Learning Objective: At the end of this lesson, the learner will be able to differentiate between Southern, Northern, and Western blotting techniques and explain their applications in molecular diagnosis and research.
Blotting techniques are molecular biology tools used to detect specific biomolecules (DNA, RNA, or proteins) within complex samples. Each type of blot targets a different molecule, but all share a common principle — separation by electrophoresis, transfer to a membrane, and detection using a labeled probe or antibody.
Mnemonic: SNoW DRoP Southern → DNA Northern → RNA Western → Protein
Southern Blot (DNA Detection)
| Step |
Description |
| 1. DNA digestion |
The DNA sample is enzymatically cleaved into smaller fragments. |
| 2. Gel electrophoresis |
Fragments are separated by size. |
| 3. Transfer |
DNA is transferred to a nitrocellulose or nylon membrane. |
| 4. Hybridization |
The membrane is exposed to a labeled DNA probe that binds to its complementary sequence. |
| 5. Visualization |
The labeled DNA is detected using film or digital imaging. |
Purpose:
- Identifies the size of specific DNA sequences
- Determines heterozygosity, or repeat expansions (e.g., CGG repeats in the FMR1 gene → Fragile X syndrome)
Northern Blot (RNA Detection)
| Feature |
Description |
| Sample type |
RNA |
| Principle |
Similar to Southern blot, but detects RNA transcripts using a labeled complementary probe. |
| Use |
Evaluates gene expression levels by studying mRNA size and abundance. |
| Clinical significance |
Detects splicing errors and transcriptional regulation abnormalities. |
Western Blot (Protein Detection)
| Feature |
Description |
| Sample type |
Protein |
| Principle |
Proteins are separated by gel electrophoresis, transferred to a membrane, and then detected using a labeled antibody specific to the target protein. |
| Use |
Identifies and quantifies specific proteins. |
| Clinical significance |
Used for HIV confirmatory testing, prion detection, and protein expression analysis in research. |
Summary Table
| Blot Type |
Analyte Detected |
Probe Used |
Main Application |
| Southern |
DNA |
DNA probe |
Gene mapping, mutation or repeat analysis |
| Northern |
RNA |
DNA/RNA probe |
Gene expression and splicing analysis |
| Western |
Protein |
Antibody |
Protein identification and quantification |
Activity:
