Extracting high-quality genomic DNA from cells is the starting point for PCR, cloning, sequencing, genotyping, and Southern blotting. While the basic chemistry is simpler than RNA extraction — DNA is far more stable — poor technique still produces sheared, contaminated, or low-yield DNA that fails in downstream applications. This guide covers the three major approaches to DNA extraction from cultured cells and tissue, with specific protocols you can follow at the bench.

Factors That Determine DNA Extraction Success

Before choosing a method, consider what you need:

Fragment size: Standard PCR and sequencing tolerate sheared DNA. Long-read sequencing (PacBio, Oxford Nanopore) requires high-molecular-weight DNA (> 50 kb). Southern blotting needs intact, non-degraded genomic DNA.
Purity: A260/A280 of 1.7–1.9 is typical for pure DNA. Enzymatic applications (restriction digestion, ligation) are sensitive to protein and salt contamination.
Yield: A confluent T-75 flask of HeLa cells (~10⁷ cells) typically yields 30–60 µg genomic DNA.
Throughput: Processing 96 samples demands a different approach than processing 6.

Method 1: Spin Column Kit (Recommended for Most Applications)

Column-based kits like the DNeasy Blood & Tissue Kit (Qiagen, Cat# 69504) or the PureLink Genomic DNA Mini Kit (Thermo Fisher, Cat# K182001) offer the best balance of speed, purity, and reproducibility.

Protocol: DNeasy Blood & Tissue Kit

Starting Material

Up to 5 × 10⁶ cultured cells or 25 mg tissue.

Step-by-Step

Cell harvest: Trypsinize adherent cells, pellet at 300 × g for 5 minutes. Remove all media. For suspension cells, pellet directly.
Lysis: Resuspend the cell pellet in 200 µL PBS. Add 20 µL Proteinase K (supplied with kit, ~600 mAU/mL) and 200 µL Buffer AL (lysis buffer). Vortex thoroughly for 15 seconds.
Incubation: Place at 56°C for 10 minutes. For tissue, extend to 1–3 hours or until completely dissolved. Vortex occasionally.
Ethanol addition: Add 200 µL of 96–100% ethanol. Vortex immediately. A wispy precipitate may form — this is normal.
Column binding: Transfer the entire mixture (~620 µL) to a DNeasy spin column in a 2 mL collection tube. Centrifuge at ≥ 6,000 × g for 1 minute. Discard flow-through and collection tube.
Wash 1: Place column in a new collection tube. Add 500 µL Buffer AW1. Centrifuge at ≥ 6,000 × g for 1 minute. Discard flow-through.
Wash 2: Add 500 µL Buffer AW2. Centrifuge at 20,000 × g for 3 minutes to dry the membrane. This high-speed spin is critical — residual ethanol inhibits PCR and restriction enzymes.
Elution: Transfer column to a clean 1.5 mL tube. Add 100–200 µL Buffer AE (10 mM Tris-Cl, 0.5 mM EDTA, pH 9.0) directly to the membrane. Incubate 1 minute at room temperature. Centrifuge at ≥ 6,000 × g for 1 minute.

For maximum yield, perform a second elution with another 100 µL. For maximum concentration, elute once with 50 µL (expect ~70% recovery vs. 95% with 200 µL).

Expected Results

| Parameter | Typical Value | |-----------|--------------| | Yield | 4–10 µg per 10⁶ cells | | A260/A280 | 1.7–1.9 | | Fragment size | 20–50 kb | | Time | 30–45 minutes |

Method 2: Phenol-Chloroform Extraction

The traditional method. Still used when maximum DNA yield or very high molecular weight is required (e.g., long-read sequencing, BAC library construction).

Protocol

Lysis: Resuspend up to 10⁷ cells in 500 µL DNA Lysis Buffer (10 mM Tris-HCl pH 8.0, 100 mM NaCl, 10 mM EDTA, 0.5% SDS). Add 10 µL Proteinase K (20 mg/mL stock, e.g., Thermo Fisher Cat# EO0491). Mix gently by inverting — do not vortex.
Digestion: Incubate at 55°C for 3 hours to overnight. For tissue, overnight digestion with periodic gentle mixing gives best results.
RNase treatment (optional): Add 5 µL RNase A (10 mg/mL, DNase-free) and incubate at 37°C for 30 minutes.
Phenol-chloroform extraction: Add 500 µL phenol:chloroform:isoamyl alcohol (25:24:1, pH 8.0, e.g., Sigma Cat# P2069). Invert gently 20–30 times. Do not vortex — mechanical shearing fragments DNA. Centrifuge at 12,000 × g for 10 minutes at room temperature. Carefully transfer the upper aqueous phase to a fresh tube.
Chloroform wash: Add 500 µL chloroform:isoamyl alcohol (24:1). Invert 10 times. Centrifuge 5 minutes. Transfer aqueous phase.
Ethanol precipitation: Add 1/10 volume (50 µL) of 3 M sodium acetate pH 5.2 and 2.5 volumes (1.25 mL) of ice-cold 100% ethanol. For high-molecular-weight DNA, you should see a stringy white precipitate you can spool on a glass rod or pipette tip.
Wash: Centrifuge at 12,000 × g for 10 minutes at 4°C. Wash pellet with 1 mL 70% ethanol. Centrifuge 5 minutes.
Resuspend: Air-dry 5 minutes. Resuspend in 50–200 µL TE buffer (10 mM Tris-HCl pH 8.0, 1 mM EDTA) or nuclease-free water. Allow to dissolve at 4°C overnight for high-molecular-weight DNA — aggressive pipetting will shear it.

When to Use This Method

You need fragments > 100 kb (long-read sequencing, optical mapping)
You need maximum yield from limited samples
Column capacity is insufficient for your input
Budget constraints preclude kits

Downsides

Uses hazardous organic solvents (fume hood required)
More hands-on time
Phenol carryover inhibits many enzymes

Method 3: Salting-Out (Non-Organic) Extraction

A cost-effective, phenol-free alternative popular in genetics and biobanking labs processing large numbers of blood or cell samples.

Protocol

Lysis: Resuspend cell pellet in 3 mL Nuclei Lysis Buffer (10 mM Tris-HCl pH 8.2, 400 mM NaCl, 2 mM EDTA). Add 200 µL 10% SDS and 500 µL Proteinase K (1 mg/mL). Incubate overnight at 37°C.
Salt precipitation: Add 1 mL saturated NaCl (approximately 6 M). Shake vigorously for 15 seconds. Centrifuge at 3,500 × g for 15 minutes at room temperature. Proteins and SDS precipitate as a white pellet.
DNA precipitation: Transfer supernatant to a fresh tube. Add 2 volumes of room-temperature 100% ethanol. Invert gently — DNA precipitates as visible strands. Spool with a pipette tip or glass rod.
Wash and resuspend: Wash in 70% ethanol. Air-dry briefly. Resuspend in TE buffer.

This method yields DNA of comparable quality to phenol-chloroform extraction at a fraction of the cost and without hazardous solvents.

DNA Extraction from Specific Cell and Tissue Types

Adherent Cultured Cells

Trypsinize with 0.05% trypsin-EDTA at 37°C for 3–5 minutes. Neutralize with serum-containing media. Count cells if possible — knowing your input helps troubleshoot yield issues.

Suspension Cells

Pellet directly from culture. Be aware that some suspension cell lines (e.g., Jurkat, K562) have different ploidy, which affects expected yield per cell.

Fresh/Frozen Tissue

Mince tissue finely with a sterile scalpel on a cold surface. For column kits, do not exceed 25 mg — overloading clogs the membrane. For difficult tissues (skin, muscle), extend Proteinase K digestion overnight. For frozen tissue, do not thaw before adding lysis buffer — cut while frozen to minimize nuclease activity.

Buccal Swabs and Saliva

Low cell counts require sensitive protocols. Use the Oragene DNA kit (DNA Genotek) for saliva. For buccal swabs, process the same day or store dry at −20°C. Typical yield: 1–10 µg from a saliva sample, 0.5–3 µg from a buccal swab.

Troubleshooting DNA Extraction Issues

Low Yield

Insufficient lysis: Extend Proteinase K incubation time or increase enzyme concentration.
Overloaded column: Column membranes have binding capacity limits (typically 20–30 µg for mini columns). Reduce input or use a midi/maxi prep format.
Elution problems: Pre-warm elution buffer to 56°C. Use a lower-volume elution and re-elute.

Low A260/A280 (< 1.7)

Protein contamination: Incomplete Proteinase K digestion. Add more enzyme and incubate longer.
Phenol carryover: Perform an additional chloroform-only extraction.

Sheared DNA

Mechanical shearing: Never vortex genomic DNA. Use wide-bore pipette tips for high-molecular-weight applications. Avoid repeated freeze-thaw cycles.
Nuclease degradation: Ensure sufficient EDTA in lysis buffer to chelate Mg²⁺ and inhibit nucleases.

RNA Contamination

Visible as a low-molecular-weight smear on an agarose gel. Treat with 10 µg/mL RNase A at 37°C for 30 minutes. Most column kits include RNase A in the lysis buffer for this reason.

DNA Won't Dissolve

High-molecular-weight DNA is viscous and slow to dissolve. Incubate at 4°C overnight rather than pipetting aggressively. Gentle heating at 55°C for 10–15 minutes also helps.

Storing Extracted DNA

Short-term (weeks): 4°C in TE buffer or nuclease-free water.
Long-term (years): −20°C or −80°C in TE buffer. Avoid nuclease-free water for long-term storage — the slight buffering capacity of TE prevents acid hydrolysis.
Avoid repeated freeze-thaw: Aliquot into working stocks.
Quantify before storing: Use a Qubit fluorometer (Thermo Fisher) for accurate dsDNA quantification — NanoDrop overestimates when RNA or free nucleotides are present.

How LabProtocol.co Can Help

Every DNA extraction protocol requires adjustments based on your cell type, starting material amount, and downstream application. LabProtocol.co generates tailored protocols that account for these variables automatically, so you spend less time adapting published methods and more time running experiments. Get started free and generate a protocol customized to your exact setup.

Summary

For most routine applications, spin column kits offer the best speed-to-quality ratio.
Phenol-chloroform extraction remains the method of choice when fragment size or maximum yield matters most.
Salting-out is underrated — it produces excellent DNA at minimal cost.
Proteinase K digestion time is the single most impactful variable across all methods.
Quantify DNA with a Qubit, not just a NanoDrop, especially before committing to expensive downstream workflows.