Q.1 What is the primary purpose of Gram staining in microbiology?
To determine the motility of bacteria
To differentiate bacteria based on cell wall composition
To measure bacterial growth rate
To identify antibiotic resistance genes
Explanation - Gram staining exploits differences in cell wall structure; Gram-positive bacteria retain crystal violet and appear purple, while Gram-negative bacteria do not and appear pink after counterstaining.
Correct answer is: To differentiate bacteria based on cell wall composition
Q.2 Which technique is used to separate DNA fragments by size in a laboratory?
Thin-layer chromatography
Polyacrylamide gel electrophoresis
Western blotting
Flow cytometry
Explanation - Polyacrylamide gel electrophoresis (PAGE) allows separation of DNA, RNA, or proteins based on size and charge by applying an electric field across a gel matrix.
Correct answer is: Polyacrylamide gel electrophoresis
Q.3 Which reagent is commonly used as a counterstain in Gram staining?
Crystal violet
Safranin
Iodine
Methanol
Explanation - Safranin is the counterstain applied after decolorization in Gram staining; it stains Gram-negative bacteria pink.
Correct answer is: Safranin
Q.4 During PCR, which component acts as the template for DNA synthesis?
DNA polymerase
Primers
Deoxynucleotide triphosphates
Template DNA
Explanation - PCR uses a specific template DNA that is copied by the polymerase using primers and dNTPs.
Correct answer is: Template DNA
Q.5 Which method is most suitable for quantifying protein concentration in a complex mixture?
Bradford assay
Gram staining
Spectrophotometric measurement at 600 nm
Microscopy
Explanation - The Bradford assay uses Coomassie Brilliant Blue dye, which binds proteins and allows spectrophotometric quantification.
Correct answer is: Bradford assay
Q.6 In a bacterial culture, the optical density at 600 nm (OD600) is a measure of:
pH of the medium
Cellular DNA content
Cell density or growth
Enzyme activity
Explanation - OD600 measures light scattering by cells, correlating with the number of cells in culture.
Correct answer is: Cell density or growth
Q.7 Which technique allows the identification of a specific antibody in a serum sample?
ELISA
PCR
Gram staining
Spectrophotometry
Explanation - Enzyme-linked immunosorbent assay (ELISA) detects antigens or antibodies using enzyme-conjugated antibodies and color change.
Correct answer is: ELISA
Q.8 Which of the following is a primary purpose of centrifugation in cell biology?
To lyse cells
To separate cellular components based on density
To stain cells
To amplify DNA
Explanation - Centrifugation uses centrifugal force to sediment components like nuclei, mitochondria, or ribosomes according to their size and density.
Correct answer is: To separate cellular components based on density
Q.9 What type of medium is used to isolate anaerobic bacteria?
Simmons citrate agar
Thioglycolate broth
MacConkey agar
Chocolate agar
Explanation - Thioglycolate broth contains reducing agents and a density gradient that supports growth of anaerobes by lowering oxygen concentration.
Correct answer is: Thioglycolate broth
Q.10 In gel electrophoresis, the direction of DNA migration is determined by:
DNA concentration
Electric field polarity
Temperature of the gel
Staining dye
Explanation - DNA is negatively charged and moves towards the positive electrode during electrophoresis.
Correct answer is: Electric field polarity
Q.11 Which enzyme is primarily responsible for adding a methyl group to DNA in bacteria?
DNA polymerase
RNA polymerase
DNA methyltransferase
Ligase
Explanation - DNA methyltransferases transfer a methyl group from S-adenosylmethionine to cytosine or adenine residues, affecting gene expression and restriction modification.
Correct answer is: DNA methyltransferase
Q.12 What is the main advantage of using real-time PCR over conventional PCR?
It does not require thermal cycling
It can quantify DNA as it amplifies
It uses fewer reagents
It is cheaper
Explanation - Real-time PCR monitors amplification in real-time using fluorescent dyes, allowing quantification of initial template amounts.
Correct answer is: It can quantify DNA as it amplifies
Q.13 Which of the following is NOT a type of chromatography?
Ion-exchange chromatography
Size-exclusion chromatography
Affinity chromatography
Thermocycling
Explanation - Thermocycling refers to PCR temperature cycling, not a chromatographic technique.
Correct answer is: Thermocycling
Q.14 In which type of microscope is fluorescence microscopy performed?
Light microscope
Electron microscope
Phase-contrast microscope
Scanning probe microscope
Explanation - Fluorescence microscopy uses a light microscope equipped with appropriate filters to detect fluorescently labeled samples.
Correct answer is: Light microscope
Q.15 Which reagent is commonly used to fix cells for microscopy?
Ethanol
Glutaraldehyde
Benzene
Acetone
Explanation - Glutaraldehyde cross-links proteins and preserves cellular structure for electron and fluorescence microscopy.
Correct answer is: Glutaraldehyde
Q.16 Which culture medium is selective for gram-negative bacteria, especially E. coli?
MacConkey agar
Mannitol salt agar
Blood agar
Sabouraud dextrose agar
Explanation - MacConkey agar contains bile salts and crystal violet to inhibit gram-positives and lactose for selective growth of gram-negatives.
Correct answer is: MacConkey agar
Q.17 The detection of a specific DNA sequence using fluorescent probes is called:
Southern blotting
Northern blotting
Fluorescent in situ hybridization
Western blotting
Explanation - FISH uses fluorescently labeled probes to bind complementary DNA or RNA sequences in cells or tissues.
Correct answer is: Fluorescent in situ hybridization
Q.18 Which of the following best describes a 'lag phase' in bacterial growth?
The period of exponential cell division
The phase where cells adapt to new environment
The stationary phase of growth
The death phase of culture
Explanation - During the lag phase bacteria adjust to growth conditions before entering exponential growth.
Correct answer is: The phase where cells adapt to new environment
Q.19 In microbiology, what does 'CFU' stand for?
Cellular Function Unit
Colony Forming Unit
Complementary Fragment Unit
Cytoplasmic Fragment Unit
Explanation - CFU refers to the number of viable bacterial colonies that can form from a given sample.
Correct answer is: Colony Forming Unit
Q.20 Which of the following is a primary function of the enzyme lactase?
Break down lactose into glucose and galactose
Synthesize lactose
Phosphorylate lactose
Convert lactose to lactic acid
Explanation - Lactase hydrolyzes lactose into its monosaccharide components.
