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Unit 4: Protein estimation - Practice Quiz

BTY555 — Biotechnology Laboratory-I 60 Questions
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1 Which dye is used in the Bradford method for protein estimation?

Estimation of proteins using Bradford method Easy
A. Bromophenol blue
B. Ninhydrin
C. Methylene blue
D. Coomassie Brilliant Blue G-250

2 At what wavelength is the absorbance typically measured in the Bradford assay?

Estimation of proteins using Bradford method Easy
A. nm
B. nm
C. nm
D. nm

3 When Coomassie dye binds to protein, its color changes to which of the following?

Estimation of proteins using Bradford method Easy
A. Red
B. Yellow
C. Blue
D. Green

4 Which protein is most commonly used as a standard in the Bradford assay?

Estimation of proteins using Bradford method Easy
A. Hemoglobin
B. Bovine serum albumin (BSA)
C. Casein
D. Gelatin

5 The Bradford assay is based on which type of measurement?

Estimation of proteins using Bradford method Easy
A. Colorimetric (spectrophotometric)
B. Fluorometric
C. Gravimetric
D. Titrimetric

6 In the Bradford assay, the dye binds primarily to which amino acid residues?

Estimation of proteins using Bradford method Easy
A. Sulfur-containing residues only
B. Acidic residues only
C. Neutral residues only
D. Basic and aromatic residues

7 The Bradford reagent solution is usually acidic due to the presence of which acid?

Estimation of proteins using Bradford method Easy
A. Phosphoric acid
B. Sulfuric acid
C. Acetic acid
D. Nitric acid

8 A standard curve in the Bradford assay is a plot of absorbance against which variable?

Estimation of proteins using Bradford method Easy
A. Wavelength
B. Reaction time
C. Temperature
D. Protein concentration

9 The Bradford assay was developed by which scientist?

Estimation of proteins using Bradford method Easy
A. Frederick Sanger
B. Oliver Lowry
C. Robert Koch
D. Marion Bradford

10 In a colorimetric Bradford assay, absorbance is generally proportional to what?

Estimation of proteins using Bradford method Easy
A. Cuvette color
B. Volume of water
C. Amount of protein present
D. Room temperature

11 Which of the following is a major advantage of the Bradford assay?

Estimation of proteins using Bradford method Easy
A. It needs radioactive labels
B. It takes several hours
C. It is rapid and simple
D. It requires no reagents

12 A blank in the Bradford assay typically contains which of the following?

Estimation of proteins using Bradford method Easy
A. Only protein
B. Only distilled water
C. Buffer or water plus dye reagent (no protein)
D. Concentrated acid

13 The stable blue color in the Bradford assay usually develops within about how long?

Estimation of proteins using Bradford method Easy
A. A few minutes
B. Several hours
C. One week
D. One day

14 Which type of substance is a common interfering agent in the Bradford assay?

Estimation of proteins using Bradford method Easy
A. Detergents such as SDS
B. Distilled water
C. Glucose
D. Table salt

15 The dye-protein interaction in the Bradford assay is primarily based on which type of binding?

Estimation of proteins using Bradford method Easy
A. Non-covalent (electrostatic and hydrophobic)
B. Hydrogen gas formation
C. Covalent bonding
D. Ionic precipitation

16 The unbound (free) form of Coomassie dye appears which color?

Estimation of proteins using Bradford method Easy
A. Green
B. Reddish-brown
C. Colorless
D. Bright blue

17 Which instrument is used to measure absorbance in the Bradford assay?

Estimation of proteins using Bradford method Easy
A. Autoclave
B. pH meter
C. Centrifuge
D. Spectrophotometer

18 The Bradford assay is used to determine which property of a sample?

Estimation of proteins using Bradford method Easy
A. Solution pH
B. Enzyme temperature
C. DNA concentration
D. Protein concentration

19 To find the protein concentration of an unknown sample, its absorbance is compared against what?

Estimation of proteins using Bradford method Easy
A. The dye expiry date
B. The room humidity
C. The cuvette weight
D. The standard calibration curve

20 In which year was the Bradford protein assay first published?

Estimation of proteins using Bradford method Easy
A.
B.
C.
D.

21 In the Bradford assay, the dye Coomassie Brilliant Blue G-250 shifts its absorbance maximum upon binding to protein. What is the wavelength at which the protein-bound dye is measured?

