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Alkaline Lysis Plasmid Miniprep (Birnboim & Doly method)

Classic chemical miniprep for plasmid DNA from small-volume bacterial cultures, using alkaline lysis and differential precipitation. The foundational open-source alternative to commercial spin-column kits (QIAprep, Thermo GeneJET, Monarch). Replicates the input plasmid into working stocks so future reagent-production batches don't burn through the single Addgene shipment. Written for a novice audience; requires careful pipetting but no specialised equipment beyond a benchtop microcentrifuge.

sample_prep

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Effective: 2026-04-20

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Initial release. Atomic technique for the open-reagent production stack. Content from standard molecular biology practice (Birnboim & Doly 1979 plus ~45 years of field refinement). Fresh original prose; foundational references cited.

Procedure Details

Safety & Hazards

PC1 / CL1 containment required for GMO strains.
NaOH (alkaline lysis buffer). 0.2 M solution is moderately caustic. Gloves and eye protection; wash spills immediately with water.
SDS. Low acute toxicity but respiratory irritant as powder. Handle dry SDS only in a fume hood or biosafety cabinet. Solutions are safe with standard gloves.
Isopropanol / ethanol. Flammable; keep away from heat sources. Low toxicity; gloves standard.
Plasmid DNA. Once prepared, treat as biological material only in the context of what it encodes — antibiotic-resistance plasmids are not hazardous but should be discarded via autoclaved biological waste.
Autoclave bacterial waste. All pre-lysis bacterial material must be autoclaved before disposal even though the extracted DNA is no longer a GMO risk.

Preparation Notes

Buffer preparation (once, then stable for months):

P1 resuspension buffer — 50 mM Tris-HCl pH 8.0 + 10 mM EDTA. Store 4°C. Add RNase A (100 µg/mL final) just before use or store with RNase A for active use.
P2 lysis buffer — 0.2 M NaOH + 1% SDS. Prepare fresh monthly. NaOH stock degrades; SDS precipitates in cold.
P3 neutralisation buffer — 3 M potassium acetate, pH 5.5 (adjust with glacial acetic acid). Store at 4°C; precipitates in cold storage — warm to RT before use.
TE elution buffer — 10 mM Tris-HCl pH 8.0 + 1 mM EDTA. Or use nuclease-free water for elution (compatible with most downstream uses).

Other items:

Saturated overnight culture (3–5 mL) of the plasmid-carrying strain in selective media.
Isopropanol, 100% (for precipitation).
70% ethanol (for wash).
Sterile 1.5 mL microcentrifuge tubes.
Ice bucket.

Mental model: alkaline pH + SDS breaks open cells and denatures all nucleic acids and proteins. Potassium acetate neutralises, dropping pH rapidly — genomic DNA and proteins (large or precipitated) crash out, while circular plasmid DNA (small and compact) re-anneals and stays in solution. Centrifugation clears the debris; isopropanol precipitates the remaining plasmid DNA from the supernatant.

Expected yield: 2–10 µg of plasmid DNA per 3–5 mL saturated culture, depending on plasmid copy number and host strain.

Timing

Overnight (hands-off): grow saturated culture.
Procedure (30–45 min, hands-on): harvest → lyse → neutralise → precipitate → wash → elute.
Per sample: 1–12 samples in parallel straightforward; >12 benefits from a 96-well plate format (not covered in this procedure).

Equipment (Catalog) 6

Microcentrifuge Centrifugation
Specs: 13,000 × g, room temperature and 4°C
Standard benchtop
Micropipette Liquid handling
Specs: P20, P200, P1000
Filter tips
Vortex mixer Mixing
Specs: Standard benchtop
For P1 resuspension only — never for P2/P3 steps
Biosafety cabinet Safety
Specs: Class II Type A2 or equivalent
All culture handling
Freezer (−20 °C) Storage
Specs: Standard lab freezer
For plasmid storage
Shaking incubator Thermal regulation
Specs: 37°C, 225 rpm, 5 mL tube position
For overnight growth

Materials (Catalog) 7

E. coli BL21(DE3) (NEB) C2527I Bacterial strain
Qty: 3 mL overnight culture
Any plasmid-carrying strain; high-copy hosts (DH5α, XL1-Blue) give better yields
Ampicillin Antibiotic
Qty: 100 µg/mL final
Match the plasmid's selection marker
Buffer Reagent
Qty: 250 µL
50 mM Tris-HCl pH 8.0 + 10 mM EDTA + 100 µg/mL RNase A
Buffer Reagent
Qty: 250 µL
0.2 M NaOH + 1% SDS; prepare fresh monthly
Buffer Reagent
Qty: 350 µL
3 M potassium acetate pH 5.5
Buffer Reagent
Qty: 50 µL
10 mM Tris-HCl pH 8.0 + 1 mM EDTA; or nuclease-free water
Superior Broth (Athena Enzyme Systems) 0105 Growth media
Qty: 3 mL
LB broth equivalent acceptable

Protocol Parameters Captured per-assay on each run; exported as ISA-Tab Parameter Value columns

Name	Type	Required	Default	Unit	Description
`culture_volume_ml`	number	—	`3`	milliliter (UO:0000098)	Volume of saturated culture to prep from. 3 mL standard for high-copy plasmids; scale up to 10–15 mL for low-copy.
`p1_volume_ul`	number	—	`250`	microliter (UO:0000101)	P1 resuspension buffer volume. 250 µL standard for 3 mL culture.
`p2_volume_ul`	number	—	`250`	microliter (UO:0000101)	P2 lysis buffer volume. 1:1 ratio with P1.
`p2_exposure_time_min`	number	—	`3`	minute (UO:0000031)	Time in alkaline lysis before P3 neutralisation. 2–5 min typical; exceed 5 min risks plasmid damage.
`isopropanol_volume_ul`	number	—	`600`	microliter (UO:0000101)	Isopropanol volume for DNA precipitation. 0.7× supernatant volume standard.
`elution_volume_ul`	number	—	`40`	microliter (UO:0000101)	Final elution volume. 30–50 µL typical; adjust based on desired final concentration.

