# DNA Isolation by Phenol Chloroform: Principles and Protocol

## Introduction to Phenol Chloroform DNA Isolation

Phenol-chloroform extraction is a liquid-liquid organic extraction method used to purify nucleic acids from cellular lysates. The technique exploits the differential solubility of DNA, RNA, and proteins in immiscible aqueous and organic phases. When an aqueous cell lysate is mixed with a phenol-chloroform mixture and centrifuged, proteins partition into the organic phase, while DNA remains in the aqueous phase. This classic method, first developed in the 1950s and refined over subsequent decades, remains a gold standard for obtaining high-molecular-weight DNA with minimal chemical modification.

The method is fundamentally a partitioning process driven by the hydrophobic and hydrophilic properties of biomolecules. Phenol, a weak acid (pKa ≈ 10), disrupts hydrogen bonds and hydrophobic interactions that maintain [protein structure](/knowledge/bioinformatics/protein-structure-biophysical-levels-folding), causing denaturation. Denatured proteins aggregate and partition into the organic phase, while the negatively charged phosphate backbone of DNA remains hydrated in the aqueous phase. The result is a simple, inexpensive, and highly effective purification strategy that does not require specialized instrumentation.

### What is Phenol Chloroform Extraction?

Phenol-chloroform extraction is a batch-wise purification procedure. A biological sample—cultured cells, tissue homogenate, or bacterial pellet—is first lysed to release its contents. The lysate is then mixed with an equal volume of a phenol-chloroform-isoamyl alcohol mixture (typically 25:24:1 by volume). After vigorous mixing and centrifugation, the mixture separates into distinct layers: a lower organic phase containing phenol and chloroform, an upper aqueous phase containing nucleic acids, and a thin white interphase containing precipitated proteins and cell debris. The aqueous layer is carefully removed without disturbing the interphase, and the extraction is often repeated to achieve high purity. Finally, DNA is recovered by ethanol precipitation, washed, and resuspended in an appropriate buffer.

### Applications in [Molecular Biology](/blog/careers/molecular-biology)

Phenol-chloroform extraction is used whenever high-quality DNA is required for downstream applications. It is the method of choice for isolating genomic DNA from mammalian tissues, plant leaves, and Gram-positive bacteria, where cell walls require enzymatic digestion prior to lysis. The technique is also used for purifying plasmid DNA after alkaline lysis in [Miniprep Plasmid Isolation](/knowledge/molecular-biology/miniprep-plasmid-isolation) procedures, and for removing proteins from DNA samples before [restriction enzyme digestion](/knowledge/diagnostics/molecular/restriction-enzyme-digestion-protocol-troubleshooting), PCR amplification, or sequencing. The method is equally applicable to RNA purification, though the protocol requires additional precautions against RNase contamination—see [Phenol Chloroform RNA Extraction](/knowledge/molecular-biology/phenol-chloroform-rna-extraction) for the RNA-specific variant. In clinical diagnostics, phenol-chloroform extraction remains a reference method for extracting DNA from blood, buccal swabs, and formalin-fixed paraffin-embedded tissues, despite the increasing popularity of column-based kits.

## Principles of the Method

The success of phenol-chloroform extraction rests on the distinct chemical properties of the three main components: phenol, chloroform, and isoamyl alcohol. Each plays a specific role in achieving phase separation and [protein denaturation](/knowledge/molecular-biology/protein-denaturation).

### Role of Phenol

Phenol (C₆H₅OH) is an aromatic organic compound that is partially miscible with water (about 8.3 g per 100 mL at 20°C). At neutral pH, phenol acts as a powerful protein denaturant. It disrupts hydrophobic interactions and hydrogen bonds that stabilize protein tertiary structure, causing proteins to unfold. The unfolded polypeptide chains expose hydrophobic residues that interact favorably with the aromatic ring of phenol, causing the proteins to partition into the organic phase. Phenol is also an effective inhibitor of nucleases; by denaturing DNases and RNases, it protects the nucleic acids from degradation during the extraction process.