Correct answer is: Break down lactose into glucose and galactose
Q.21 What is the role of a 'negative control' in an ELISA assay?
To confirm the assay works properly
To determine the baseline signal in absence of antigen
To measure the maximum signal intensity
To calibrate the plate reader
Explanation - A negative control lacks the target antigen, ensuring any signal detected is due to background or nonspecific binding.
Correct answer is: To determine the baseline signal in absence of antigen
Q.22 Which of the following is a common method to assess bacterial motility?
Gram staining
Motility agar test
Catalase test
Oxidase test
Explanation - Motility agar contains low agar concentration allowing motile bacteria to spread, forming a diffused appearance.
Correct answer is: Motility agar test
Q.23 Which technique uses antibodies attached to a solid surface to detect antigens?
Southern blot
Western blot
Northern blot
Dot blot
Explanation - Western blotting immobilizes proteins on a membrane and uses specific antibodies for detection.
Correct answer is: Western blot
Q.24 What is the main difference between agar and agarose in laboratory use?
Agar is a solidifying agent, agarose is a staining dye
Agar is a mixture of polysaccharides, agarose is purified gelling agent
Agar is used for eukaryotes, agarose for prokaryotes
Agar is liquid at room temperature, agarose is solid
Explanation - Agar contains agarose and agaropectin; agarose is purified for use in electrophoresis.
Correct answer is: Agar is a mixture of polysaccharides, agarose is purified gelling agent
Q.25 Which of the following enzymes is used in the ligase chain reaction (LCR)?
DNA polymerase
Ligase
Taq polymerase
RNA polymerase
Explanation - LCR uses DNA ligase to join adjacent probes that hybridize to a target sequence, enabling rapid DNA detection.
Correct answer is: Ligase
Q.26 The presence of which gas can be used to identify a bacterial species in a gas chromatography analysis?
Oxygen
Hydrogen sulfide
Carbon dioxide
Nitrogen
Explanation - Certain bacteria produce H₂S, which can be detected by gas chromatography as a characteristic marker.
Correct answer is: Hydrogen sulfide
Q.27 What does 'pH indicator' do in a microbiology experiment?
Measures protein concentration
Changes color in response to acid–base changes
Quantifies DNA
Stains bacterial cells
Explanation - pH indicators change color when the medium's acidity or basicity shifts during microbial metabolism.
Correct answer is: Changes color in response to acid–base changes
Q.28 Which method is best suited for visualizing the 3D structure of a protein?
X-ray crystallography
Transmission electron microscopy
Fluorescence microscopy
Phase-contrast microscopy
Explanation - X-ray crystallography provides atomic-level resolution of protein crystal structures.
Correct answer is: X-ray crystallography
Q.29 During a serial dilution, the dilution factor of 10^3 means what?
Each step reduces concentration by 10 times
Final solution is 1000 times more dilute
Three decimal places of precision
The sample contains 1000 cells
Explanation - 10^3 indicates a thousandfold dilution from the original concentration.
Correct answer is: Final solution is 1000 times more dilute
Q.30 Which of the following is a characteristic of a 'spore-forming' bacterium?
Produces endospores for survival in harsh conditions
Grows only in liquid media
Requires oxygen to grow
Has a single circular chromosome
Explanation - Spore-forming bacteria like Bacillus and Clostridium form resistant spores to survive extreme environments.
Correct answer is: Produces endospores for survival in harsh conditions
Q.31 Which reagent is commonly used to fix nucleic acids in gel electrophoresis?
Ethanol
Acetic acid
Tris buffer
Glycerol
Explanation - Ethanol precipitates DNA, allowing it to be loaded onto the gel for separation.
Correct answer is: Ethanol
Q.32 In the Kirby–Bauer disk diffusion method, what does the diameter of the inhibition zone indicate?
Bacterial motility
Antibiotic concentration
Susceptibility of the bacteria to the antibiotic
Rate of bacterial growth
Explanation - A larger zone of inhibition indicates higher susceptibility of the test organism to the antibiotic.
Correct answer is: Susceptibility of the bacteria to the antibiotic
Q.33 Which of the following is a commonly used DNA stain for agarose gel visualization?
Crystal violet
Coomassie Brilliant Blue
Ethidium bromide
Gram's iodine
Explanation - Ethidium bromide intercalates into DNA and fluoresces under UV light, allowing gel imaging.
Correct answer is: Ethidium bromide
Q.34 What is the purpose of a 'positive control' in a PCR assay?
Ensure the PCR reaction worked correctly
Measure background fluorescence
Detect contamination
Calibrate the thermal cycler
Explanation - A positive control contains known target DNA to confirm the PCR reagents and conditions are functional.
Correct answer is: Ensure the PCR reaction worked correctly
Q.35 Which of the following best describes a 'selective' growth medium?
Allows growth of all microorganisms
Inhibits certain types of microorganisms
Provides nutrients for bacterial growth
Has no antibiotics or inhibitors
Explanation - Selective media contain substances that suppress the growth of specific organisms while allowing others to thrive.
Correct answer is: Inhibits certain types of microorganisms
Q.36 Which enzyme is used in the process of DNA replication to remove RNA primers?
DNA polymerase I
DNA ligase
RNA polymerase
Helicase
Explanation - DNA polymerase I has 5'→3' exonuclease activity that removes RNA primers during replication.
Correct answer is: DNA polymerase I
Q.37 Which of the following is a major advantage of using a microplate reader in ELISA?
High throughput quantification
Direct microscopic observation
Low cost
No need for reagents
Explanation - Microplate readers allow simultaneous measurement of multiple wells, increasing speed and accuracy.
Correct answer is: High throughput quantification
Q.38 What type of microscopy uses a vacuum and an electron beam to image samples?
Confocal microscopy
Atomic force microscopy
Electron microscopy
Light microscopy
Explanation - Electron microscopes use electrons and a vacuum to achieve high-resolution imaging.
Correct answer is: Electron microscopy
Q.39 Which component of the Gram staining procedure is responsible for decolorizing the bacterial cell?
Crystal violet
Iodine
Ethanol
Safranin
Explanation - Ethanol extracts the lipid layer of gram-negative bacteria, removing the crystal violet-iodine complex.
