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Southern Blotting


Southern Blotting 



Introduction


Southern blotting is a molecular biology technique used for the detection of specific DNA sequences in a complex mixture of DNA. It was developed by Edwin M. Southern in 1975. The method involves restriction digestion of DNA, separation by gel electrophoresis, transfer (blotting) onto a membrane, and hybridization with a labeled DNA probe.

Principle of Southern Blotting

The technique is based on the principle of complementary base pairing. A single-stranded labeled DNA probe hybridizes specifically with its complementary DNA sequence immobilized on a membrane. Detection of the label confirms the presence and size of the target DNA fragment.
Steps Involved in Southern Blotting.

1. Isolation of DNA


Genomic DNA is extracted from cells or tissues.
DNA must be pure and intact to ensure accurate results.

2. Restriction Enzyme  Digestion

DNA is digested using specific restriction endonucleases.
Produces DNA fragments of varying lengths.
Choice of enzyme determines fragment size.


3. Agarose Gel Electrophoresis
Digested DNA fragments are separated based on size in an agarose gel.
Smaller fragments migrate faster than larger fragments.
Ethidium bromide or safe dyes are used for visualization.
4. Denaturation of DNA

Gel is treated with an alkaline solution (NaOH).
Double-stranded DNA is converted into single-stranded DNA.
Required for probe hybridization.

5. Neutralization

Gel is treated with a neutralizing buffer.
Prevents damage to the membrane and probe.
6. Transfer (Blotting) of DNA to Membrane

DNA fragments are transferred from the gel onto a membrane (usually nitrocellulose or nylon).
Methods of Transfer:
Capillary transfer (most common)
Vacuum blotting
Electroblotting
The DNA becomes permanently bound to the membrane.

7. Fixation of DNA


DNA is fixed to the membrane by:
Baking at 80°C, or
UV cross-linking
Prevents loss of DNA during hybridization.

8. Probe Preparation

A single-stranded DNA probe complementary to the target sequence is prepared.
Labeling methods:
Radioactive (³²P)
Non-radioactive (biotin, digoxigenin, fluorescent dyes)


9. Hybridization


Membrane is incubated with the labeled probe under optimal conditions.
Probe binds specifically to complementary DNA sequences.


10. Washing


Excess and non-specifically bound probes are removed.
Ensures high specificity.

11. Detection


Depends on probe label:
Autoradiography (radioactive probes)
Chemiluminescence / colorimetric detection (non-radioactive probes)
Bands appear corresponding to the target DNA.
Diagram (Flow of Southern Blotting)


DNA extraction → Restriction digestion → Gel electrophoresis →
Denaturation → Transfer to membrane → Fixation →
Probe hybridization → Washing → Detection


Applications of Southern Blotting


Detection of specific genes in genomic DNA
Gene mapping and genome analysis
Diagnosis of genetic diseases (e.g., sickle cell anemia)
DNA fingerprinting and forensic analysis
Identification of mutations and deletions
Verification of transgenic organisms
Study of gene rearrangements.


Advantages


Highly specific and sensitive
Detects gene structure and copy number
Useful for large DNA fragments


Limitations


Time-consuming procedure
Requires large amounts of DNA
Use of radioactive probes is hazardous
Low throughput compared to PCR-based methods

Conclusion


Southern blotting is a classical and powerful technique in molecular biology for the detection and analysis of specific DNA sequences. Although newer techniques like PCR and sequencing are widely used, Southern blotting remains important for gene structure analysis, mutation detection, and genomic studies.