However, phenol alone is not sufficient for efficient extraction. Pure phenol is dense (density ≈ 1.07 g/mL) and viscous, making pipetting difficult. It also retains up to 10–20% water, which can cause DNA loss into the organic phase. For these reasons, phenol is typically equilibrated with buffer (usually Tris-HCl, pH 8.0) to saturate it with water and adjust its pH. At pH 8.0, DNA remains in the aqueous phase because the phosphate groups are fully ionized and the DNA is highly hydrophilic. At acidic pH, DNA partitions into the organic phase—a property exploited in some RNA extraction protocols but undesirable for DNA isolation.

### Role of Chloroform and Isoamyl Alcohol

Chloroform (CHCl₃) is added to the phenol to improve the efficiency of the extraction. Chloroform has several functions. First, it increases the density of the organic phase, ensuring clean and rapid phase separation upon centrifugation. Second, it reduces the amount of water retained in the phenol phase, minimizing DNA loss. Third, chloroform is a more effective denaturant than phenol alone for some proteins, particularly histones, which are tightly associated with DNA in chromatin. Fourth, chloroform removes phenol from the aqueous phase after extraction; because phenol is more soluble in chloroform than in water, the final chloroform wash removes residual phenol that would otherwise inhibit downstream enzymes.

Isoamyl alcohol (3-methyl-1-butanol) is included at a ratio of 1 part per 24 parts chloroform. Its primary function is to reduce foaming and to stabilize the interphase. Isoamyl alcohol is a surfactant that decreases the surface tension at the interface between the aqueous and organic phases, preventing the formation of emulsions that trap DNA and make phase separation difficult. It also helps to prevent the denaturation of DNA at the interface.

### Phase Separation and DNA Partitioning

When the phenol-chloroform-isoamyl alcohol mixture (25:24:1) is added to an aqueous cell lysate and mixed, a two-phase system forms. The organic phase, composed of phenol and chloroform, has a density of approximately 1.27 g/mL, while the aqueous phase has a density of approximately 1.0 g/mL. Upon centrifugation (typically 12,000–16,000 × g for 5–10 minutes), the denser organic phase settles at the bottom of the tube, and the aqueous phase forms the upper layer. Denatured proteins accumulate at the interface between the two phases, forming a visible white precipitate.

DNA, with its highly negatively charged phosphate backbone, is strongly hydrophilic and remains dissolved in the aqueous phase. The partitioning of DNA into the aqueous phase is essentially quantitative under neutral to slightly alkaline conditions (pH 7.0–8.0). The extraction efficiency is high: a single phenol-chloroform extraction removes approximately 90–95% of proteins, and two to three successive extractions can remove virtually all protein contamination. The aqueous phase is then carefully removed with a pipette, taking care not to disturb the interphase, and the extraction is repeated if necessary.

## Required Reagents and Materials

The following reagents and equipment are required for phenol-chloroform DNA isolation. All solutions should be prepared using nuclease-free water and [molecular biology](/blog/careers/molecular-biology) grade reagents. For guidance on preparing buffers and stock solutions, refer to [Buffer Preparation](/knowledge/molecular-biology/buffer-preparation).

### Reagents and Buffers

| Reagent | Composition | Purpose |
|---------|-------------|---------|
| Lysis buffer | 10 mM Tris-HCl (pH 8.0), 100 mM NaCl, 25 mM EDTA (pH 8.0), 0.5% SDS, 100 µg/mL proteinase K | Cell lysis and protein digestion |
| Proteinase K | 20 mg/mL stock solution in 10 mM Tris-HCl (pH 7.5) | Digests proteins, including nucleases |
| Phenol-chloroform-isoamyl alcohol | 25:24:1 (v/v/v), equilibrated with Tris-HCl (pH 8.0) | Organic extraction of proteins |
| Chloroform-isoamyl alcohol | 24:1 (v/v) | Final organic wash to remove residual phenol |
| 3 M Sodium acetate | pH 5.2 | Provides monovalent cations for ethanol precipitation |
| Absolute ethanol (100%) | Ice-cold | DNA precipitation |
| 70% Ethanol | Prepared in nuclease-free water | Washing the DNA pellet |
| TE buffer | 10 mM Tris-HCl (pH 8.0), 1 mM EDTA | Resuspension and storage of DNA |

Additional materials: sterile microcentrifuge tubes (1.5 mL or 2.0 mL), aerosol-resistant pipette tips, a microcentrifuge capable of 16,000 × g, a vortex mixer, a water bath or heat block set to 56°C, and a UV spectrophotometer or NanoDrop for quality assessment.