Estimation of proteins using Bradford method Medium
A. nm
B. nm
C. nm
D. nm

22 The Bradford dye binds primarily to which types of amino acid residues in a protein?

Estimation of proteins using Bradford method Medium
A. Acidic residues such as aspartate and glutamate
B. Sulfur-containing residues such as cysteine and methionine
C. Basic and aromatic residues such as arginine, lysine, and tryptophan
D. Nonpolar aliphatic residues such as alanine and valine

23 A student prepares a standard curve using BSA and finds it deviates from linearity at higher concentrations. What is the best practice to obtain accurate results for a concentrated sample?

Estimation of proteins using Bradford method Medium
A. Dilute the sample so its reading falls within the linear range
B. Add more dye reagent to the concentrated sample
C. Increase the incubation time to 1 hour
D. Measure at a longer wavelength

24 Which of the following substances is a well-known interferent that can cause errors in the Bradford assay?

Estimation of proteins using Bradford method Medium
A. Glucose
B. Detergents such as SDS
C. Sodium chloride
D. Glycerol

25 A sample gives an absorbance of at nm. From the standard curve equation (where is in ), what is the protein concentration?

Estimation of proteins using Bradford method Medium
A.
B.
C.
D.

26 Why is bovine serum albumin (BSA) commonly used as the standard in the Bradford method?

Estimation of proteins using Bradford method Medium
A. It absorbs strongly at nm on its own
B. It is inexpensive, stable, and gives a well-characterized response
C. It binds Coomassie dye irreversibly
D. It has no aromatic residues and gives a flat response

27 The Bradford assay reaction develops color rapidly. What is the recommended approach for reading the samples?

Estimation of proteins using Bradford method Medium
A. Read immediately without any incubation
B. Incubate briefly (about 5 minutes) and read within a stable time window
C. Incubate for at least 2 hours before reading
D. Read only after 24 hours of storage

28 Compared to the Lowry method, a key advantage of the Bradford assay is that it is:

Estimation of proteins using Bradford method Medium
A. Unaffected by all detergents and buffers
B. Faster and involves fewer reagent-addition steps
C. Based on copper reduction chemistry
D. Completely insensitive to protein composition

29 A blank in the Bradford assay is prepared using:

Estimation of proteins using Bradford method Medium
A. Dye reagent alone in an empty cuvette
B. BSA standard at the highest concentration
C. Protein sample without dye reagent
D. Buffer or water plus dye reagent, without protein

30 Two proteins at identical concentrations can give different absorbance values in the Bradford assay. This is best explained by differences in their:

Estimation of proteins using Bradford method Medium
A. Content of basic and aromatic amino acid residues
B. Molecular weight only
C. Solubility in water
D. Total number of peptide bonds

31 In a standard curve, absorbance is plotted on the y-axis. What is plotted on the x-axis?

Estimation of proteins using Bradford method Medium
A. Wavelength
B. Known protein concentration of standards
C. Volume of dye reagent
D. Incubation time

32 A student forgets to prepare a blank and instead zeroes the spectrophotometer against air. What is the most likely effect on the measured values?

Estimation of proteins using Bradford method Medium
A. Absorbance readings will be systematically too high
B. Absorbance readings will be systematically too low
C. The wavelength maximum will shift to nm
D. Readings will be unaffected

33 The color change in the Bradford assay is due to the dye shifting from which form to which form upon binding protein?

Estimation of proteins using Bradford method Medium
A. From colorless to blue neutral
B. From blue anionic to red cationic
C. From reddish-brown cationic to blue anionic
D. From green neutral to red cationic

34 If glassware used for the assay is contaminated with residual detergent from washing, what error is most likely?

Estimation of proteins using Bradford method Medium
A. Falsely low protein estimates only
B. A shift of the peak to nm
C. No effect because detergents are inert
D. Interference and inaccurate, often falsely high, readings

35 A standard curve gives the equation . An unknown sample reads . What is ?

Estimation of proteins using Bradford method Medium
A.
B.
C.
D.

36 Why does the Bradford assay generally require the reaction to be measured relatively quickly rather than left for many hours?

Estimation of proteins using Bradford method Medium
A. The dye evaporates from solution
B. The protein degrades the dye enzymatically
C. The protein-dye complex can aggregate and precipitate over time
D. The color intensifies indefinitely

37 Which pair of factors most directly ensures a reliable Bradford result?

Estimation of proteins using Bradford method Medium
A. Using free amino acids as the standard
B. Measuring at nm without a blank
C. High temperature and long incubation
D. Consistent dye-to-sample ratio and readings within the linear range

38 The Bradford reagent is typically prepared in an acidic solution. What is the primary purpose of the acidic conditions?