Correct answer is: Ethanol
Q.40 What does the term 'optical density' refer to in microbiology?
The intensity of light absorption by a sample
The density of bacterial colonies on an agar plate
The thickness of the agar layer
The refractive index of a bacterial culture
Explanation - OD is measured spectrophotometrically and is proportional to bacterial cell concentration.
Correct answer is: The intensity of light absorption by a sample
Q.41 Which of the following best describes the principle of a 'sandwich' ELISA?
The antigen is captured between two antibodies
The antibody is immobilized and antigen binds directly
The detection antibody binds to the antigen, forming a complex
The enzyme is conjugated to the antigen directly
Explanation - A capture antibody first immobilizes the antigen, then a detection antibody binds the antigen forming a 'sandwich'.
Correct answer is: The antigen is captured between two antibodies
Q.42 In microbiological safety, what does the term 'biosafety level 2' (BSL-2) signify?
Pathogens that can be transmitted through the air
Moderately hazardous pathogens requiring specific precautions
Highly dangerous pathogens that require a biosafety cabinet
Non-pathogenic organisms
Explanation - BSL-2 labs handle organisms that pose moderate risks and require basic biosafety practices.
Correct answer is: Moderately hazardous pathogens requiring specific precautions
Q.43 Which of the following is a common technique for measuring enzymatic activity?
Titration
Spectrophotometry
Mass spectrometry
Chromatography
Explanation - Enzymatic reactions often produce or consume a chromogenic substrate measurable by spectrophotometry.
Correct answer is: Spectrophotometry
Q.44 What is the function of the 'carrier' in a DNA sequencing reaction?
To amplify the DNA
To stabilize the DNA polymerase
To prevent the DNA from sticking to tube walls
To provide a fluorescent signal
Explanation - Carrier molecules such as BSA or tRNA reduce DNA loss due to adhesion to surfaces.
Correct answer is: To prevent the DNA from sticking to tube walls
Q.45 Which of the following is the primary function of the enzyme catalase?
Breaks down hydrogen peroxide
Synthesizes DNA
Phosphorylates proteins
Hydrolyzes starch
Explanation - Catalase decomposes hydrogen peroxide into water and oxygen, protecting cells from oxidative damage.
Correct answer is: Breaks down hydrogen peroxide
Q.46 What is the purpose of using a 'buffer' in a biochemical experiment?
To provide energy to cells
To maintain a stable pH
To fix cells
To stain DNA
Explanation - Buffers resist changes in pH, ensuring optimal activity for enzymes and reactions.
Correct answer is: To maintain a stable pH
Q.47 Which of the following is a method to separate proteins based on charge?
Ion exchange chromatography
Size exclusion chromatography
Affinity chromatography
Thin layer chromatography
Explanation - Ion exchange chromatography separates molecules by their charge interactions with a charged resin.
Correct answer is: Ion exchange chromatography
Q.48 Which of the following is the most appropriate technique for detecting bacterial endotoxins?
Gram staining
Limulus amebocyte lysate (LAL) assay
PCR
Western blot
Explanation - The LAL assay uses horseshoe crab blood to detect lipopolysaccharide endotoxins in samples.
Correct answer is: Limulus amebocyte lysate (LAL) assay
Q.49 The process of separating genetic material using electrophoresis in an agarose gel is called:
SDS-PAGE
PAGE
Agarose gel electrophoresis
Western blotting
Explanation - Agarose gels are used for DNA and RNA separation, whereas PAGE is for proteins.
Correct answer is: Agarose gel electrophoresis
Q.50 Which of the following is used as a substrate to test for the presence of urease enzyme?
Urea solution
Blood agar
MacConkey agar
Mannitol salt agar
Explanation - Urease hydrolyzes urea producing ammonia, which raises the pH and changes the indicator color.
Correct answer is: Urea solution
Q.51 In the context of DNA sequencing, what is a 'clonal amplification' step?
Amplification of DNA by PCR
Formation of DNA microarrays
Preparation of single DNA molecules for sequencing
Cloning of DNA into plasmids
Explanation - Clonal amplification (e.g., emulsion PCR) creates many copies of a single DNA fragment for accurate sequencing reads.
Correct answer is: Preparation of single DNA molecules for sequencing
Q.52 Which of the following statements is true regarding a 'selective media'?
It is used to isolate viruses from bacteria
It contains antibiotics to prevent bacterial growth
It allows growth of some microorganisms while inhibiting others
It is used exclusively for fungi
Explanation - Selective media contain inhibitors or selective agents that suppress unwanted species.
Correct answer is: It allows growth of some microorganisms while inhibiting others
Q.53 What is the primary reason for adding SDS (sodium dodecyl sulfate) to a protein sample in SDS-PAGE?
To preserve protein structure
To denature proteins and give them a uniform negative charge
To fix proteins to the gel
To stain proteins during electrophoresis
Explanation - SDS coats proteins with negative charge, allowing separation by size in a polyacrylamide gel.
Correct answer is: To denature proteins and give them a uniform negative charge
Q.54 Which of the following is a method of detecting RNA viruses in a sample?
PCR
ELISA
Gram staining
Microscopy
Explanation - Reverse transcription PCR (RT-PCR) amplifies viral RNA after conversion to cDNA.
Correct answer is: PCR
Q.55 The presence of which compound indicates a positive result in a carbohydrate fermentation test?
Blue color
Red color
Green color
Clear solution
Explanation - Redox indicators (e.g., phenol red) change to red when fermentation acids lower the pH.
Correct answer is: Red color
Q.56 In microbiology, a 'culture medium' primarily provides what?
A surface for cells to attach
Nutrients and growth factors for microorganisms
A staining solution
An antibiotic to kill bacteria
Explanation - Culture media supply essential nutrients like carbon, nitrogen, vitamins, and minerals.
Correct answer is: Nutrients and growth factors for microorganisms
Q.57 What is the purpose of an 'antigen–antibody conjugate' in ELISA?
To amplify DNA
To produce a visible or fluorescent signal
To fix cells
To separate proteins by size
Explanation - Enzyme-conjugated antibodies catalyze colorimetric or fluorescent reactions, indicating antigen presence.
Correct answer is: To produce a visible or fluorescent signal
Q.58 Which of the following is the correct sequence of steps in a standard PCR protocol?