### Safety Considerations

Phenol is highly corrosive and toxic. It causes severe chemical burns on contact with skin and is readily absorbed through the skin, potentially causing systemic toxicity. Chloroform is a suspected carcinogen and a central nervous system depressant. Both compounds are volatile and should be handled in a fume hood. The following precautions are mandatory:

1. Wear nitrile gloves, a laboratory coat, and safety goggles at all times. Nitrile gloves are preferred over latex because phenol penetrates latex rapidly.
2. Work in a certified chemical fume hood when handling phenol and chloroform. Do not pipette phenol by mouth; always use a mechanical pipette aid.
3. If phenol contacts the skin, wash immediately with copious amounts of water and then with 70% polyethylene glycol (PEG 400) if available. Do not use ethanol, which increases absorption.
4. Dispose of phenol-chloroform waste in a designated organic waste container. Do not pour down the sink.
5. Store phenol-chloroform solutions in amber glass bottles, protected from light, at 4°C. Do not allow the solution to freeze, as this can cause phase separation.

## Step-by-Step Protocol

The following protocol describes the isolation of genomic DNA from cultured mammalian cells or tissue. Volumes are given for a starting sample of approximately 1 × 10⁶ cells or 25 mg of tissue. Adjust volumes proportionally for other sample sizes.

### Cell Lysis and Homogenization

1. **Harvest cells.** For adherent cells, remove the culture medium and wash the monolayer with phosphate-buffered saline (PBS). Add 0.25% trypsin-EDTA and incubate at 37°C for 2–5 minutes until cells detach. Neutralize the trypsin with serum-containing medium, collect the cells, and pellet by centrifugation at 500 × g for 5 minutes. For suspension cells, pellet directly. For tissue samples, mince the tissue finely with a sterile scalpel blade on ice.

2. **Resuspend the pellet** in 500 µL of lysis buffer (10 mM Tris-HCl pH 8.0, 100 mM NaCl, 25 mM EDTA, 0.5% SDS). Vortex briefly to ensure complete resuspension. The SDS in the lysis buffer disrupts the cell membrane and nuclear envelope, while EDTA chelates divalent cations (Mg²⁺, Ca²⁺) that are cofactors for DNases, thereby inhibiting nuclease activity.

3. **Add proteinase K** to a final concentration of 100 µg/mL (i.e., 2.5 µL of a 20 mg/mL stock solution). Mix by inverting the tube several times. Do not vortex vigorously, as this can shear high-molecular-weight DNA.

4. **Incubate at 56°C for 1–3 hours** (or overnight at 37°C) with occasional gentle mixing. Proteinase K is a broad-spectrum serine protease that digests histones and other DNA-associated proteins, releasing DNA into solution. The incubation time depends on the sample type; tissues and Gram-positive bacteria require longer digestion.

### Phenol-Chloroform Extraction

5. **Add an equal volume** (500 µL) of phenol-chloroform-isoamyl alcohol (25:24:1) to the lysate. The solution should be at room temperature; do not use ice-cold phenol, as this reduces extraction efficiency.

6. **Mix thoroughly** by inverting the tube 20–30 times or by vortexing for 10–15 seconds. The mixture should appear milky white and homogeneous. This step ensures intimate contact between the aqueous and organic phases, allowing proteins to partition into the organic phase.

7. **Centrifuge at 12,000 × g for 10 minutes at room temperature** (or 4°C). After centrifugation, three layers should be visible: a clear upper aqueous phase, a white interphase containing precipitated proteins, and a lower yellow organic phase.

8. **Carefully transfer the upper aqueous phase** (approximately 450–500 µL) to a fresh microcentrifuge tube using a pipette with a fine tip. Do not disturb the interphase. If the interphase is disturbed, re-centrifuge the sample for 5 minutes before proceeding.

9. **Repeat the extraction** (steps 5–8) with fresh phenol-chloroform-isoamyl alcohol if the interphase is thick or if high protein content is expected (e.g., from tissues). Two extractions are usually sufficient for most samples.

10. **Perform a chloroform-isoamyl alcohol extraction** (24:1) by adding an equal volume of this mixture to the aqueous phase. Mix and centrifuge as above. This step removes residual phenol from the aqueous phase, which would otherwise interfere with ethanol precipitation and inhibit downstream enzymes.