Estimation of proteins using Bradford method Medium
A. To stabilize the dye and promote the color shift upon protein binding
B. To shift the peak absorbance to nm
C. To neutralize the protein charge
D. To denature the protein completely

39 A researcher must estimate very low protein amounts (in the low range). Compared to the standard Bradford, the micro-assay format achieves higher sensitivity mainly by:

Estimation of proteins using Bradford method Medium
A. Using larger sample volumes relative to dye and reading at a longer path length
B. Measuring at nm instead of nm
C. Reducing incubation to a few seconds
D. Using a higher dye concentration

40 If a sample's absorbance reads higher than the highest standard on the curve, the correct interpretation is that the:

Estimation of proteins using Bradford method Medium
A. Sample concentration exceeds the curve's range and should be diluted and re-assayed
B. Concentration can be reported directly by extrapolation
C. Reading is invalid because of dye degradation
D. Sample contains no protein

41 In the Bradford assay, Coomassie Brilliant Blue G-250 exists in three forms. Upon binding protein, the dye shifts predominantly to the anionic blue form. What is the corresponding shift in absorption maximum that the assay measures?

Estimation of proteins using Bradford method Hard
A. From nm to nm
B. From nm to nm
C. From nm to nm
D. From nm to nm

42 The Bradford dye binds proteins primarily through interactions with specific residues. Which combination of amino acid residues contributes most to dye binding, explaining why protein-to-protein variability occurs?

Estimation of proteins using Bradford method Hard
A. Arginine and, to a lesser extent, lysine and histidine (basic/aromatic residues)
B. Aspartate and glutamate (acidic residues) via hydrogen bonding
C. Glycine and alanine through hydrophobic backbone packing
D. Cysteine and methionine through disulfide exchange

43 A standard curve is prepared with BSA over – . Beyond about – , the plot of versus concentration bends toward the x-axis. What is the underlying cause of this non-linearity?

Estimation of proteins using Bradford method Hard
A. Photobleaching of the bound dye during measurement
B. Precipitation of BSA at high concentration removing it from solution
C. Depletion of free dye as most dye becomes protein-bound, so absorbance no longer increases proportionally
D. The blue dye form re-converts to the red form at high protein levels

44 A researcher measures a sample and obtains , but the linear range of the standard curve only extends to . What is the most scientifically sound corrective action?

Estimation of proteins using Bradford method Hard
A. Extrapolate the standard curve linearly beyond to read off the value
B. Report the concentration corresponding to as the answer
C. Increase the dye volume to raise the linear range and re-read the same tube
D. Dilute the sample, re-assay, and multiply the result by the dilution factor

45 Which of the following interfering agents is most notorious for causing serious errors in the Bradford assay, and why?

Estimation of proteins using Bradford method Hard
A. Glycerol, which absorbs strongly at nm on its own
B. Sodium chloride, which quenches the blue color by ionic strength effects
C. Glucose, which reduces the dye chemically to a colorless form
D. Detergents such as SDS and Triton X-100, which interact with the dye and shift the equilibrium, producing large color changes independent of protein

46 Why is BSA often considered a poor choice as the sole standard when quantifying an unknown protein by Bradford, and IgG sometimes preferred?

Estimation of proteins using Bradford method Hard
A. IgG binds no dye and gives a truer blank
B. BSA is not soluble in acidic Bradford reagent
C. BSA gives an unusually low color yield, so it overestimates other proteins
D. BSA gives an unusually high color yield, so it underestimates most other proteins; IgG's response is closer to a typical protein average

47 A linear standard curve fits , where is in . A sample diluted before assay gives . What is the protein concentration in the original (undiluted) sample?

Estimation of proteins using Bradford method Hard
A.
B.
C.
D.

48 Why must all Bradford tubes be read within a consistent, relatively short time window (typically – min) after adding reagent?

Estimation of proteins using Bradford method Hard
A. Evaporation concentrates the sample enough to change the reading within minutes
B. The protein-dye complex is metastable and color intensity drifts over time, so timing differences introduce systematic error
C. The cuvette plastic reacts with the dye after prolonged contact
D. The dye chemically digests the protein, destroying it after 20 min

49 The Bradford reagent is prepared in phosphoric acid and methanol/ethanol. What is the primary functional role of the strongly acidic environment?

Estimation of proteins using Bradford method Hard
A. It prevents bacterial contamination of the reagent during storage
B. It increases the refractive index to enhance absorbance readings
C. It hydrolyzes the protein into peptides that bind dye more strongly
D. It stabilizes the doubly protonated cationic dye form so that protein binding produces a clear shift to the anionic blue form

50 Comparing Bradford with the Lowry and BCA assays, which statement about Bradford's key advantage and limitation is correct?