Denaturation → Annealing → Extension
Extension → Denaturation → Annealing
Annealing → Extension → Denaturation
Denaturation → Extension → Annealing
Explanation - PCR cycles first melt the DNA, then primers anneal, followed by extension by polymerase.
Correct answer is: Denaturation → Annealing → Extension
Q.59 Which of the following is a key advantage of using real-time PCR compared to traditional PCR?
No need for thermal cycler
Lower cost
Simultaneous detection and quantification
Higher amplification efficiency
Explanation - Real-time PCR measures fluorescence during amplification, enabling quantitative analysis of target DNA.
Correct answer is: Simultaneous detection and quantification
Q.60 Which of the following reagents is commonly used to fix bacterial cells for TEM imaging?
Ethanol
Glutaraldehyde
Acetic acid
Benzene
Explanation - Glutaraldehyde cross-links proteins, preserving cellular ultrastructure for transmission electron microscopy.
Correct answer is: Glutaraldehyde
Q.61 The 'Müller–Hinton agar' is used for which of the following?
Testing for antibiotic susceptibility
Culturing anaerobic bacteria
Selecting for Gram-positive bacteria
Staining bacterial cells
Explanation - Müller–Hinton agar is a standard medium for performing disk diffusion tests.
Correct answer is: Testing for antibiotic susceptibility
Q.62 Which of the following best describes the term 'chromogenic substrate'?
A substrate that turns color when hydrolyzed by an enzyme
A substrate that binds DNA
A substrate used in chromatography
A substrate that emits fluorescence
Explanation - Chromogenic substrates produce a colored product upon enzymatic cleavage, allowing visual detection.
Correct answer is: A substrate that turns color when hydrolyzed by an enzyme
Q.63 What is the main role of the 'phospholipid bilayer' in a cell membrane?
To provide structural support to the cell
To regulate the passage of substances in and out of the cell
To store genetic information
To generate energy for the cell
Explanation - The phospholipid bilayer forms a selective barrier controlling transport.
Correct answer is: To regulate the passage of substances in and out of the cell
Q.64 Which of the following is a common method for measuring the concentration of DNA in a sample?
Coulometry
Nanodrop spectrophotometry
Chromatography
PCR amplification
Explanation - Nanodrop devices measure absorbance at 260 nm to estimate DNA concentration.
Correct answer is: Nanodrop spectrophotometry
Q.65 The term 'catalytic efficiency' in enzymology refers to:
The ratio of kcat to KM
The maximum velocity of an enzyme
The enzyme's stability at high temperatures
The amount of product formed over time
Explanation - Catalytic efficiency is expressed as kcat/KM and reflects how efficiently an enzyme converts substrate.
Correct answer is: The ratio of kcat to KM
Q.66 Which of the following best describes an 'emulsion PCR' (ePCR)?
PCR performed in a water-in-oil emulsion to isolate individual DNA molecules
PCR performed at elevated temperatures
PCR performed with an emulsified DNA template
PCR that amplifies RNA directly
Explanation - ePCR creates isolated compartments allowing clonal amplification of individual DNA strands.
Correct answer is: PCR performed in a water-in-oil emulsion to isolate individual DNA molecules
Q.67 Which of the following is a key difference between Gram-positive and Gram-negative bacteria?
Gram-positive have an outer membrane, Gram-negative do not
Gram-negative have an outer membrane, Gram-positive do not
Gram-positive are motile, Gram-negative are not
Gram-negative are larger in size
Explanation - The outer membrane of Gram-negative bacteria contains lipopolysaccharide, absent in Gram-positives.
Correct answer is: Gram-negative have an outer membrane, Gram-positive do not
Q.68 What is the purpose of a 'negative stain' in electron microscopy?
To stain the background and reveal the shape of the specimen
To fix the specimen to the grid
To enhance contrast by staining the specimen itself
To provide a fluorescent marker
Explanation - Negative staining surrounds the specimen with heavy metal salts, creating a silhouette against a dark background.
Correct answer is: To stain the background and reveal the shape of the specimen
Q.69 Which of the following is used to separate proteins based on their isoelectric point?
SDS-PAGE
Isoelectric focusing
Affinity chromatography
Size exclusion chromatography
Explanation - Isoelectric focusing separates proteins at the pH where their net charge is zero.
Correct answer is: Isoelectric focusing
Q.70 Which of the following reagents is used to fix proteins onto a nitrocellulose membrane in a Western blot?
Methanol
Ethanol
Acetone
Formaldehyde
Explanation - Methanol aids in protein precipitation and immobilization onto the membrane during transfer.
Correct answer is: Methanol
Q.71 A 'plaque assay' is used to determine the titer of:
Bacterial cultures
Viruses
Fungi
Protozoa
Explanation - Plaque assays quantify infectious viral particles by counting clear zones on a cell monolayer.
Correct answer is: Viruses
Q.72 Which of the following is a key feature of a 'selective and differential medium' in microbiology?
It allows only one species to grow and provides no differentiation
It supports growth of all organisms and differentiates them by color
It inhibits unwanted species and differentiates based on metabolic traits
It differentiates by shape but does not support growth
Explanation - Selective/differential media both select for specific microbes and provide phenotypic clues.
Correct answer is: It inhibits unwanted species and differentiates based on metabolic traits
Q.73 Which of the following is a primary advantage of using microfluidic devices in microbiology?
Large sample volumes are required
They allow high-throughput analysis with minimal reagent use
They are only used for plant cells
They eliminate the need for centrifugation
Explanation - Microfluidics enable precise control of small volumes for rapid, parallel assays.
Correct answer is: They allow high-throughput analysis with minimal reagent use
Q.74 Which of the following is a hallmark of bacterial biofilm formation?
Cells detach immediately after growth
Cells produce extracellular polymeric substances
Biofilm cells are always planktonic
Biofilms are only formed in the presence of antibiotics
Explanation - Biofilms consist of cells embedded in a self-produced matrix of polysaccharides, proteins, and DNA.
Correct answer is: Cells produce extracellular polymeric substances
Q.75 The term 'inoculation loop' is used primarily for:
Measuring pH
Storing antibiotics
Transferring small amounts of liquid cultures
Staining bacterial cells
Explanation - An inoculation loop is a metal rod used to transfer tiny amounts of bacterial liquid culture.