### Precipitation and Washing of DNA

11. **Transfer the aqueous phase** (approximately 400–450 µL) to a fresh microcentrifuge tube. Add 0.1 volumes of 3 M sodium acetate (pH 5.2), i.e., 40–45 µL, and mix gently. The sodium acetate provides Na⁺ ions that neutralize the negative charges on the DNA phosphate backbone, reducing electrostatic repulsion and facilitating precipitation.

12. **Add 2.5 volumes of ice-cold absolute ethanol** (approximately 1 mL). Mix by inverting the tube several times. A visible white DNA precipitate may form immediately for high-concentration samples. For low-concentration samples, precipitation may require incubation at −20°C for 30–60 minutes or overnight.

13. **Centrifuge at 16,000 × g for 15 minutes at 4°C.** The DNA will form a pellet at the bottom of the tube. The pellet may be invisible for low-yield samples, so orient the tube in the centrifuge consistently (e.g., hinge facing outward) to know where the pellet will be.

14. **Carefully remove the supernatant** with a pipette, taking care not to dislodge the pellet. Add 500 µL of ice-cold 70% ethanol to wash the pellet. This removes residual salt and phenol.

15. **Centrifuge at 16,000 × g for 5 minutes at 4°C.** Remove the supernatant as before. Briefly air-dry the pellet for 5–10 minutes at room temperature. Do not over-dry, as this makes the DNA difficult to resuspend.

16. **Resuspend the DNA pellet** in 50–100 µL of TE buffer (10 mM Tris-HCl pH 8.0, 1 mM EDTA) or nuclease-free water. If the DNA does not dissolve readily, incubate at 37°C for 10–15 minutes with gentle tapping. Store the DNA at 4°C for short-term use or at −20°C for long-term storage.

## Expected Results and Quality Assessment

A successful phenol-chloroform extraction should yield high-molecular-weight DNA with an A260/A280 ratio between 1.8 and 2.0. The yield depends on the starting material: approximately 5–10 µg of DNA per 1 × 10⁶ cultured mammalian cells, or 1–2 µg per mg of tissue.

### Quantification by UV Spectroscopy

DNA concentration is determined by measuring absorbance at 260 nm (A260). An absorbance of 1.0 at 260 nm corresponds to approximately 50 µg/mL of double-stranded DNA. The concentration is calculated as:

**DNA concentration (µg/mL) = A260 × 50 × dilution factor**

The purity is assessed by the ratio of absorbance at 260 nm to absorbance at 280 nm (A260/A280). Pure DNA has a ratio of 1.8–2.0. A ratio below 1.8 indicates protein or phenol contamination, while a ratio above 2.0 may indicate RNA contamination. The A260/A230 ratio (expected 2.0–2.2) is a secondary measure of purity; low values indicate contamination by chaotropic salts, carbohydrates, or residual phenol. For accurate measurements, the DNA sample should be diluted in TE buffer rather than water, as low ionic strength can affect absorbance readings.

### Agarose Gel Analysis

Agarose gel electrophoresis is used to assess DNA integrity and molecular weight. Run 200–500 ng of DNA on a 0.8% agarose gel in 1× TAE buffer (40 mM Tris-acetate, 1 mM EDTA) at 80–100 V for 30–60 minutes. Stain with ethidium bromide or a safer alternative such as SYBR Safe, and visualize under UV light.

High-quality genomic DNA appears as a single high-molecular-weight band near the top of the gel, with minimal smearing. Smearing indicates DNA degradation (shearing or nuclease activity). A bright band of low-molecular-weight material may indicate RNA contamination. RNA can be removed by treating the sample with RNase A (10 µg/mL) at 37°C for 30 minutes, followed by re-extraction with phenol-chloroform.

## Advantages and Limitations Compared to Other Methods

### Advantages

Phenol-chloroform extraction offers several distinct advantages over commercial column-based kits:

1. **High molecular weight DNA.** The method yields DNA of very high molecular weight (50–200 kb), which is essential for applications such as [long-read sequencing](/knowledge/bioinformatics/long-read-sequencing-technologies-pacbio-and-oxford-nanopore) (PacBio, Oxford Nanopore), [pulsed-field gel electrophoresis](/knowledge/diagnostics/molecular/pulsed-field-gel-electrophoresis), and [genomic library](/blog/guides/genomic-library) construction. Column-based methods typically shear DNA to 20–50 kb.