1. Southern blotting is used to detect
A. RNA
B. Protein
C. DNA
D. Lipids
✅ Answer: C
2. Southern blotting was developed by
A. Watson
B. Crick
C. Edwin M. Southern
D. Mullis
✅ Answer: C
3. The principle of Southern blotting is based on
A. Antigen–antibody reaction
B. Complementary base pairing
C. Enzyme–substrate interaction
D. Protein folding
✅ Answer: B
4. Which enzyme is used to cut DNA in Southern blotting?
A. Ligase
B. Restriction endonuclease
C. DNA polymerase
D. RNA polymerase
✅ Answer: B
5. DNA fragments are separated by
A. SDS-PAGE
B. Agarose gel electrophoresis
C. Paper chromatography
D. TLC
✅ Answer: B
6. Smaller DNA fragments move
A. Slower
B. Faster
C. Same as larger fragments
D. Do not move
✅ Answer: B
7. DNA is denatured using
A. Acid
B. Heat only
C. Alkali (NaOH)
D. Alcohol
✅ Answer: C
8. Denaturation converts DNA into
A. Circular DNA
B. Double-stranded DNA
C. Single-stranded DNA
D. RNA
✅ Answer: C
9. Which membrane is commonly used?
A. Cellulose acetate
B. Nitrocellulose
C. Agarose
D. Polyacrylamide
✅ Answer: B
10. Nylon membranes are preferred because they
A. Are cheaper
B. Have high binding capacity
C. Cannot bind DNA
D. Are radioactive
✅ Answer: B
11. DNA transfer by capillary action is called
A. Electroblotting
B. Vacuum blotting
C. Capillary blotting
D. Dot blotting
✅ Answer: C
12. DNA is fixed to the membrane by
A. Cooling
B. Heating or UV cross-linking
C. Freezing
D. Staining
✅ Answer: B
13. A probe is
A. Protein
B. RNA enzyme
C. Labeled single-stranded DNA
D. Lipid
✅ Answer: C
14. Probe binds to target DNA by
A. Covalent bonding
B. Hydrogen bonding
C. Ionic bonding
D. Metallic bonding
✅ Answer: B
15. Which probe is radioactive?
A. Biotin
B. Digoxigenin
C. ³²P
D. FITC
✅ Answer: C
16. Non-radioactive probes are detected by
A. Autoradiography
B. Chemiluminescence
C. X-ray diffraction
D. UV spectroscopy
✅ Answer: B
17. Autoradiography uses
A. UV light
B. X-ray film
C. Visible light
D. Electron microscope
✅ Answer: B
18. Washing step removes
A. DNA
B. Gel
C. Non-specifically bound probe
D. Enzyme
✅ Answer: C
19. Southern blot is mainly used for
A. Protein estimation
B. RNA sequencing
C. Gene identification
D. Lipid analysis
✅ Answer: C
20. Which disease can be diagnosed by Southern blotting?
A. Diabetes
B. Sickle cell anemia
C. Tuberculosis
D. Cancer (all types)
✅ Answer: B
21. Southern blotting helps in
A. DNA fingerprinting
B. Forensic science
C. Paternity testing
D. All of the above
✅ Answer: D
22. The size of DNA fragments is determined by
A. Probe
B. Marker ladder
C. Membrane
D. Buffer
✅ Answer: B
23. Which step ensures specificity?
A. Digestion
B. Electrophoresis
C. Hybridization
D. Transfer
✅ Answer: C
24. Compared to PCR, Southern blotting is
A. Faster
B. More sensitive
C. Slower
D. Cheaper
✅ Answer: C
25. Main disadvantage of Southern blotting
A. Low specificity
B. High cost of enzymes
C. Time-consuming
D. Cannot detect DNA
✅ Answer: C
26. Which buffer is used for denaturation?
A. Tris buffer
B. NaCl
C. NaOH
D. EDTA
✅ Answer: C
27. The probe should be
A. Double stranded
B. Single stranded
C. Circular
D. Supercoiled
✅ Answer: B
28. Southern blotting detects
A. DNA sequence and size
B. Only DNA size
C. Only DNA presence
D. RNA sequence
✅ Answer: A
29. Which is NOT a blotting technique?
A. Southern
B. Northern
C. Western
D. Eastern chromatography
✅ Answer: D
30. Northern blotting detects
A. DNA
B. RNA
C. Protein
D. Lipids
✅ Answer: B
31. Western blotting detects
A. DNA
B. RNA
C. Protein
D. Carbohydrates
✅ Answer: C
32. Transfer of DNA onto membrane is called
A. Hybridization
B. Blotting
C. Denaturation
D. Digestion
✅ Answer: B
33. Agarose concentration affects
A. Probe binding
B. DNA migration
C. Membrane binding
D. Detection
✅ Answer: B
34. Southern blotting requires
A. Very small DNA quantity
B. Large amount of DNA
C. No DNA
D. RNA only
✅ Answer: B
35. Which labeling is safest?
A. Radioactive
B. Biotin
C. ³²P
D. Tritium
✅ Answer: B
36. Hybridization temperature depends on
A. DNA size
B. GC content
C. Enzyme
D. Membrane
✅ Answer: B
37. High stringency washing means
A. Low temperature
B. High salt
C. High temperature & low salt
D. No washing
✅ Answer: C
38. Southern blotting is less used today due to
A. Low accuracy
B. Replacement by PCR
C. No membrane available
D. No probes
✅ Answer: B
39. DNA fingerprinting uses
A. Variable regions of DNA
B. Conserved genes
C. RNA
D. Proteins
✅ Answer: A
40. Which method is fastest?
A. Southern blot
B. Northern blot
C. PCR
D. Western blot
✅ Answer: C
41. A positive result appears as
A. Colorless area
B. Band on membrane
C. Liquid solution
D. Crystal
✅ Answer: B
42. Membrane orientation must be
A. Random
B. Inverted
C. Correctly marked
D. Washed away
✅ Answer: C
43. Southern blotting detects
A. Gene deletion
B. Gene insertion
C. Gene rearrangement
D. All of the above
✅ Answer: D
44. DNA is transferred from gel to membrane in
A. Wet condition
B. Dry condition
C. Frozen condition
D. Vacuum only
✅ Answer: A
45. Which chemical is used for visualization in gel?
A. Methanol
B. Ethidium bromide
C. Acetone
D. Formaldehyde
✅ Answer: B
46. Southern blotting is useful in
A. Transgenic confirmation
B. GMO detection
C. Gene copy number analysis
D. All of the above
✅ Answer: D
47. Blotting paper is used to
A. Stop reaction
B. Absorb buffer
C. Stain DNA
D. Digest DNA
✅ Answer: B
48. Nitrocellulose membrane binds DNA by
A. Covalent bonds
B. Hydrogen bonds
C. Hydrophobic interactions
D. Ionic bonds
✅ Answer: C
49. The first blotting technique developed was
A. Northern
B. Western
C. Southern
D. Eastern
✅ Answer: C
50. Southern blotting is a
A. Protein technique
B. DNA hybridization technique
C. RNA sequencing method
D. Cell culture method
✅ Answer: B







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