi completes processing by trimming, adding oligosaccharides, and ensuring proteins reach their correct destinations (plasma membrane, lysosomes, or secretion). This makes it vital for proper folding, stability, and cell communication.

Q2.

Which of the following best describes the role of cyclin-dependent kinases (CDKs) in the cell cycle?
A) They degrade cyclins
B) They provide checkpoints for DNA repair
C) They activate cell cycle progression when bound to cyclins
D) They stabilize microtubules during mitosis

Answer: C) They activate cell cycle progression when bound to cyclins
Explanation: CDKs are serine/threonine kinases that require cyclin binding to become active. Specific cyclin-CDK complexes regulate transitions between cell cycle phases (e.g., Cyclin D–CDK4/6 drives G1 phase, Cyclin E–CDK2 initiates S phase). They function as “engines” of the cell cycle. Unregulated CDK activity leads to uncontrolled division, a hallmark of cancer. DNA repair checkpoints involve additional proteins like ATM/ATR, not CDKs directly.

Q3.

Which DNA repair pathway corrects thymine dimers caused by UV radiation?
A) Mismatch repair
B) Base excision repair
C) Nucleotide excision repair
D) Non-homologous end joining

Answer: C) Nucleotide excision repair
Explanation: UV light induces covalent crosslinking of adjacent thymine residues, forming bulky distortions in DNA. Nucleotide excision repair (NER) recognizes these lesions, removes a short single-stranded DNA segment containing the damage, and fills in the gap using DNA polymerase and ligase. This pathway is defective in xeroderma pigmentosum patients, making them highly sensitive to sunlight and prone to skin cancer. Mismatch repair handles replication errors, not UV lesions.

Q4.

In oxidative phosphorylation, the final electron acceptor in the electron transport chain is:
A) NAD+
B) Oxygen
C) Cytochrome c
D) ATP

Answer: B) Oxygen
Explanation: Oxygen serves as the terminal electron acceptor in mitochondrial oxidative phosphorylation. It accepts electrons from cytochrome c oxidase (Complex IV) and combines with protons to form water. This step maintains electron flow through the chain and sustains proton pumping into the intermembrane space, creating the electrochemical gradient used by ATP synthase. Without oxygen, electron flow halts, forcing cells into anaerobic metabolism with far lower ATP yield.

Q5.

Which of the following molecules is synthesized in the nucleolus?
A) mRNA
B) rRNA
C) tRNA
D) snRNA

Answer: B) rRNA
Explanation: The nucleolus is a specialized sub-nuclear structure dedicated to ribosome biogenesis. It synthesizes and processes ribosomal RNA (28S, 18S, and 5.8S rRNA in eukaryotes), which are then assembled with ribosomal proteins imported from the cytoplasm. Although tRNAs and snRNAs are transcribed elsewhere in the nucleus, rRNA synthesis dominates nucleolar activity, making it a critical site for protein production capacity of the cell.

Q6.

Which type of RNA is directly responsible for carrying amino acids to the ribosome during translation?
A) mRNA
B) tRNA
C) rRNA
D) snRNA

Answer: B) tRNA
Explanation: Transfer RNAs (tRNAs) function as adaptor molecules in translation. Each tRNA carries a specific amino acid at its 3′ end (CCA sequence) and has an anticodon loop that base-pairs with codons on mRNA. This ensures that the genetic code is accurately converted into a polypeptide chain. mRNA provides the sequence, rRNA forms structural and catalytic components of ribosomes, and snRNA primarily functions in splicing, not amino acid delivery.

Q7.

What feature distinguishes prokaryotic DNA replication from eukaryotic DNA replication?
A) Bidirectional replication forks
B) Requirement of RNA primers
C) Multiple origins of replication
D) Semi-conservative mechanism

Answer: C) Multiple origins of replication
Explanation: Prokaryotic chromosomes are circular and typically initiate replication at a single origin (OriC in E. coli). Eukaryotic genomes are much larger and linear, requiring multiple origins along each chromosome to replicate efficiently within S phase. Both systems share bidirectional forks, RNA primers (made by primase), and the semi-conservative mechanism described by Meselson and Stahl. The key difference is origin multiplicity in eukaryotes.

Q8.

The “fluid mosaic model” of membranes emphasizes:
A) Rigid lipid bilayers with immobile proteins
B) Membranes as dynamic structures with mobile proteins and lipids
C) Random distribution of lipids without domains
D) Exclusively protein-based permeability

Answer: B) Membranes as dynamic structures with mobile proteins and lipids
Explanation: The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a dynamic bilayer of phospholipids in which proteins (integral, peripheral, glycoproteins) are embedded and free to move laterally. This fluidity allows signaling, transport, and repair. Cholesterol modulates fluidity, and lipid rafts provide localized domains. The model has since been refined but remains foundational in understanding membrane biology.