2. **Cost-effectiveness.** The reagents are inexpensive and can be prepared in-house. For laboratories processing large numbers of samples, the cost per extraction is substantially lower than that of commercial kits.

3. **Scalability.** The method can be scaled from microgram quantities (1.5 mL tubes) to milligram quantities (15 mL or 50 mL tubes) without loss of quality.

4. **Removal of contaminants.** The method effectively removes proteins, polysaccharides, and lipids, including contaminants that often clog silica columns.

5. **No specialized equipment.** A standard microcentrifuge and a fume hood are the only major equipment requirements.

### Limitations

1. **Hazardous reagents.** Phenol and chloroform are toxic and require careful handling and proper waste disposal. This is a significant drawback for teaching laboratories and high-throughput settings.

2. **Time-consuming.** The multiple extraction and precipitation steps make the method labor-intensive. A typical extraction takes 2–4 hours, compared to 30–60 minutes for a column-based kit.

3. **Low throughput.** The method is difficult to automate, making it unsuitable for processing hundreds of samples simultaneously.

4. **Risk of contamination.** The multi-step procedure increases the risk of cross-contamination between samples. Careful technique and the use of aerosol-resistant tips are essential.

5. **Residual phenol.** Incomplete removal of phenol can inhibit downstream enzymes. The chloroform wash step is critical for eliminating this problem.

Compared to other organic extraction methods, such as the [Phenol Chloroform Method of DNA Extraction](/knowledge/molecular-biology/phenol-chloroform-method-of-dna-extraction) variants that use guanidinium thiocyanate for chaotropic lysis, the classic SDS-proteinase K method described here is gentler and yields larger DNA fragments. For applications requiring very high purity, such as [Phenol Extraction of Proteins](/knowledge/molecular-biology/phenol-extraction-of-proteins) for proteomics, the organic phase can be retained and processed separately.

## Common Pitfalls and Troubleshooting

### Low DNA Yield

**Symptom:** The final DNA concentration is lower than expected, or no pellet is visible after ethanol precipitation.

**Possible causes and solutions:**
- **Incomplete cell lysis.** Ensure that the lysis buffer contains fresh SDS and that proteinase K is active. Proteinase K should be stored at −20°C and protected from repeated freeze-thaw cycles.
- **DNA lost during phase transfer.** When removing the aqueous phase, leave a small volume (10–20 µL) behind to avoid disturbing the interphase. Do not attempt to collect every drop.
- **Incomplete precipitation.** Ethanol precipitation requires sufficient salt (0.1 volumes of 3 M sodium acetate) and cold temperature. Incubate at −20°C for at least 30 minutes, or overnight for dilute samples.
- **DNA pellet dislodged.** When removing the 70% ethanol wash, do not aspirate too vigorously. If the pellet is loose, centrifuge again for 5 minutes before removing the supernatant.
- **DNA not resuspended.** High-molecular-weight DNA can be difficult to dissolve. Incubate at 37°C for 15–30 minutes with gentle tapping. Do not vortex, as this will shear the DNA.

### Protein or RNA Contamination

**Symptom:** A260/A280 ratio below 1.8, or a smear of low-molecular-weight material on the agarose gel.

**Possible causes and solutions:**
- **Insufficient protein digestion.** Increase the proteinase K concentration to 200 µg/mL and extend the incubation time to 4 hours or overnight.
- **Insufficient organic extraction.** Perform an additional phenol-chloroform extraction step. If the interphase is thick, repeat the extraction until the interphase is clear.
- **RNA contamination.** Treat the sample with DNase-free RNase A (10 µg/mL) at 37°C for 30 minutes, then re-extract with phenol-chloroform and re-precipitate.
- **Phenol contamination.** Ensure that the chloroform-isoamyl alcohol wash step is performed. Residual phenol absorbs at 280 nm and depresses the A260/A280 ratio.

### Emulsion Formation

**Symptom:** A milky, cloudy layer forms between the aqueous and organic phases, and the phases do not separate cleanly after centrifugation.