Correct answer is: Transferring small amounts of liquid cultures
Q.76 What is the purpose of adding a 'carrier' during DNA purification?
To increase DNA concentration in the solution
To stabilize the DNA and prevent loss on silica columns
To catalyze DNA polymerization
To provide a fluorescent signal
Explanation - Carrier molecules such as glycogen help DNA precipitate and bind to purification matrices.
Correct answer is: To stabilize the DNA and prevent loss on silica columns
Q.77 Which of the following best describes the function of the enzyme 'DNA polymerase I' in DNA replication?
Adds nucleotides to the 5' end of DNA strands
Removes RNA primers and fills gaps with DNA
Helices the DNA double helix
Stabilizes the DNA double helix
Explanation - DNA polymerase I has 5'→3' polymerase activity and 3'→5' exonuclease for proofreading.
Correct answer is: Removes RNA primers and fills gaps with DNA
Q.78 What is the main reason to perform a 'serial dilution' before plating bacterial cultures?
To sterilize the culture
To reduce the number of colonies to a countable range
To increase bacterial growth rate
To change the medium composition
Explanation - Serial dilutions create lower concentrations that produce 30–300 colonies for accurate enumeration.
Correct answer is: To reduce the number of colonies to a countable range
Q.79 Which of the following is a major benefit of using 'DNA microarray' technology?
It can amplify a single gene copy
It allows simultaneous measurement of expression levels of thousands of genes
It is used only for detecting single nucleotide polymorphisms
It provides high-resolution protein structures
Explanation - DNA microarrays can assess global gene expression patterns in a single experiment.
Correct answer is: It allows simultaneous measurement of expression levels of thousands of genes
Q.80 In microbiology, the 'catalase test' differentiates which two groups of bacteria?
Gram-positive and Gram-negative
Aerobic and anaerobic
Catalase-positive and catalase-negative
Oxidase-positive and oxidase-negative
Explanation - Catalase-positive bacteria produce bubbles in hydrogen peroxide, while catalase-negative do not.
Correct answer is: Catalase-positive and catalase-negative
Q.81 Which of the following best describes the principle of 'chromatography'?
Separation based on density differences
Separation based on charge interactions
Separation based on movement through a stationary phase
Separation based on fluorescence emission
Explanation - Chromatography separates components as they move through a stationary phase under the influence of a mobile phase.
Correct answer is: Separation based on movement through a stationary phase
Q.82 What is the purpose of using an 'agarose gel' in electrophoresis?
To separate proteins by charge
To separate nucleic acids by size
To fix cells for microscopy
To culture bacteria
Explanation - Agarose gels form a porous matrix that allows DNA fragments to be separated based on length.
Correct answer is: To separate nucleic acids by size
Q.83 Which of the following is a primary function of the enzyme 'DNA ligase'?
Helices the DNA double helix
Seals nicks in the phosphodiester backbone
Adds nucleotides to the 3' end of DNA strands
Unwinds DNA strands
Explanation - DNA ligase joins DNA strands by forming phosphodiester bonds, repairing nicks.
Correct answer is: Seals nicks in the phosphodiester backbone
Q.84 Which of the following is used to differentiate between bacterial species based on their carbohydrate fermentation pattern?
Miller's nitrate agar
Triple Sugar Iron (TSI) agar
Congo red agar
MacConkey agar
Explanation - TSI agar indicates fermentation of glucose, lactose, sucrose and production of hydrogen sulfide.
Correct answer is: Triple Sugar Iron (TSI) agar
Q.85 What does the term 'optical density' (OD) indicate in a bacterial culture?
Number of bacteria per milliliter
Concentration of dissolved oxygen
pH level of the medium
Color intensity of the culture
Explanation - OD measures light scattering by cells, correlating with cell concentration.
Correct answer is: Number of bacteria per milliliter
Q.86 Which of the following best describes the 'halo test' in microbiology?
A test for enzyme activity that results in a clear zone around colonies
A test for bacterial motility
A test for antibiotic resistance
A test for carbohydrate fermentation
Explanation - The halo test detects enzymatic activity such as protease or lipase by clearing a substrate zone.
Correct answer is: A test for enzyme activity that results in a clear zone around colonies
Q.87 What is the primary function of the 'agar plate' in microbiology?
To provide a solid surface for bacterial growth
To separate proteins
To measure pH changes
To fix cells for microscopy
Explanation - Agar plates allow colonies to form and be counted or observed.
Correct answer is: To provide a solid surface for bacterial growth
Q.88 In a 'sandwich ELISA', what is the role of the capture antibody?
To detect the enzyme's activity
To bind to the antigen and immobilize it on the plate
To produce a color change
To act as a substrate
Explanation - The capture antibody attaches to the antigen, allowing subsequent detection by a second antibody.
Correct answer is: To bind to the antigen and immobilize it on the plate
Q.89 Which of the following is a common use for 'polyethylene glycol' (PEG) in molecular biology?
As a DNA polymerase
To precipitate proteins and nucleic acids
As a fluorescent dye
As an antimicrobial agent
Explanation - PEG induces precipitation of macromolecules, used in purification protocols.
Correct answer is: To precipitate proteins and nucleic acids
Q.90 Which of the following enzymes is used to degrade bacterial cell walls for the extraction of DNA?
Lysostaphin
Lysozyme
Protease
Lipase
Explanation - Lysozyme breaks down peptidoglycan, facilitating lysis of many bacteria.
Correct answer is: Lysozyme
Q.91 The 'OD600' measurement is typically taken at what wavelength?
400 nm
520 nm
600 nm
700 nm
Explanation - Optical density at 600 nm is a standard for estimating bacterial cell density.
Correct answer is: 600 nm
Q.92 What does the 'Citrate utilization test' detect?
Ability to metabolize citrate as a sole carbon source
Resistance to antibiotics
Production of H₂S
Production of indole
Explanation - The test uses citrate as the sole carbon source and monitors pH change.
Correct answer is: Ability to metabolize citrate as a sole carbon source
Q.93 Which of the following best describes the term 'chromatography'?
The process of separating molecules based on size only
The process of separating molecules based on charge only
The process of separating molecules based on multiple properties through a stationary phase
The process of separating molecules based on light absorption
Explanation - Chromatography uses a stationary and mobile phase to separate components based on various interactions.
Correct answer is: The process of separating molecules based on multiple properties through a stationary phase
Q.94 What is the role of a 'syringe filter' in preparing bacterial cultures?