Q9.

During apoptosis, which family of proteases executes cleavage of cellular proteins?
A) Ubiquitin ligases
B) Caspases
C) Proteasomes
D) Cathepsins

Answer: B) Caspases
Explanation: Caspases (cysteine-aspartic proteases) are central executioners of programmed cell death. Initiator caspases (e.g., Caspase-8, -9) activate downstream effector caspases (Caspase-3, -6, -7), which dismantle the cell by cleaving nuclear lamins, cytoskeletal proteins, and repair enzymes. This controlled proteolysis ensures clean removal of dying cells without inflammation. Proteasomes degrade ubiquitinated proteins but do not drive apoptosis directly.

Q10.

What is the role of telomerase in eukaryotic cells?
A) Repairs DNA breaks via homologous recombination
B) Adds repetitive DNA sequences to chromosome ends
C) Functions as a helicase in DNA replication
D) Removes RNA primers from Okazaki fragments

Answer: B) Adds repetitive DNA sequences to chromosome ends
Explanation: Telomerase is a ribonucleoprotein enzyme that extends telomeres by adding short repetitive sequences (TTAGGG in humans) to the 3′ end of chromosomes. It carries its own RNA template and reverse transcriptase activity. This compensates for the “end replication problem” of lagging strand synthesis. Stem cells and germ cells maintain active telomerase, while most somatic cells repress it, contributing to aging. In contrast, many cancer cells reactivate telomerase to achieve immortality.

Q11.

Which cytoskeletal element is primarily responsible for intracellular transport of vesicles and organelles?
A) Microfilaments
B) Microtubules
C) Intermediate filaments
D) Septins

Answer: B) Microtubules
Explanation: Microtubules, composed of α- and β-tubulin dimers, form polarized tracks that motor proteins use for transport. Kinesins move cargo toward the plus end (usually periphery), while dyneins move toward the minus end (centrosome). This directional system allows efficient vesicle delivery, organelle positioning, and mitotic spindle function. Microfilaments mainly control cell shape and motility, while intermediate filaments provide structural integrity.

Q12.

Which of the following is an example of a second messenger in cell signaling?
A) G-protein
B) cAMP
C) Receptor tyrosine kinase
D) Ras

Answer: B) cAMP
Explanation: Cyclic AMP (cAMP) is a small, diffusible second messenger produced by adenylyl cyclase following activation of G-protein coupled receptors (GPCRs). It transmits the extracellular signal into the cytoplasm, primarily activating protein kinase A (PKA), which phosphorylates target proteins. Unlike first messengers (hormones, ligands), cAMP works inside cells to amplify signals. G-proteins and Ras are signal transducers, not second messengers.

Q13.

What is the main function of the signal recognition particle (SRP) during protein synthesis?
A) Terminate translation
B) Target ribosome-nascent chain complexes to the ER membrane
C) Assist in tRNA charging
D) Facilitate RNA splicing

Answer: B) Target ribosome-nascent chain complexes to the ER membrane
Explanation: The SRP binds to a hydrophobic signal peptide emerging from a ribosome translating a secretory or membrane protein. It pauses translation and directs the ribosome to the ER by binding its receptor. Once docked, translation resumes, and the growing polypeptide is translocated into the ER lumen or membrane. This ensures proteins destined for secretion or organelles are properly localized.

Q14.

In eukaryotic transcription, which RNA polymerase synthesizes mRNA?
A) RNA polymerase I
B) RNA polymerase II
C) RNA polymerase III
D) Reverse transcriptase

Answer: B) RNA polymerase II
Explanation: RNA polymerase II transcribes protein-coding genes into precursor mRNA (pre-mRNA) in eukaryotes. It also produces some snRNAs involved in splicing. RNA polymerase I transcribes rRNA, and RNA polymerase III transcribes tRNA and 5S rRNA. This specialization ensures efficient control over different RNA types. Transcription factors (TFIID, TFIIH, etc.) recruit Pol II to promoters, while co-activators regulate expression.

Q15.

Which experimental technique separates proteins based on molecular weight using an electric field?
A) Western blot
B) SDS-PAGE
C) ELISA
D) Chromatography

Answer: B) SDS-PAGE
Explanation: SDS-PAGE (sodium dodecyl sulfate–polyacrylamide gel electrophoresis) denatures proteins, coats them with negative charge, and separates them in a gel according to size. Smaller proteins migrate faster. This method is often followed by Western blotting for detection. ELISA measures protein concentration via antibody binding, not separation. Chromatography separates based on size, charge, or affinity but not strictly molecular weight.

Q16.

Which organelle is the main site of fatty acid β-oxidation in animal cells?
A) Lysosome
B) Peroxisome
C) Mitochondrion
D) Endosome

Answer: C) Mitochondrion
Explanation: Mitochondria are the primary site of β-oxidation in animal cells, breaking down long-chain fatty acids into acetyl-CoA units, which feed the TCA cycle and oxidative phosphorylation. Peroxisomes also perform β-oxidation but specialize in very-long-chain and branched fatty acids, producing hydrogen peroxide as a byproduct. Thus, while both organelles contribute, mitochondria generate most ATP from fatty acid metabolism.

Q17.

The lac operon in E. coli is an example of:
A) Positive regulation only
B) Negative regulation only
C) Both positive and negative regulation
D) Constitutive expression

Answer: C) Both positive and negative regulation
Explanation: The lac operon is controlled negatively by the lac repressor, which blocks transcription when lactose is absent. It is also positively regulated by CRP (cAMP receptor protein), which enhances transcription when glucose is scarce. This dual regulation ensures the operon is highly expressed only when lactose is available and glucose is not, demonstrating elegant bacterial gene regulation.

Q18.

Which of the following best describes facilitated diffusion?
A) Requires ATP
B) Moves molecules down their concentration gradient using transport proteins
C) Transports molecules against gradient
D) Exclusively for gases

Answer: B) Moves molecules down their concentration gradient using transport proteins
Explanation: Facilitated diffusion enables large or polar molecules (e.g., glucose, ions) to cross membranes via carrier proteins or channels, without requiring ATP. Unlike active transport, it cannot move molecules against the gradient. Examples include GLUT transporters for glucose or ion channels for Na+ and K+. This process allows specificity and faster transport than simple diffusion, while remaining energy-independent.

Q19.

What is the function of chaperone proteins in cells?
A) Degrade misfolded proteins
B) Facilitate proper protein folding and prevent aggregation
C) Cleave signal peptides
D) Attach ubiquitin to damaged proteins

Answer: B) Facilitate proper protein folding and prevent aggregation
Explanation: Molecular chaperones such as Hsp70 and chaperonins assist in protein folding by stabilizing unfolded chains, preventing misfolding and aggregation, especially under stress. They do not encode folding information but provide the environment for correct conformations. Misfolded proteins that escape chaperones are targeted for degradation via ubiquitination and proteasomes. Thus, chaperones maintain proteostasis and cell health.

Q20.

Which technique allows amplification of a specific DNA sequence in vitro?
A) PCR
B) Southern blotting
C) DNA sequencing
D) Northern blotting

Answer: A) PCR
Explanation: Polymerase chain reaction (PCR) exponentially amplifies specific DNA sequences using primers, DNA polymerase (usually Taq or high-fidelity enzymes), nucleotides, and thermal cycling. Southern blotting detects DNA, Northern blotting detects RNA, and sequencing determines nucleotide order but does not amplify. PCR revolutionized molecular biology, enabling cloning, diagnostics, forensic testing, and modern genome editing approaches.

Q21.

Which of the following organelles is involved in detoxifying hydrogen peroxide?
A) Lysosome
B) Peroxisome
C) Mitochondrion
D) Golgi apparatus

Answer: B) Peroxisome
Explanation: Peroxisomes contain enzymes such as catalase and oxidases that metabolize hydrogen peroxide, a toxic byproduct of fatty acid oxidation and other reactions. Catalase breaks it into water and oxygen. While mitochondria produce reactive oxygen species during respiration, they do not detoxify H₂O₂ as efficiently. Thus, peroxisomes are crucial for redox balance and cellular detoxification.

Q22.

Which component of the cytoskeleton anchors epithelial cells to each other at desmosomes?
A) Actin filaments
B) Microtubules
C) Intermediate filaments
D) Septins

Answer: C) Intermediate filaments
Explanation: Intermediate filaments, such as keratins in epithelial cells, attach to desmosomes to provide mechanical strength and resilience against stress. This stabilizes tissue structure, especially in skin. Actin filaments are linked to adherens junctions, while microtubules are mainly involved in intracellular transport. Intermediate filaments thus act like a flexible skeleton, ensuring tissue integrity.

Q23.

