Comprehensive Guide to DNA Extraction and Purification

DNA extraction and purification involves the removal of pure DNA from cells, serving as a fundamental preliminary step for numerous molecular diagnostic analyses.
It is a critical requirement for diagnosing diseases, identifying genetic disorders, and detecting pathogens such as viruses and bacteria in the environment.
Common molecular techniques necessitating high-purity DNA include:
- PCR Reactions.
- DNA Sequencing.
- Genetic Cloning.
- Fluorescent in situ Hybridization (FISH).
- DNA Fingerprinting.

While various commercial kits, such as Qiagen, are widely available for DNA extraction, the underlying process for genomic DNA extraction generally consists of three fundamental phases:
- Lysis: Breaking open the cell and nuclear membranes.
- Precipitation: Separating the DNA from other cellular components and aggregating it into a solid form.
- Purification: Cleaning the isolated DNA of any remaining contaminants.

The primary goal of the lysis phase is to break down all cellular membranes and the nucleus (if present) to release the genomic DNA.
Lysing mechanisms vary depending on the sample type because different organisms possess distinct cellular structures (e.g., cell walls versus plasma membranes).
Specific lysis methods include:
- Physical Methods: These involve mechanical force to disrupt cells. Examples include grinding with a mortar and pestle under liquid nitrogen, freeze and thaw cycles, or shaking/vortexing samples with beads.
- Chemical Methods: Common detergents such as Sodium dodecyl Sulfate (SDS) are utilized to disrupt cellular membranes chemically.
- Enzymatic Treatments: Specific enzymes target different cellular components, including lysozyme, lipase, collagenase, and proteinase K.
Application by Sample Type:
- Plant cells, bacteria, and yeast often require physical or enzymatic methods first to break down the highly structured peptidoglycan layer of the cell wall.
- Animal samples—such as blood, cheek cells, hair follicles, and small tissue pieces—typically undergo cellular disruption through chemical methods.
- Modern commercial kits often combine enzymatic and chemical methods in tandem for increased efficiency.

Once membranes are broken, DNA must be isolated from proteins, sugars, and cellular debris.
DNA Solubility:
- DNA is soluble in water.
- DNA is insoluble in the presence of salt and alcohol.
Process Steps:
- Salts are added to interrupt the hydrogen bonds between water and DNA molecules.
- Ice-cold alcohol (ethanol or isopropanol) is carefully added to the sample.
- In the presence of cations (from the salt), ethanol induces a structural change in the DNA molecules, causing them to aggregate and precipitate out of the aqueous solution.
Removing Debris:
- To ensure a clean sample, Protease (such as proteinase K), phenol, or chloroform is used to denature and degrade DNA-associated proteins and other cellular proteins prior to the addition of alcohol.

After precipitation, DNA is separated from the aqueous solution, allowing for further cleaning.
Purification involves rinsing the precipitated DNA with additional alcohol (typically $70\%$ ethanol) to remove any remaining salts or cellular debris.
Final Resuspension:
- The purified DNA is resuspended in a stable medium, such as nuclease-free water or an alkaline buffer (e.g., TE Buffer).
- Once resuspended, the genomic DNA is ready for downstream use, storage, transportation, or diagnostic testing.

Measuring Concentration:
- The aromatic rings of nitrogenous bases in DNA absorb UV light at a wavelength of $260\,nm$ .
- By shining a beam of UV light through the DNA solution, the proportion of light absorbed determines the specific concentration of DNA.
Measuring Purity:
- Aromatic proteins absorb UV light at $280\,nm$ .
- Purity is assessed using the absorbance ratio at $260/280\,nm$ .
- A ratio of $1.8$ is considered pure DNA; the universally accepted standard range for pure DNA is $1.8 - 2.0$ .
- A ratio less than $1.8$ indicates the sample is contaminated with protein.
Advanced Quality Checks (NanoDrop):
- Concentration is measured in $ng/\mu L$ .
- $260/280$ ratio: Measures protein contamination (Normal range: $1.8 - 2.0$ ).
- $260/230$ ratio: Measures salt contamination (Normal range: $2.0 - 2.2$ ).

RBC Lysis Buffer:
- Ammonium chloride ( $155\,mM$ ): Used to disrupt the osmotic balance of Red Blood Cell membranes.
- Sodium bicarbonate ( $10\,mM$ ): Maintains a slightly alkaline $pH$ of $7.4$ .
- EDTA ( $0.1\,mM$ ): Inactivates DNase enzymes to prevent DNA degradation.
WBC Lysis Buffer:
- Tris-HCl ( $10\,mM$ ): Maintains a $pH$ of $8.0$ .
- NaCl ( $400\,mM$ ): Neutralizes the negatively charged phosphate backbone of DNA.
- EDTA ( $2\,mM$ ): Inactivates DNase.
Precipitation Reagents:
- Protein precipitation salt: Sodium chloride ( $6\,M$ ).
- DNA precipitating alcohol: Ice-cold $100\%$ ethanol or isopropanol.
Washing and Resuspension:
- Washing alcohol: $70\%$ ethanol.
- Resuspension buffer: TE buffer ( $pH = 8.0$ ) containing Tris-HCl ( $10\,mM$ ) and EDTA ( $1\,mM$ ), or sterile nuclease-free water.

DNA integrity is verified through agarose gel electrophoresis, typically using a gel concentration of $0.8\% - 1.0\%$ agarose.
Solvent/Buffer: Tris-borate-EDTA (TBE) buffer with a $pH = 8.3 - 8.5$ .
Staining: Ethidium bromide is added as an intercalating dye/stain to visualize DNA under UV light.
Interpretation of Results:
- High-quality genomic DNA: Appears as a single, clean, sharp, high-molecular weight band at the top of the gel.
- Degraded/Sheared DNA: Appears as a long, blurry smear, indicating the DNA was broken during the extraction process.
- RNA Contamination: Appears as a bright, fuzzy cloud at the bottom of the gel.
Electrophoresis Dynamics:
- Samples are loaded into wells at the negative electrode.
- DNA moves toward the positive electrode.
- Shorter DNA fragments move faster and further than longer DNA fragments.
- A DNA ladder is used to determine the size of fragments in base pairs ( $bp$ ), with markers ranging from $250\,bp$ to $10,000\,bp$ .

Question 1: Kristen received a basket of strawberries from her Grandad Bob and wants to confirm if the fruits are genetically modified or organic.
- From what type of cells is she going to isolate the DNA? Answer: Plant cells.
- Mention one preferred lysing method: Answer: Grinding with a mortar and pestle under liquid nitrogen.
- What steps might she use to extract the DNA? Answer: Lysis, Precipitation, and Purification.
- Mention one technique she can use her extracted DNA for: Answer: PCR Reactions or DNA Sequencing.
Question 2: Ronnie is showing signs of fever, cough, fatigue, muscle and body ache. His doctor suspects COVID-19 and takes a buccal swab for testing.
- From what cells is the DNA going to be extracted? Answer: Buccal (cheek) cells.
- What extraction method is likely to be used for this procedure? Answer: Chemical methods.
- What steps are we going to use to extract the DNA? Answer: Lysis, Precipitation, and Purification.