Procedure Steps (Version 0.1.0)

Grow a 3–5 mL saturated overnight culture of your plasmid-carrying strain in selective media (37°C, 225 rpm, 14–16 h).

Pipette 1.5 mL of the culture into a 1.5 mL microcentrifuge tube. Centrifuge at 13,000 × g for 1 minute at room temperature. Discard supernatant into autoclavable waste.

Add another 1.5 mL of culture to the same tube. Centrifuge again; discard supernatant. You now have the combined pellet from 3 mL of culture.

Resuspend the pellet in 250 µL of cold P1 buffer (with RNase A). Vortex briefly or pipette up and down until the pellet is fully dispersed — no clumps visible. This step must be thorough.

Add 250 µL of P2 lysis buffer. Invert the tube 5–6 times gently (do NOT vortex — vortexing shears genomic DNA and contaminates the prep). The solution should become clear and viscous within 30 seconds. Do not exceed 5 minutes in lysis buffer — prolonged alkaline exposure damages the plasmid.

Add 350 µL of cold P3 neutralisation buffer. Invert 5–6 times gently. A white precipitate will form immediately — this is denatured protein + genomic DNA + SDS complexes.

Centrifuge at 13,000 × g for 10 minutes at room temperature.

Transfer the clear supernatant (~800 µL) to a new 1.5 mL microcentrifuge tube, avoiding the pellet and any floating debris. Use a P1000 pipette with a steady hand.

Add 600 µL of 100% isopropanol. Invert 5–6 times to mix.

Centrifuge at 13,000 × g for 10 minutes at 4°C (or room temperature if no refrigerated centrifuge available). A small white pellet of plasmid DNA should be visible at the bottom of the tube.

Discard the supernatant carefully — the DNA pellet may come loose. Drain the tube upside down on a paper towel for 30 seconds.

Add 500 µL of 70% ethanol to wash the pellet. Do not resuspend — just add the ethanol and invert gently once.

Centrifuge at 13,000 × g for 5 minutes.

Discard the ethanol. Spin the tube briefly (30 s at 13,000 × g) to collect residual ethanol, and remove it with a P20 pipette tip.

Air-dry the pellet for 5–10 minutes at room temperature with the tube open (watch for pellet becoming transparent — that's fully dry). Over-drying makes DNA hard to re-dissolve; under-drying leaves ethanol that inhibits downstream enzymes.

Resuspend the pellet in 30–50 µL of TE buffer or nuclease-free water. Pipette up and down several times; vortex briefly. Incubate at room temperature for 10 minutes to fully dissolve.

Measure DNA concentration on a Nanodrop or Qubit. Record the concentration and 260/280 ratio as a Sample annotation in LibreBiotech.

Store at −20°C for medium-term (months to years) or −80°C for indefinite storage. Label the tube with plasmid identifier + prep date + concentration.

Completion Notes

Expected outcome. 30–50 µL of plasmid DNA in TE buffer at concentration 50–500 ng/µL, OD260/OD280 ratio of 1.8–2.0 (indicates clean DNA). Typical yield for a pUC-derived high-copy plasmid: 5–10 µg from 3 mL culture. Low-copy plasmids (pSC101, BAC): 0.5–2 µg — may need larger starting culture.

QC checks.

Nanodrop. 260/280 ratio should be 1.8–2.0. Lower indicates protein contamination; higher indicates RNA contamination (add more RNase A).
Agarose gel. Run 2 µL of the prep on a 1% agarose gel with ladder. Expected: one or two clean bands (supercoiled + relaxed forms of the plasmid). Smear below indicates RNA carry-over; band above expected size indicates genomic DNA contamination.
Restriction digest (optional but recommended). Cut 200 ng with a known single-cutter; run on agarose gel. Expected band size confirms plasmid identity.

Storage. −20°C for years; −80°C for decades. Multiple freeze-thaws are fine for DNA (unlike RNA or protein).

Use as transformation input. 1–10 ng per transformation is typical — see procedure 60 (Heat-shock Transformation).

Troubleshooting.

Symptom	Likely cause	Fix
No DNA pellet visible after isopropanol precipitation	Insufficient cells / plasmid; pipetting error	Verify overnight culture OD600 >2; redo with larger starting volume
White/fluffy precipitate instead of clean pellet	Salt carryover from P3	Add more isopropanol; longer centrifugation; consider column purification as follow-up
Low yield (<1 µg)	Low-copy plasmid; strain without selection pressure	Use DH5α / XL1-Blue (higher copy); verify antibiotic was fresh
High OD260/OD280 ratio (>2.0)	RNA contamination	Add RNase A to P1 buffer; longer resuspension
Low OD260/OD280 ratio (<1.7)	Protein contamination	Repeat with cleaner P3 precipitation step; longer centrifugation
Cloudy / brown pellet	Phenolic compounds from stressed culture	Fresher culture; culture shorter overnight (14 h instead of 18 h)

References

Birnboim HC, Doly J (1979). A rapid alkaline extraction procedure for screening recombinant plasmid DNA. Nucleic Acids Res 7(6):1513–23. (Original method paper). DOI paper
Sambrook J, Russell DW (2001). Molecular Cloning: A Laboratory Manual, 3rd edition. Cold Spring Harbor Laboratory Press. (Standard reference for alkaline lysis). book
LibreBiotech procedure 60 — Heat-shock Transformation (downstream use of miniprep DNA). Link protocol