Methods for Mitochondrial DNA Extraction

The core principle of mitochondrial DNA (mtDNA) extraction is: isolate intact and pure mitochondria first, then lyse mitochondria to extract DNA. Different from nuclear genomic DNA extraction, mtDNA extraction requires milder conditions to maintain mitochondrial structural integrity.

Below are several major extraction methods, ranging from classic laboratory protocols to commercial kit‑based workflows:

Method 1: Classical Stepwise Centrifugation (Differential Centrifugation + Density‑Gradient Centrifugation)

This is the most traditional method with the highest purity, suitable for research requiring high‑quality mtDNA (e.g., next‑generation sequencing).

Basic Workflow:

1. Tissue or Cell Disruption:

• Materials: Fresh or frozen tissues (e.g., liver, muscle) or large‑scale cultured cells (e.g., HeLa cells).
• Buffer: Use homogenization buffer (containing sucrose or mannitol for isotonic maintenance to prevent mitochondrial swelling and rupture; EDTA for metal ion chelation; Tris‑HCl for pH stabilization; BSA is sometimes added to protect mitochondrial membranes).
• Procedure: Perform all steps on ice; gently disrupt cells using a glass homogenizer or appropriately fitted electric homogenizer to release organelles.

2. Differential Centrifugation for Debris Removal:

• Low‑speed centrifugation (~600 ×g, 4°C, 10 min): Remove unbroken cells, nuclei, and large membrane fragments. The supernatant contains mitochondria, microsomes, etc.
• High‑speed centrifugation (~11,000 ×g, 4°C, 10 min): Pellet mitochondria. Discard the supernatant containing ribosomes and soluble proteins.
• Washing: Resuspend the mitochondrial pellet in homogenization buffer and repeat high‑speed centrifugation 1–2 times to remove contaminants.

3. Density‑Gradient Centrifugation (for further purity improvement):

• Layer crude mitochondrial pellets onto Percoll or sucrose density‑gradient solution.
• After ultracentrifugation, mitochondria are enriched at a specific density band (~1.19 g/cm³), while lysosomes, peroxisomes and other organelles localize to other bands.
• Collect the mitochondrial band, dilute and centrifuge to obtain highly pure mitochondria.

4. Mitochondrial Lysis and DNA Extraction:

• Resuspend purified mitochondrial pellets in mitochondrial lysis buffer (containing SDS and Proteinase K) to thoroughly lyse mitochondrial membranes.
• Subsequently remove proteins, RNA and other contaminants via standard phenol‑chloroform extraction or silica column purification, and precipitate or elute mtDNA.
• RNase treatment is essential to remove abundant mitochondrial RNA.

Method 2: Kit‑Based Extraction (Most Commonly Used and Convenient)

Most laboratories currently use commercial kits based on modified differential centrifugation and column purification principles for simplified operation.

Operating Steps

Preparation: Add 600 μL (for 50‑prep kit) or 1100 μL (for 100‑prep kit) DNase reaction buffer to DNase I, aliquot appropriately and store at −20°C. Store mitochondrial lysis buffer at room temperature; dissolve precipitates in a 37°C water bath if present. Cool the centrifuge to 4°C (2–8°C). If a refrigerated centrifuge is unavailable, room‑temperature centrifugation is acceptable with 10‑min steps shortened to 5 min, though DNA quality and yield may be slightly compromised.

