Isolation of DNA — Revision Notes

⚡ 30-Second Revision

Lysis: — Break open cells (detergents, enzymes: Lysozyme for bacteria, Cellulase for plants).
Contaminant Removal:

- Proteins: Proteinase K (digests proteins, inactivates nucleases). - RNA: RNase (degrades RNA).

DNA Precipitation: — Chilled Ethanol/Isopropanol + Salt (e.g., Sodium Acetate).
Washing: — 70% Ethanol (removes residual salts).
Rehydration: — TE Buffer (Tris for pH, EDTA for DNase inhibition).
Key Principle: — DNA is insoluble in cold alcohol in presence of salts.

2-Minute Revision

DNA isolation is the crucial first step in genetic engineering, aiming to extract pure DNA from biological samples. The process begins with lysis, where cell and nuclear membranes are disrupted using detergents (like SDS) and sometimes specific enzymes (e.

g., cellulase for plants, lysozyme for bacteria) to release the cellular contents. Following lysis, contaminants must be removed. Proteinase K is added to digest proteins and inactivate nucleases, while RNase degrades RNA.

After purification, the DNA is still dissolved in solution. To make it collectible, chilled ethanol or isopropanol is added along with a salt (like sodium acetate). The salt neutralizes the DNA's negative charge, and DNA, being insoluble in cold alcohol, precipitates out as visible strands.

This precipitated DNA is then collected by centrifugation, washed with 70% ethanol to remove residual salts, briefly air-dried, and finally rehydrated in a buffer like TE buffer (Tris-EDTA) for stable storage.

This systematic approach ensures a high-quality DNA sample for downstream applications.

5-Minute Revision

Mastering DNA isolation is fundamental for NEET, as it underpins all recombinant DNA technologies. The process systematically purifies DNA from a complex cellular mixture. It starts with cell lysis, where physical (grinding), chemical (detergents like SDS to dissolve membranes), and enzymatic (e.

g., lysozyme for bacteria, cellulase for plants to break cell walls) methods are employed to rupture cells and nuclei. This releases DNA along with other cellular components. The next critical stage is removal of contaminants.

Proteins, including DNA-degrading nucleases, are digested by Proteinase K. RNA, which can interfere with DNA analysis, is degraded by RNase. Lipids are typically removed during lysis by detergents.

Once purified, the DNA is still dissolved. To make it visible and collectible, DNA precipitation is performed. This involves adding a salt (e.g., sodium acetate) to neutralize the negative charges on the DNA backbone, followed by chilled ethanol or isopropanol.

DNA is insoluble in cold alcohol, causing it to aggregate and precipitate. The precipitated DNA is then pelleted by centrifugation, washed with 70% ethanol to remove residual salts and impurities, and briefly air-dried.

Finally, the pure DNA pellet is rehydrated in a suitable buffer, commonly TE buffer (Tris-EDTA). Tris maintains a stable pH, while EDTA chelates divalent cations, inhibiting DNase activity and protecting the DNA from degradation.

Understanding the 'why' behind each step and the specific function of each reagent is key for NEET success.

Prelims Revision Notes

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Definition: — DNA isolation is the process of extracting pure DNA from biological samples.
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Purpose: — Essential first step for PCR, cloning, sequencing, genetic engineering, diagnostics.
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General Steps (Sequential Order):

* Lysis: Breaking open cells to release DNA. * Contaminant Removal: Separating DNA from proteins, RNA, lipids, etc. * DNA Precipitation: Making DNA insoluble to collect it. * Washing: Removing residual impurities. * Rehydration: Dissolving DNA for storage/use.

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Key Reagents and Their Functions:

* Detergents (e.g., SDS, Triton X-100): Disrupt cell/nuclear membranes (lysis), denature proteins. * Enzymes for Cell Walls: * Lysozyme: Degrades bacterial cell walls (peptidoglycan). * Cellulase: Degrades plant cell walls (cellulose).

* Chitinase: Degrades fungal cell walls (chitin). * Proteinase K: Digests proteins (including histones and nucleases), inactivates nucleases. * RNase (Ribonuclease): Degrades RNA into smaller fragments.

* Salts (e.g., Sodium Acetate, NaCl): Provide cations to neutralize DNA's negative charge, aiding precipitation. * Chilled Ethanol/Isopropanol: Precipitates DNA (DNA is insoluble in cold alcohol).

* 70% Ethanol: Washes DNA pellet, removes residual salts while keeping DNA precipitated. * TE Buffer (Tris-EDTA): * Tris: Maintains stable pH. * EDTA: Chelates divalent cations ($\text{Mg}^{2+}$, $\text{Ca}^{2+}$), inhibiting DNase activity to protect DNA.

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Physical Methods: — Grinding in liquid nitrogen (for tough tissues like plants) for mechanical lysis.
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Centrifugation: — Used to pellet cellular debris after lysis, and to pellet precipitated DNA.
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Key Principle of Precipitation: — DNA's negative charge is neutralized by salts, making it aggregate and become insoluble in cold alcohol.

Vyyuha Quick Recall

To remember the main steps of DNA isolation: Lions Prefer Roasted Peanuts, Washed Regularly.

Lysis
Protein/RNA removal
Precipitation
Washing
Rehydration