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Western Blotting


Western Blotting (Immunoblotting)


Introduction


Western blotting, also known as immunoblotting, is a widely used analytical technique for the detection, identification, and quantification of specific proteins in a complex biological sample. The technique combines protein separation by gel electrophoresis with specific antigen–antibody interaction.
The method was developed by Towbin et al. (1979) and is called “Western” in analogy to Southern blotting (DNA) and Northern blotting (RNA).


Principle


The principle of Western blotting involves:
Separation of proteins based on molecular weight using SDS-PAGE
Transfer (blotting) of separated proteins onto a membrane
Specific detection of the target protein using primary and secondary antibodies
Visualization using enzymatic or fluorescent detection systems
👉 Antigen–antibody specificity is the core principle of Western blotting.


Steps Involved in Western Blotting

1. Sample Preparation

Protein samples are extracted from cells or tissues
Lysis buffers (RIPA, Tris buffer) are used
Protease inhibitors prevent protein degradation
Proteins are denatured using:
SDS
β-mercaptoethanol or DTT
Heat (95°C)


2. Protein Separation – SDS-PAGE


Proteins are separated based on molecular weight
SDS gives proteins a uniform negative charge
Smaller proteins migrate faster through the gel
Molecular weight markers (protein ladder) are run simultaneously


3. Transfer of Proteins (Blotting)


Proteins are transferred from gel to membrane
Membranes used:
Nitrocellulose
PVDF (Polyvinylidene difluoride)
Transfer Methods
Method
Description
Wet transfer
High efficiency, commonly used
Semi-dry transfer
Faster, less buffer
Dry transfer
Rapid, modern method


4. Blocking


Membrane is incubated with blocking agents to prevent non-specific binding
Common blocking agents:
BSA
Non-fat dry milk
Casein


5. Antibody Incubation


Primary Antibody
Binds specifically to the target protein
Can be monoclonal or polyclonal
Secondary Antibody
Binds to primary antibody
Conjugated with:
Horseradish peroxidase (HRP)
Alkaline phosphatase (AP)
Fluorescent dyes


6. Detection


Detection is done by adding substrate that reacts with enzyme on secondary antibody.
Detection Methods
Method
Description
Colorimetric
Visible color formation
Chemiluminescent
Light emission (most sensitive)
Fluorescent
Multiplex detection


7. Visualization and Analysis


Bands appear corresponding to the target protein
Band intensity reflects protein quantity
Molecular weight is determined using protein ladder
Applications of Western Blotting
Detection of specific proteins in complex samples
Diagnosis of diseases (e.g., HIV confirmatory test)
Detection of post-translational modifications
Protein expression studies
Validation of gene expression data
Study of signaling pathways
Quality control in recombinant protein production

Advantages


High specificity and sensitivity
Can detect low abundance proteins
Confirms protein size and identity
Widely accepted diagnostic tool


Limitations
Time-consuming and labor-intensive
Requires high-quality antibodies
Semi-quantitative
Prone to non-specific binding
Expensive reagents

Post-translational modifications

Precautions
Avoid protein degradation
Proper blocking is essential
Antibody dilution must be optimized
Avoid air bubbles during transfer
Wash steps should be adequate


Conclusion


Western blotting is a powerful, reliable, and specific technique for protein analysis. Due to its accuracy and versatility, it remains a gold standard method in molecular biology, biomedical research, and clinical diagnostics.







1. Western blotting is mainly used for detection of
A. DNA
B. RNA
C. Protein
D. Lipids
✅ Answer: C
2. Western blotting is also known as
A. Southern blot
B. Northern blot
C. Immunoblot
D. Dot blot
✅ Answer: C
3. The principle of Western blotting is based on
A. DNA–DNA hybridization
B. RNA–RNA hybridization
C. Antigen–antibody interaction
D. Enzyme–substrate binding
✅ Answer: C
4. Which electrophoresis technique is used in Western blotting?
A. Agarose gel electrophoresis
B. Native PAGE
C. SDS-PAGE
D. Isoelectric focusing
✅ Answer: C
5. SDS in SDS-PAGE provides proteins with
A. Positive charge
B. Negative charge
C. Neutral charge
D. Variable charge
✅ Answer: B
6. Protein separation in SDS-PAGE is based on
A. Charge
B. Shape
C. Size (molecular weight)
D. Solubility
✅ Answer: C
7. Which membrane is commonly used in Western blotting?
A. Cellulose acetate
B. Nylon
C. Nitrocellulose
D. Agarose
✅ Answer: C
8. Another membrane used in Western blotting is
A. PET
B. PVDF
C. Teflon
D. Polystyrene
✅ Answer: B
9. Purpose of blotting in Western blotting is
A. Protein amplification
B. Protein transfer to membrane
C. Protein digestion
D. Protein staining
✅ Answer: B
10. Blocking step prevents
A. Protein denaturation
B. Non-specific antibody binding
C. Protein transfer
D. Enzyme inactivation
✅ Answer: B
11. Common blocking agent used is
A. Agarose
B. BSA
C. Ethanol
D. Acetone
✅ Answer: B
12. Primary antibody binds to
A. Secondary antibody
B. Enzyme
C. Target protein
D. Substrate
✅ Answer: C
13. Secondary antibody binds to
A. Antigen
B. Primary antibody
C. Membrane
D. Substrate
✅ Answer: B
14. Enzyme commonly attached to secondary antibody
A. DNA polymerase
B. Restriction enzyme
C. Horseradish peroxidase
D. RNA polymerase
✅ Answer: C
15. Chemiluminescent detection uses
A. Fluorescent dye
B. Radioisotopes
C. Light emission
D. Color precipitation
✅ Answer: C
16. Which is the most sensitive detection method?
A. Colorimetric
B. Radioactive
C. Chemiluminescent
D. Coomassie staining
✅ Answer: C
17. Protein ladder is used to determine
A. Protein concentration
B. Protein purity
C. Molecular weight
D. Protein activity
✅ Answer: C
18. Wet transfer method is
A. Less efficient
B. Highly efficient
C. Very slow
D. Obsolete
✅ Answer: B
19. Semi-dry transfer uses
A. Large buffer volume
B. No electricity
C. Less buffer
D. High temperature
✅ Answer: C
20. Which disease diagnosis uses Western blot as confirmatory test?
A. Tuberculosis
B. Malaria
C. HIV
D. Dengue
✅ Answer: C
21. Which reducing agent is used in sample preparation?
A. EDTA
B. DTT
C. NaCl
D. Urea
✅ Answer: B
22. β-mercaptoethanol is used to
A. Break peptide bonds
B. Reduce disulfide bonds
C. Add charge
D. Stain proteins
✅ Answer: B
23. PVDF membrane must be activated using
A. Water
B. Acetone
C. Methanol
D. Ethanol
✅ Answer: C
24. Which buffer is commonly used for protein transfer?
A. TAE buffer
B. TBE buffer
C. Tris-Glycine buffer
D. PBS
✅ Answer: C
25. Which step follows blocking?
A. Detection
B. Sample loading
C. Primary antibody incubation
D. Transfer
✅ Answer: C
26. Western blotting is a
A. Quantitative technique
B. Semi-quantitative technique
C. Non-analytical method
D. Staining method
✅ Answer: B
27. Which blotting technique detects RNA?
A. Southern
B. Western
C. Northern
D. Eastern
✅ Answer: C
28. Eastern blotting is used for
A. DNA
B. RNA
C. Proteins
D. Post-translational modifications
✅ Answer: D
29. Which antibody is enzyme-labeled?
A. Primary
B. Secondary
C. Both
D. None
✅ Answer: B
30. Colorimetric detection produces
A. Light
B. Fluorescence
C. Colored precipitate
D. Heat
✅ Answer: C
31. Purpose of washing steps is to
A. Fix proteins
B. Remove unbound antibodies
C. Stain proteins
D. Denature proteins
✅ Answer: B
32. SDS is a
A. Reducing agent
B. Detergent
C. Buffer
D. Enzyme
✅ Answer: B
33. Which technique confirms protein size and identity?
A. ELISA
B. Western blot
C. PCR
D. Northern blot
✅ Answer: B
34. Protein degradation is prevented by
A. SDS
B. Protease inhibitors
C. BSA
D. Methanol
✅ Answer: B
35. Which antibody has higher specificity?
A. Polyclonal
B. Monoclonal
C. Both same
D. None
✅ Answer: B
36. Over-blocking can cause
A. High background
B. Signal loss
C. Protein degradation
D. Gel breakage
✅ Answer: B
37. Which detection method allows multiplexing?
A. Colorimetric
B. Chemiluminescent
C. Fluorescent
D. Radioactive
✅ Answer: C
38. Western blotting cannot detect
A. Protein expression
B. Molecular weight
C. DNA mutation
D. Protein modification
✅ Answer: C
39. Loading control protein commonly used
A. Actin
B. DNA polymerase
C. Restriction enzyme
D. RNA polymerase
✅ Answer: A
40. Which step is most critical for specificity?
A. Gel preparation
B. Blocking
C. Antibody incubation
D. Sample heating
✅ Answer: C
41. Which blotting uses radioactive probes?
A. Western
B. Southern
C. Northern
D. Both B and C
✅ Answer: D
42. Nitrocellulose membrane has
A. High mechanical strength
B. Low protein binding
C. High protein binding
D. No protein binding
✅ Answer: C
43. Signal intensity depends on
A. Gel thickness
B. Protein amount
C. Buffer volume
D. Transfer time only
✅ Answer: B
44. Which step confirms successful transfer?
A. Blocking
B. Antibody binding
C. Ponceau S staining
D. Detection
✅ Answer: C
45. Western blotting is used in
A. Proteomics
B. Clinical diagnostics
C. Molecular biology research
D. All of the above
✅ Answer: D
46. Which buffer is used for washing membrane?
A. TE buffer
B. TBS-T
C. TAE
D. SSC
✅ Answer: B
47. Primary antibody is usually
A. Enzyme-linked
B. Fluorescent
C. Unlabeled
D. Radioactive
✅ Answer: C
48. Which technique preceded Western blotting?
A. ELISA
B. Southern blot
C. Northern blot
D. PCR
✅ Answer: B
49. Protein bands appear as
A. Spots
B. Smears
C. Distinct bands
D. Wells
✅ Answer: C
50. Western blotting combines electrophoresis with
A. Hybridization
B. Immunodetection
C. Sequencing
D. Amplification
✅ Answer: B

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