Which of the following best describes endocytosis?
A) Export of proteins from the cell
B) Uptake of extracellular material by vesicle formation
C) Passive diffusion of molecules across membranes
D) Osmotic water movement

Answer: B) Uptake of extracellular material by vesicle formation
Explanation: Endocytosis allows cells to engulf extracellular molecules, fluids, or particles into vesicles. Types include phagocytosis (large particles), pinocytosis (fluid uptake), and receptor-mediated endocytosis (specific ligands like LDL). This process maintains nutrient uptake, receptor regulation, and immune defense. Exocytosis is the reverse, exporting material outside the cell. Diffusion and osmosis involve no vesicles.

Q24.

Which enzyme is responsible for removing RNA primers during DNA replication in prokaryotes?
A) DNA polymerase I
B) DNA polymerase III
C) Primase
D) Ligase

Answer: A) DNA polymerase I
Explanation: In E. coli, DNA polymerase I has 5′→3′ exonuclease activity that removes RNA primers and replaces them with DNA nucleotides. DNA polymerase III is the primary replication enzyme but cannot remove primers. Ligase later seals nicks between fragments. This proofreading and primer-removal function of Pol I is essential for replication fidelity in prokaryotes.

Q25.

What is the role of p53 in cells?
A) Promotes DNA replication
B) Functions as a tumor suppressor by inducing cell cycle arrest or apoptosis
C) Stimulates angiogenesis
D) Acts as a growth factor receptor

Answer: B) Functions as a tumor suppressor by inducing cell cycle arrest or apoptosis
Explanation: p53 is called the “guardian of the genome” because it senses DNA damage and halts the cell cycle to allow repair. If repair fails, it triggers apoptosis. Loss of p53 function is common in cancers, allowing uncontrolled division despite mutations. Unlike oncogenes that drive proliferation, p53 prevents tumor development by enforcing genetic integrity.

Q26.

Which of the following proteins forms gap junctions between animal cells?
A) Connexins
B) Integrins
C) Cadherins
D) Selectins

Answer: A) Connexins
Explanation: Gap junctions are intercellular channels formed by connexin proteins, permitting direct passage of ions and small metabolites between adjacent cells. This electrical and metabolic coupling is critical in cardiac and smooth muscle tissues. Cadherins mediate adherens junctions, integrins connect cells to the extracellular matrix, and selectins facilitate leukocyte rolling in immunity.

Q27.

Which of the following mechanisms is used to regulate gene expression in eukaryotes?
A) DNA acetylation
B) Histone acetylation
C) DNA ligation
D) Ribosome recycling

Answer: B) Histone acetylation
Explanation: Acetylation of lysine residues on histones neutralizes their positive charge, loosening DNA-histone interactions and promoting transcriptional activation. Histone acetyltransferases (HATs) add acetyl groups, while histone deacetylases (HDACs) remove them, repressing transcription. DNA acetylation is not a regulatory mechanism; instead, DNA methylation silences genes. Epigenetic modifications like acetylation make chromatin more or less accessible to transcriptional machinery.

Q28.

Which type of vesicular transport moves materials from the ER to the Golgi apparatus?
A) Clathrin-coated vesicles
B) COPI-coated vesicles
C) COPII-coated vesicles
D) Caveolin-coated vesicles

Answer: C) COPII-coated vesicles
Explanation: COPII vesicles mediate anterograde transport, carrying proteins from the ER to the Golgi. COPI vesicles mediate retrograde transport (Golgi back to ER). Clathrin-coated vesicles are involved in endocytosis and transport from the trans-Golgi to endosomes. This vesicular specificity ensures accurate trafficking, a fundamental part of the secretory pathway.

Q29.

Which experimental technique allows visualization of live cells with minimal damage using light scattering?
A) Transmission electron microscopy (TEM)
B) Scanning electron microscopy (SEM)
C) Phase-contrast microscopy
D) Fluorescence microscopy

Answer: C) Phase-contrast microscopy
Explanation: Phase-contrast microscopy enhances contrast in transparent, unstained specimens by converting phase shifts in light passing through a specimen into differences in brightness. This allows visualization of living cells without fixation or staining. TEM and SEM require fixed samples. Fluorescence microscopy provides molecular specificity but often requires dyes that can stress cells.

Q30.

Which DNA double-strand break repair mechanism is error-prone but fast?
A) Homologous recombination
B) Non-homologous end joining (NHEJ)
C) Mismatch repair
D) Nucleotide excision repair

Answer: B) Non-homologous end joining (NHEJ)
Explanation: NHEJ directly ligates broken DNA ends without a homologous template, making it quick but error-prone due to possible insertions or deletions. In contrast, homologous recombination uses a sister chromatid as a template, ensuring accuracy but limited to S/G2 phase. NHEJ operates throughout the cell cycle and is crucial for immune system diversity (V(D)J recombination).