To sterilize the culture by removing cells
To concentrate bacterial cells
To dilute bacterial cultures
To change the pH of the medium
Explanation - Syringe filters with 0.22 µm pores remove bacteria, sterilizing liquids.
Correct answer is: To sterilize the culture by removing cells
Q.95 Which of the following reagents is commonly used as a fluorescent dye to stain DNA in gels?
Coomassie Brilliant Blue
Crystal violet
DAPI
Gram's iodine
Explanation - DAPI binds to AT-rich regions of DNA and fluoresces blue under UV light.
Correct answer is: DAPI
Q.96 The 'spot test' in microbiology is primarily used to detect:
Bacterial motility
Antibiotic sensitivity
Bacterial viability on agar surface
Protein expression levels
Explanation - Spot tests involve depositing a small volume of culture onto agar to observe growth and viability.
Correct answer is: Bacterial viability on agar surface
Q.97 Which of the following is a common buffer used in DNA extraction protocols?
Tris-HCl
Acetone
Ethanol
Acetic acid
Explanation - Tris-HCl maintains pH during DNA extraction, protecting nucleic acids.
Correct answer is: Tris-HCl
Q.98 Which of the following best describes the 'plaque forming units (PFU)' method?
Measurement of bacterial colony counts
Measurement of fungal spore counts
Measurement of viral infectious particle counts
Measurement of cell viability in a tissue culture
Explanation - PFU counts determine the number of infectious viral particles capable of forming plaques.
Correct answer is: Measurement of viral infectious particle counts
Q.99 What is the main purpose of adding 'agarose' to a DNA sample before electrophoresis?
To fix the DNA in place
To create a porous matrix that slows down DNA migration
To stain the DNA for visualization
To digest the DNA into smaller fragments
Explanation - Agarose forms a gel that separates DNA fragments by size during electrophoresis.
Correct answer is: To create a porous matrix that slows down DNA migration
Q.100 Which of the following enzymes is used to remove 5' RNA primers during DNA replication?
DNA polymerase I
RNA polymerase
Ligase
Helicase
Explanation - DNA polymerase I has 5'→3' exonuclease activity to remove RNA primers.
Correct answer is: DNA polymerase I
Q.101 What does the term 'isoelectric point (pI)' refer to in protein chemistry?
The temperature at which a protein denatures
The pH at which a protein has no net charge
The optimal pH for enzyme activity
The concentration of a protein in solution
Explanation - At the isoelectric point, the positive and negative charges balance, affecting solubility.
Correct answer is: The pH at which a protein has no net charge
Q.102 Which of the following best describes the function of 'BSA' in molecular biology?
To act as a reducing agent
To serve as a blocking agent preventing nonspecific binding
To stabilize enzymes by acting as a cofactor
To serve as a fluorescent dye
Explanation - BSA blocks surfaces and reduces nonspecific adsorption of proteins and nucleic acids.
Correct answer is: To serve as a blocking agent preventing nonspecific binding
Q.103 Which of the following is an example of a 'differential medium'?
MacConkey agar
Chocolate agar
Blood agar
Müller–Hinton agar
Explanation - MacConkey agar differentiates lactose fermenters (pink colonies) from non-fermenters (colorless).
Correct answer is: MacConkey agar
Q.104 Which of the following is the most accurate method to determine the pH of a bacterial culture?
Use of a pH meter
Observation of colony color
Counting colony-forming units
Measuring optical density
Explanation - A pH meter provides direct measurement of hydrogen ion concentration.
Correct answer is: Use of a pH meter
Q.105 Which of the following is a key advantage of using 'real-time PCR' compared to conventional PCR?
It uses no reagents
It can amplify DNA without a thermal cycler
It provides quantitative data during amplification
It is faster than gel electrophoresis
Explanation - Real-time PCR measures fluorescence during amplification, allowing quantification.
Correct answer is: It provides quantitative data during amplification
Q.106 In a typical 'enzyme assay', what does the term 'substrate' refer to?
The enzyme being measured
The cofactor required by the enzyme
The molecule upon which the enzyme acts
The product formed by the enzyme
Explanation - The substrate is the specific molecule that the enzyme converts into product.
Correct answer is: The molecule upon which the enzyme acts
Q.107 Which of the following best describes the 'limulus amebocyte lysate (LAL) assay'?
A method for detecting bacterial endotoxins
A technique for DNA sequencing
A method for measuring protein concentration
A staining method for Gram-negative bacteria
Explanation - LAL assay uses horseshoe crab blood to detect lipopolysaccharide endotoxins in samples.
Correct answer is: A method for detecting bacterial endotoxins
Q.108 Which of the following is a common 'positive control' in a Western blot experiment?
A sample known to express the target protein
A sample with no protein
A sample with a different protein
A sample stained with Gram's iodine
Explanation - A positive control confirms the blotting and detection procedures are working.
Correct answer is: A sample known to express the target protein
Q.109 What is the primary purpose of adding 'ethanol' during DNA precipitation?
To denature proteins
To precipitate DNA by reducing solubility
To sterilize the solution
To add fluorescence
Explanation - Ethanol decreases DNA solubility, allowing it to aggregate and precipitate.
Correct answer is: To precipitate DNA by reducing solubility
Q.110 Which of the following is the most suitable method to determine the presence of a plasmid in a bacterial cell?
Gram staining
PCR amplification of plasmid-specific genes
Microscopy with Gram's iodine
Electrophoresis of cell lysate
Explanation - PCR targeting plasmid-encoded sequences confirms plasmid presence.
Correct answer is: PCR amplification of plasmid-specific genes
Q.111 Which of the following enzymes is used to remove RNA primers from Okazaki fragments?
DNA polymerase I
Helicase
Ligase
RNA polymerase
Explanation - DNA polymerase I has 5'→3' exonuclease activity that removes RNA primers.
Correct answer is: DNA polymerase I
Q.112 Which of the following best describes the term 'agar' in microbiology?
A synthetic polymer used for gel electrophoresis
A natural polysaccharide used to solidify culture media
A dye used for staining cells
An antimicrobial agent
Explanation - Agar is extracted from seaweed and provides a gel matrix for bacterial growth.
Correct answer is: A natural polysaccharide used to solidify culture media
Q.113 What does the 'Citrate utilization test' determine in microbiological identification?