**Possible causes and solutions:**
- **Excessive vortexing.** Over-vigorous mixing creates emulsions. Mix by inverting the tube 20–30 times rather than vortexing at high speed.
- **High protein or lipid content.** Samples with high lipid content (e.g., brain tissue, adipose tissue) are prone to emulsion formation. Increase the isoamyl alcohol concentration (e.g., use 25:24:1 with extra isoamyl alcohol) or centrifuge at higher speed (16,000 × g) for 15 minutes.
- **Insufficient centrifugation.** Increase the centrifugation time or speed. If the emulsion persists, centrifuge at 16,000 × g for 20 minutes at 4°C.
- **The sample is too viscous.** Dilute the lysate with additional lysis buffer before adding phenol-chloroform.

### DNA Shearing

**Symptom:** The DNA appears as a smear on the agarose gel rather than a distinct high-molecular-weight band.

**Possible causes and solutions:**
- **Vigorous mixing or pipetting.** Always mix by gentle inversion, not vortexing. Use wide-bore pipette tips when transferring DNA solutions.
- **Mechanical disruption during tissue homogenization.** Use a Dounce homogenizer with a loose-fitting pestle rather than a blender or sonicator.
- **Nuclease contamination.** Ensure that all buffers are nuclease-free and that EDTA is present in the lysis buffer and TE buffer. EDTA chelates Mg²⁺, which is required for DNase activity.
- **Repeated freeze-thaw cycles.** Store DNA in aliquots and avoid repeated freezing and thawing, which causes strand breakage.

## Practical Summary and Key Takeaways

Phenol-chloroform extraction is a robust, time-tested method for isolating high-quality DNA. The technique relies on the differential partitioning of biomolecules between an aqueous phase and an organic phase composed of phenol, chloroform, and isoamyl alcohol. Proteins are denatured and extracted into the organic phase, while DNA remains in the aqueous phase. The DNA is then recovered by ethanol precipitation.

The method is particularly valuable when high-molecular-weight DNA is required, such as for long-read sequencing or genomic library construction. It is also the method of choice when cost is a concern or when samples contain contaminants that clog silica columns. However, the use of hazardous organic solvents requires strict adherence to safety protocols, and the multi-step procedure demands careful technique to avoid contamination and DNA loss.

For a commercial alternative that avoids organic solvents, column-based kits offer speed and convenience at the cost of lower molecular weight and higher expense. The choice between methods depends on the specific requirements of the downstream application.

## Frequently Asked Questions

### Why is phenol chloroform used in DNA isolation?

Phenol-chloroform is used because it effectively separates proteins from nucleic acids based on differential solubility. Phenol denatures proteins and causes them to partition into the organic phase, while DNA remains in the aqueous phase due to its hydrophilic phosphate backbone. Chloroform enhances phase separation and removes residual phenol from the aqueous phase. This combination achieves efficient deproteinization of cell lysates, yielding DNA of high purity and high molecular weight.

### What is the role of isoamyl alcohol in phenol chloroform extraction?

Isoamyl alcohol is added to the chloroform at a ratio of 1:24 (v/v) to reduce foaming and to stabilize the interphase between the aqueous and organic phases. It acts as a surfactant that decreases surface tension at the interface, preventing the formation of emulsions that trap DNA and complicate phase separation. Without isoamyl alcohol, the extraction mixture would foam excessively during mixing, and the interphase would be less distinct, increasing the risk of cross-contamination between phases.

### How does phenol chloroform extraction separate DNA from proteins?

The separation is based on the hydrophobic effect. Phenol disrupts the hydrogen bonds and hydrophobic interactions that maintain protein tertiary structure, causing proteins to unfold. The unfolded proteins expose hydrophobic amino acid residues that interact favorably with the aromatic ring of phenol, causing them to partition into the organic phase. DNA, with its negatively charged, highly hydrophilic phosphate backbone, remains hydrated and dissolved in the aqueous phase. Upon centrifugation, the denser organic phase settles at the bottom, the aqueous phase forms the top layer, and denatured proteins accumulate at the interface.