Estimation of proteins using Bradford method Hard
A. It measures peptide bonds directly and is unaffected by amino acid composition
B. It is slow and multistep but completely insensitive to reducing agents
C. It requires copper reduction and is compatible with all detergents
D. It is fast and simple but is highly sensitive to detergents and shows large protein-to-protein variability

51 A student notices the reagent blank itself gives a noticeable blue tint before any protein is added. Which is the most likely cause and appropriate remedy?

Estimation of proteins using Bradford method Hard
A. Contaminating protein or dye aggregation in the reagent; filter or prepare fresh reagent and re-blank
B. The blank is normal and should simply be ignored
C. Methanol in the reagent is fluorescing; switch to a UV lamp
D. The spectrophotometer wavelength is set too low; increase to 700 nm

52 Two protein samples of identical mass concentration () are assayed; one is rich in arginine, the other rich in glycine. What do you predict for their values?

Estimation of proteins using Bradford method Hard
A. Neither develops any color because pure single proteins do not bind dye
B. The arginine-rich protein gives a higher than the glycine-rich protein
C. Both give identical because concentrations are equal
D. The glycine-rich protein gives a higher

53 In quantifying an unknown, why is it methodologically important that the standard protein be dissolved in the same buffer/matrix as the unknown sample?

Estimation of proteins using Bradford method Hard
A. Different buffers change the molecular weight of the standard protein
B. It is only for convenience and has no effect on accuracy
C. Buffer components can shift dye binding or background, so matching the matrix ensures the standard curve reflects the sample's conditions
D. Matching buffers is required to prevent the dye from precipitating

54 A researcher wants to detect very low protein amounts () reliably. Which modification of the standard Bradford protocol is most appropriate?

Estimation of proteins using Bradford method Hard
A. Increase the acid concentration tenfold to sharpen the color
B. Read at nm instead of nm to boost signal
C. Add SDS to enhance dye binding at low concentrations
D. Use the micro-assay format with a higher sample-to-reagent ratio and read at nm using a longer path length

55 The relationship between and protein concentration is best described within the working range by which mathematical behavior, and why is a curve rather than a strict straight line often observed?

Estimation of proteins using Bradford method Hard
A. Perfectly linear across the entire concentration range with zero intercept
B. Inversely proportional, decreasing as concentration rises
C. Approximately linear at low concentrations but curving due to a nonlinear ratio of bound to free dye as binding sites and dye become limiting
D. Exponentially increasing without bound at all concentrations

56 A sample contains SDS which cannot be removed. Which strategy best mitigates its interference in a Bradford assay?

Estimation of proteins using Bradford method Hard
A. Heat the sample to to destroy the SDS
B. Add extra Coomassie dye to overwhelm the SDS effect
C. Prepare the standard curve in the same SDS so the interference is matched, or precipitate protein to remove SDS before assay
D. Simply subtract a fixed value from every reading

57 During an assay a student obtains standards with but every sample reads slightly negative after blank subtraction. What is the most probable explanation?

Estimation of proteins using Bradford method Hard
A. Coomassie dye emits light at nm, giving negative values
B. The spectrophotometer measures transmittance, not absorbance
C. The blank absorbance was higher than the samples, likely from a contaminated or mismatched blank
D. The proteins in the samples have negative absorbance coefficients

58 Why does the Bradford assay tolerate reducing agents like -mercaptoethanol and DTT far better than the Lowry or BCA assays?

Estimation of proteins using Bradford method Hard
A. Bradford is performed at a pH where reductants are inactive
B. Bradford uses copper reduction that reductants happen to enhance
C. Bradford relies on a physical dye-binding color change, not on redox reactions that reductants would disrupt
D. Reducing agents chemically bind Coomassie and boost its color

59 A diluted unknown reads and its replicate reads , a large discrepancy. Assuming the spectrophotometer is fine, which experimental error most plausibly explains this?

Estimation of proteins using Bradford method Hard
A. Coomassie dye has two different absorption maxima that alternate
B. Inconsistent mixing/incubation timing or air bubbles in one cuvette, causing uneven color development
C. The protein spontaneously changed concentration between reads
D. The path length of the cuvette changed between measurements

60 For best analytical practice, at what wavelength and against what reference should Bradford absorbance be measured, and how should the concentration be derived?

Estimation of proteins using Bradford method Hard
A. At nm against the sample itself, reading concentration directly
B. At nm against water, using the extinction coefficient of tryptophan
C. At nm against a protein standard, with no blank subtraction needed
D. At nm against a reagent blank, deriving concentration from a same-day standard curve within the linear range
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