Ability to metabolize citrate as the sole carbon source
Production of H₂S
Production of indole
Ability to hydrolyze starch
Explanation - The test indicates if a bacterium can utilize citrate for growth.
Correct answer is: Ability to metabolize citrate as the sole carbon source
Q.114 Which of the following best describes a 'selective medium' in microbiology?
A medium that supports growth of all organisms equally
A medium that selectively inhibits growth of certain organisms
A medium that contains no nutrients
A medium that only contains antibiotics
Explanation - Selective media include inhibitors to favor the growth of specific microorganisms.
Correct answer is: A medium that selectively inhibits growth of certain organisms
Q.115 Which of the following is a common method for visualizing bacterial colonies under a microscope?
Gram staining
PCR
ELISA
Mass spectrometry
Explanation - Gram staining differentiates bacterial cell wall types for microscopic examination.
Correct answer is: Gram staining
Q.116 Which of the following is the main function of a 'DNA ladder' in gel electrophoresis?
To serve as a molecular weight standard
To stain the DNA
To fix the gel
To increase the resolution
Explanation - A DNA ladder contains fragments of known sizes, enabling estimation of unknown DNA fragment lengths.
Correct answer is: To serve as a molecular weight standard
Q.117 Which of the following best describes the 'Luminex assay'?
A method to detect DNA mutations
A bead-based multiplex assay for detecting multiple analytes simultaneously
A gel-based DNA sequencing method
A microscopy technique for visualizing bacteria
Explanation - Luminex uses fluorescently labeled beads to detect several targets in one sample.
Correct answer is: A bead-based multiplex assay for detecting multiple analytes simultaneously
Q.118 What is the main function of the 'Tris-HCl' buffer in nucleic acid work?
To denature nucleic acids
To provide a stable pH during reactions
To act as a fluorescent probe
To fix cells for microscopy
Explanation - Tris-HCl maintains a constant pH, essential for enzyme activity and nucleic acid stability.
Correct answer is: To provide a stable pH during reactions
Q.119 Which of the following is used to detect the presence of a specific protein in a sample?
PCR
ELISA
Gram staining
Culture on agar
Explanation - ELISA uses antibodies and enzymatic detection to identify specific proteins.
Correct answer is: ELISA
Q.120 Which of the following best describes the purpose of a 'syringe filter' in microbiology?
To remove cells from a liquid for sterilization
To concentrate bacterial cells
To change the pH of a medium
To fix cells for microscopy
Explanation - A 0.22 µm filter removes bacterial cells, sterilizing liquids for downstream use.
Correct answer is: To remove cells from a liquid for sterilization
Q.121 In microbiology, which of the following best describes a 'halo test'?
Detection of enzymatic activity resulting in a clear zone around colonies
Measurement of bacterial growth in liquid culture
Assessment of antibiotic resistance by inhibition zone size
Detection of bacterial motility on soft agar
Explanation - A halo forms where the substrate is degraded by the secreted enzyme.
Correct answer is: Detection of enzymatic activity resulting in a clear zone around colonies
Q.122 Which of the following is the most common method for separating proteins based on their isoelectric point?
SDS-PAGE
Isoelectric focusing
Affinity chromatography
Size exclusion chromatography
Explanation - Proteins migrate to the pH where they have no net charge and thus separate by isoelectric point.
Correct answer is: Isoelectric focusing
Q.123 Which of the following is used as a fluorescent nucleic acid stain in gel imaging?
Coomassie Brilliant Blue
DAPI
Crystal violet
Safranin
Explanation - DAPI binds to DNA and fluoresces blue under UV light, useful for gel imaging.
Correct answer is: DAPI
Q.124 What is the purpose of the 'carrier' molecule in DNA purification protocols?
To stabilize the DNA and prevent loss on silica columns
To serve as a fluorescent probe
To act as a reducing agent
To digest RNA contamination
Explanation - Carrier molecules help precipitate DNA and increase binding efficiency during purification.
Correct answer is: To stabilize the DNA and prevent loss on silica columns
Q.125 Which of the following is a key feature of the 'agar diffusion' method for antibiotic susceptibility testing?
It measures growth in liquid culture only
It uses disks containing antibiotics placed on agar to measure inhibition zones
It requires fluorescent labeling of bacteria
It only works for anaerobic bacteria
Explanation - The Kirby–Bauer disk diffusion method measures the zone of inhibition around antibiotic disks.
Correct answer is: It uses disks containing antibiotics placed on agar to measure inhibition zones
Q.126 Which of the following enzymes is used to break down peptidoglycan and lyse bacterial cells?
Lysozyme
Protease
Lipase
Amylase
Explanation - Lysozyme hydrolyzes the β‑1,4 linkages in peptidoglycan, disrupting cell walls.
Correct answer is: Lysozyme
Q.127 What is the main function of the 'citrate test' in bacterial identification?
To detect citrate utilization as a sole carbon source
To test for hydrogen sulfide production
To test for indole production
To assess growth at low temperatures
Explanation - Citrate utilization is a biochemical trait used to differentiate bacterial species.
Correct answer is: To detect citrate utilization as a sole carbon source
Q.128 Which of the following best describes a 'chromogenic medium'?
A medium that produces a color change when a specific enzyme is present
A medium that is transparent for microscopic observation
A medium that contains antibiotics to prevent contamination
A medium that supports growth of only one species
Explanation - Chromogenic media contain substrates that change color when cleaved by bacterial enzymes.
Correct answer is: A medium that produces a color change when a specific enzyme is present
Q.129 Which of the following best describes the 'E. coli K-12' strain?
A pathogenic strain used in vaccine development
A laboratory strain commonly used for cloning and genetics
A strain resistant to all known antibiotics
A strain that grows only in anaerobic conditions
Explanation - E. coli K-12 is a nonpathogenic strain frequently used in research and biotechnology.
Correct answer is: A laboratory strain commonly used for cloning and genetics
Q.130 Which of the following is NOT typically used in the 'ELISA' detection method?
Enzyme-conjugated antibody
Substrate solution
Microtiter plate
Gram stain
Explanation - Gram staining is a microscopy technique, not part of ELISA.
Correct answer is: Gram stain
Q.131 Which of the following best describes a 'carrier' protein used in ELISA?