1. Sample Preparation
   • Tissue homogenization: Weigh 100–200 mg fresh tissue (e.g., liver, brain, myocardium), rinse with PBS or normal saline to remove blood, blot dry with filter paper, mince into small pieces and transfer to a small glass homogenizer. Add 1.0 mL ice‑cold cell lysis buffer and grind 20 times in an ice bath at 0°C;
   • Cultured cell homogenization: Digest cells, wash with PBS, collect cells by centrifugation at 800×g for 5–10 min and count. Each extraction requires 5×10⁷ cells. Resuspend cells in 1.0 mL ice‑cold cell lysis buffer, transfer to a small glass homogenizer and grind 30–40 times in an ice bath at 0°C;
2. Transfer tissue or cell homogenate to a centrifuge tube and centrifuge at 1000×g for 5 min at 4°C;
3. Transfer the supernatant to a new tube and centrifuge again at 1000×g for 5 min at 4°C;
4. Transfer the supernatant to a new tube and centrifuge at 12,000×g for 10 min at 4°C. The supernatant contains cytosolic components for cytosolic protein extraction. Transfer the supernatant to a new tube; mitochondria form a pellet at the bottom;
5. Resuspend the mitochondrial pellet in 5 mL mitochondrial washing buffer and centrifuge at 1000×g for 5 min at 4°C;
6. Transfer the supernatant to a new tube and centrifuge at 12,000×g for 10 min at 4°C. Discard the supernatant; highly pure mitochondria pellet at the bottom;
7. Resuspend mitochondria in 100 μL DNase reaction buffer, mix well, add 10 μL DNase I solution (see Preparation), incubate at 37°C for 10 min to digest nuclear DNA adsorbed on the mitochondrial surface. Centrifuge at 12,000×g for 5 min at 4°C. Discard the supernatant as completely as possible, resuspend the pellet in 200 μL TE buffer, and centrifuge at 12,000×g for 5 min to remove residual DNase;
8. Resuspend the obtained pellet in 200 μL TE buffer. Add 10 μL RNase A and 200 μL mitochondrial lysis buffer, mix gently (do not pipette vigorously), incubate for 1–2 min (no more than 5 min), then add 150 μL protein precipitation solution and mix rapidly. Centrifuge at 12,000×g for 5 min at 4°C. This step further removes nuclear DNA;
9. Transfer the supernatant to a new tube (for restriction digestion analysis: perform one extraction with equal‑volume phenol‑chloroform‑isoamyl alcohol (25:24:1), followed by two chloroform extractions; this step removes trace proteins and saccharides but causes mtDNA loss and may be omitted for PCR applications). Add 6‑fold volume isopropanol (or 2.5‑fold volume ethanol if isopropanol is unavailable; split into two tubes if overfilled) plus 5–10 μL nucleic acid coprecipitant, mix well, precipitate at −20°C for ~30 min (optional), then centrifuge at 12,000×g for 10 min at 4°C;
10. Discard the supernatant, wash with 1 mL 70% ethanol, and centrifuge at 12,000×g for 10 min at 4°C. Repeat 70% ethanol washing once;
11. Discard the supernatant, centrifuge for another 1 min and remove residual supernatant without touching the pellet, then air‑dry with the lid open for 5–10 min;
12. Add 20–30 μL TE buffer, flick the tube gently, and incubate at 37°C for 5 min to dissolve mtDNA;
13. Perform DNA agarose gel electrophoresis and store at −20°C for downstream experiments.

Method 3: Alkaline Lysis (Rapid, for PCR‑Based Applications)

Suitable for rapid mtDNA detection with low purity requirements (e.g., genotyping, deletion screening).

Brief Workflow

1. Directly lyse cells or tissues using alkaline lysis buffer (e.g., NaOH‑SDS).
2. Precipitate proteins and chromosomal DNA with high‑salt solution.
3. Small‑sized mtDNA remains in the supernatant (covalently closed circular form rapidly renatures after alkaline denaturation) and is precipitated using isopropanol or ethanol.
4. Wash the pellet with 70% ethanol and dissolve.

Key Notes & Optimization Suggestions

1. Sample Freshness: Starting material must be fresh or rapidly frozen at −80°C. Mitochondria are highly vulnerable in apoptotic or necrotic cells, leading to mtDNA release and degradation.
2. Low‑Temperature Operation Throughout: Except for lysis and incubation steps, perform all procedures on ice using pre‑cooled buffers and centrifuges to inhibit nuclease activity.
3. Avoid Mechanical Damage: Use gentle homogenization or vortexing; excessive shear force disrupts mitochondria.
4. Elimination of Nuclear DNA Contamination:
   • DNase I Treatment: The most effective method to remove nuclear contamination.
   • Long‑Fragment PCR Validation: Amplify a long mtDNA fragment (>8 kb) using specific primers. Severe nuclear contamination results in failed long‑fragment amplification (nuclear DNA is easily sheared during extraction).
5. Elimination of RNA Contamination: Perform thorough digestion using DNase‑free RNase A.
6. Concentration and Integrity Detection:
   • Agarose Gel Electrophoresis: Pure animal mtDNA shows a distinct band at ~16.5 kb. Smearing or small fragments indicate degradation; higher‑molecular‑weight bands suggest nuclear DNA contamination.
   • UV Spectrophotometry: Measure A260/A280 (~1.8) and A260/A230 (>2.0) ratios to evaluate protein and salt contamination.
   • qPCR: Quantify with nuclear‑ and mtDNA‑specific primers to precisely calculate nuclear DNA contamination ratio.

Summary & Selection Recommendations

Absin Mitochondrial DNA Isolation Kit Recommendation

Contact Absin

Absin provides antibodies, proteins, ELISA kits, cell culture, detection kits, and other research reagents. If you have any product needs, please contact us.

Absin Bioscience Inc. worldwide@absin.cn

Follow us on Facebook: Absin Bio