Experiment 02Plasmid Biophysics • B.Sc. / M.Sc.
Isolation of Plasmid DNA by the Alkaline Lysis Method (Birnboim & Doly)
🎯 Aim: To isolate covalently closed circular (ccc) plasmid DNA from recombinant E. coli harboring pUC19 using the alkaline SDS lysis protocol and selectively separate supercoiled plasmid from chromosomal DNA and cellular proteins.
🔬 Theoretical Principle & Differential Denaturation
The alkaline lysis method (Birnboim & Doly, 1979) relies on the differential denaturation and renaturation properties of small covalently closed circular (ccc) plasmid DNA versus large linear chromosomal DNA fragments:
- Solution I (Resuspension Buffer): $50,text{mM}$ Glucose maintains isotonic osmotic pressure to prevent premature osmotic bursting; $25,text{mM}$ Tris-HCl ($text{pH } 8.0$) buffers the system; $10,text{mM}$ EDTA chelates $text{Mg}^{2+}$, destabilizing cell walls and inhibiting cellular DNases. RNase A digests bacterial RNA.
- Solution II (Alkaline Lysis Buffer): $0.2,text{N} text{NaOH}$ raises pH to $approx 12.0–12.5$. At this extreme alkaline threshold, all DNA base pairing breaks down, denaturing both plasmid and chromosomal DNA into single strands. $1%$ SDS lyses cells, releases contents, and denatures proteins. Crucially, because plasmid strands are covalently closed topological circles, their complementary single-stranded circular loops cannot physically separate.
- Solution III (Neutralization Buffer): $3,text{M}$ Potassium Acetate ($text{pH } 5.5$) rapidly neutralizes the alkali. Because small plasmid loops were never physically detached, their complementary bases re-align instantaneously and snap back into native double-stranded supercoiled topology. In contrast, large linear chromosomal fragments cannot find their distant complementary partners in time, entangling into a chaotic insoluble meshwork with potassium dodecyl sulfate (KDS) and cellular proteins.
Figure 2.1: Alkaline Lysis Mechanism (Plasmid vs Chromosomal DNA Fractionation)
📋 Step-by-Step Experimental Protocol
- Harvesting: Pellet $1.5,text{mL}$ of overnight LB culture of E. coli (pUC19) at $12,000,text{rpm}$ for 1 minute. Aspirate all supernatant.
- Resuspension (Sol I): Add $100,mutext{L}$ of ice-cold Solution I ($50,text{mM}$ glucose, $25,text{mM}$ Tris-HCl pH 8.0, $10,text{mM}$ EDTA, $100,mutext{g/mL}$ RNase A). Vortex vigorously until cell pellet is completely suspended.
- Lysis (Sol II): Add $200,mutext{L}$ of freshly prepared Solution II ($0.2,text{N} text{NaOH}, 1% text{SDS}$). Gently invert the tube 5–6 times. The suspension becomes clear and viscous. Do not vortex! Keep on ice for no more than 3–5 minutes.
- Neutralization (Sol III): Add $150,mutext{L}$ of ice-cold Solution III ($3,text{M}$ Potassium Acetate, $text{pH } 5.5$). Immediately invert the tube gently 6–8 times. A thick, white flocculant precipitate of KDS and chromosomal DNA forms instantly. Incubate on ice for 5 minutes.
- Clarification: Centrifuge at $13,000,text{rpm}$ for 10 minutes at $4^circtext{C}$. The white precipitate forms a tight pellet. Transfer the clear supernatant containing native plasmid DNA to a fresh tube.
- Precipitation & Wash: Add 2 volumes of absolute ethanol ($~800,mutext{L}$). Incubate at $-20^circtext{C}$ for 20 minutes. Centrifuge at $13,000,text{rpm}$ for 10 minutes. Wash pellet with $500,mutext{L}$ of $70%$ ethanol, dry, and dissolve in $50,mutext{L}$ sterile TE buffer ($text{pH } 8.0$).
💬 Viva-Voce Questions & Model Answers
Q1: What happens if Solution II incubation exceeds 5 minutes?
Prolonged exposure to $text{pH} > 12.5$ causes irreversible alkaline denaturation of plasmid DNA, converting it into permanently denatured single-stranded closed circles that cannot renature properly, appearing as an unwanted fast-migrating smear on agarose gels that cannot be cleaved by restriction enzymes.
Q2: Why must the tube be inverted gently rather than vortexed after adding Solution II?
Vortexing shears the high-molecular-weight chromosomal DNA into small linear fragments. These small sheared fragments renature quickly alongside the plasmid and remain soluble in the supernatant, contaminating the isolated plasmid preparation.
Q3: Why is potassium acetate used instead of sodium acetate in Solution III?
Potassium ions ($text{K}^+$) react with dodecyl sulfate anions ($text{DS}^-$) from SDS to form potassium dodecyl sulfate (KDS), which is insoluble in cold aqueous solutions and precipitates out as a dense white solid, trapping denatured proteins and chromosomal DNA within the clot.