### Why is the aqueous phase on top in phenol chloroform extraction?

The aqueous phase is on top because it is less dense than the organic phase. The phenol-chloroform mixture has a density of approximately 1.27 g/mL, while the aqueous phase has a density of approximately 1.0 g/mL. During centrifugation, the denser organic phase migrates to the bottom of the tube, and the less dense aqueous phase forms the upper layer. This density difference is the basis for the physical separation of the two phases.

### What is the purpose of ethanol precipitation in phenol chloroform DNA isolation?

Ethanol precipitation is used to concentrate the DNA and remove residual contaminants such as salts, phenol, and small organic molecules. The addition of monovalent cations (e.g., sodium acetate) neutralizes the negative charges on the DNA phosphate backbone, reducing electrostatic repulsion. Ethanol, being less polar than water, reduces the dielectric constant of the solution, allowing the DNA to aggregate and precipitate. The precipitated DNA is then collected by centrifugation, washed with 70% ethanol to remove residual salt, and resuspended in an appropriate buffer.

### How can I check the purity of DNA isolated by phenol chloroform?

Purity is assessed by UV spectrophotometry. Measure the absorbance at 260 nm (A260) and 280 nm (A280). The A260/A280 ratio should be between 1.8 and 2.0 for pure DNA. A ratio below 1.8 indicates protein or phenol contamination, while a ratio above 2.0 suggests RNA contamination. The A260/A230 ratio (expected 2.0–2.2) provides additional information; low values indicate contamination by chaotropic salts, carbohydrates, or residual organic solvents. DNA integrity is assessed by agarose gel electrophoresis, where high-quality DNA appears as a single high-molecular-weight band.

### What are common mistakes in phenol chloroform DNA isolation?

Common mistakes include: (1) disturbing the interphase when removing the aqueous phase, leading to protein contamination; (2) over-vortexing, which causes DNA shearing and emulsion formation; (3) insufficient proteinase K digestion, resulting in low yield and protein contamination; (4) incomplete removal of phenol, which inhibits downstream enzymes; (5) over-drying the DNA pellet, making it difficult to resuspend; and (6) using phenol that is not equilibrated to pH 8.0, which can cause DNA to partition into the organic phase. Careful technique and adherence to the protocol minimize these issues.

## Key Takeaways

- Phenol-chloroform extraction is a liquid-liquid partitioning method that separates DNA from proteins based on differential solubility in aqueous and organic phases.
- Phenol denatures proteins; chloroform enhances phase separation and removes residual phenol; isoamyl alcohol prevents emulsion formation.
- DNA remains in the aqueous phase (top layer) and is recovered by ethanol precipitation.
- The method yields high-molecular-weight DNA (50–200 kb) suitable for long-read sequencing and genomic library construction.
- Purity is assessed by A260/A280 ratio (1.8–2.0) and agarose gel electrophoresis.
- The method is inexpensive and scalable but requires handling of hazardous organic solvents and is more time-consuming than column-based kits.
- Key technical points: equilibrate phenol to pH 8.0, mix gently, avoid disturbing the interphase, and perform a chloroform wash to remove residual phenol.

## Further Reading

- Wang X et al. *[Extract human DNA from maggot crop contents by phenol-chloroform method coupled with paramagnetic particle method]*. Fa yi xue za zhi. 2009. [PubMed 20225616](https://pubmed.ncbi.nlm.nih.gov/20225616/)
- Köchl S, Niederstätter H, Parson W. *DNA extraction and quantitation of forensic samples using the phenol-chloroform method and real-time PCR*. Methods in molecular biology (Clifton, N.J.). 2005. [PubMed 15570097](https://doi.org/10.1385/1-59259-867-6:013)
- Chomczynski P, Sacchi N. *The single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction: twenty-something years on*. Nature protocols. 2006. [PubMed 17406285](https://doi.org/10.1038/nprot.2006.83)
- Bandehpour M et al. *Modified Phenol/Chloroform-Free DNA Isolation from Yeast and Other Fungi by Non-Enzymatic Salting Out Method*. Current protocols. 2023. [PubMed 37097192](https://doi.org/10.1002/cpz1.749)
- Das D, Avssn R, Chittela RK. *A phenol-chloroform free method for cfDNA isolation from cell conditioned media: development, optimization and comparative analysis*. Analytical biochemistry. 2024. [PubMed 38158107](https://doi.org/10.1016/j.ab.2023.115454)



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