A protein that carries the enzyme to the substrate
A protein that blocks nonspecific binding sites
A protein that is the target analyte
A protein used to fix cells
Explanation - Blocking agents like BSA prevent nonspecific adsorption in ELISA assays.
Correct answer is: A protein that blocks nonspecific binding sites
Q.132 Which of the following best explains the function of a 'negative stain' in electron microscopy?
To stain the specimen itself
To stain the background and provide contrast
To fix the specimen to the grid
To increase the electron beam intensity
Explanation - Negative staining surrounds the specimen with heavy metals, creating a silhouette.
Correct answer is: To stain the background and provide contrast
Q.133 What is the purpose of adding 'acetic acid' during the Gram staining procedure?
To fix the dye inside the cell wall
To decolorize gram-negative bacteria
To decolorize gram-positive bacteria
To act as a counterstain
Explanation - Acetic acid extracts the crystal violet-iodine complex from gram-negative cells, enabling counterstaining.
Correct answer is: To decolorize gram-negative bacteria
Q.134 In molecular biology, what is the purpose of a 'primer' in a PCR reaction?
To catalyze the reaction
To initiate DNA synthesis by providing a 3' end
To digest the template DNA
To act as a fluorescent probe
Explanation - Primers are short DNA fragments that anneal to the template and provide a starting point for DNA polymerase.
Correct answer is: To initiate DNA synthesis by providing a 3' end
Q.135 Which of the following best describes the term 'biomass' in a bacterial culture?
The total mass of living cells per unit volume
The number of colonies formed on a plate
The optical density of the culture
The total protein content of the cells
Explanation - Biomass refers to the total mass of living organisms in a culture, often measured as dry weight.
Correct answer is: The total mass of living cells per unit volume
Q.136 Which of the following best describes the role of a 'chromatography column' in purification?
To separate molecules based on size, charge, or affinity
To fix the sample onto a surface
To measure the pH of the solution
To amplify DNA
Explanation - Columns contain stationary phases that interact differently with molecules, enabling separation.
Correct answer is: To separate molecules based on size, charge, or affinity
Q.137 In the context of microbiology, what is a 'syringe filter' primarily used for?
Sterilizing solutions by filtering out bacteria
Staining bacterial cells
Measuring optical density
Counting colony-forming units
Explanation - A syringe filter removes microbes from liquids, making them sterile.
Correct answer is: Sterilizing solutions by filtering out bacteria
Q.138 What is the purpose of using 'ethidium bromide' in agarose gel electrophoresis?
To stain DNA for visualization under UV light
To fix the gel in place
To digest the DNA
To serve as a buffer
Explanation - Ethidium bromide intercalates into DNA and fluoresces, allowing detection of DNA bands.
Correct answer is: To stain DNA for visualization under UV light
Q.139 Which of the following best describes a 'plaque assay' in virology?
A method to measure bacterial cell density
A method to measure viral infectivity by counting clear zones on a cell monolayer
A method to quantify enzyme activity
A method to assess protein folding
Explanation - Plaque assays determine the number of infectious viral particles by counting plaques.
Correct answer is: A method to measure viral infectivity by counting clear zones on a cell monolayer
Q.140 Which of the following is a key function of the enzyme 'RNA polymerase' in the cell?
Synthesize RNA from DNA template
Synthesize DNA from RNA template
Translate RNA into protein
Repair DNA strands
Explanation - RNA polymerase catalyzes transcription, creating RNA from a DNA template.
Correct answer is: Synthesize RNA from DNA template
Q.141 Which of the following is a common method to visualize bacterial cells under a light microscope?
Gram staining
PCR
Western blot
Mass spectrometry
Explanation - Gram staining provides contrast and color differentiation for bacterial cell walls.
Correct answer is: Gram staining
Q.142 Which of the following best describes the principle of a 'DNA microarray'?
Simultaneous sequencing of multiple DNA fragments
Simultaneous measurement of expression levels of thousands of genes
Purification of a single DNA fragment
Amplification of DNA by PCR
Explanation - Microarrays hybridize labeled cDNA to probes on a chip, enabling global gene expression profiling.
Correct answer is: Simultaneous measurement of expression levels of thousands of genes
Q.143 What does the 'OD600' value represent in a bacterial growth curve?
The pH of the culture medium
The optical density at 600 nm, indicating cell density
The growth rate of the bacterial culture
The amount of protein in the culture
Explanation - OD600 is a standard metric for estimating bacterial concentration in culture.
Correct answer is: The optical density at 600 nm, indicating cell density
Q.144 Which of the following is a typical use of 'SDS-PAGE' in protein analysis?
Separating DNA fragments by size
Separating proteins based on charge
Separating proteins by size after denaturation
Staining cells for microscopy
Explanation - SDS-PAGE denatures proteins, giving them a uniform negative charge, and separates them by size.
Correct answer is: Separating proteins by size after denaturation
Q.145 Which of the following best describes the purpose of a 'negative control' in a microbiology experiment?
To confirm that the assay works correctly by showing a positive result
To ensure that no contamination or false positive occurs by showing no growth or signal
To increase the sensitivity of the assay
To calibrate the equipment
Explanation - Negative controls should produce no signal, confirming assay specificity.
Correct answer is: To ensure that no contamination or false positive occurs by showing no growth or signal
Q.146 Which of the following is used to fix proteins onto a membrane during a Western blot?
Methanol
Ethanol
Acetone
Formaldehyde
Explanation - Methanol assists in the transfer of proteins from gel to membrane.
Correct answer is: Methanol
Q.147 What is the main purpose of adding a 'counterstain' in a Gram staining procedure?
To fix the dye inside the cell wall
To decolorize gram-negative bacteria
To color gram-positive bacteria purple
To stain gram-negative bacteria pink or red
Explanation - The counterstain, typically safranin, colors gram-negative bacteria after decolorization.
Correct answer is: To stain gram-negative bacteria pink or red
Q.148 Which of the following best describes a 'selective medium'?
A medium that supports growth of all microorganisms
A medium that selectively inhibits the growth of certain microorganisms while allowing others to grow
A medium that differentiates microorganisms by color
A medium that contains no nutrients
Explanation - Selective media contain inhibitors that suppress the growth of unwanted species.
Correct answer is: A medium that selectively inhibits the growth of certain microorganisms while allowing others to grow