Experiment 01Genomic Extraction • B.Sc. / M.Sc.
Isolation and Purification of High Molecular Weight Genomic DNA from Bacteria
🎯 Aim: To isolate intact, high molecular weight genomic DNA from overnight cultures of Escherichia coli using enzymatic cell-wall digestion, SDS-Proteinase K lysis, organic solvent extraction (Phenol:Chloroform:Isoamyl alcohol), and cold ethanol spooling.
🔬 Theoretical Principle & Biochemical Fractionation
Bacterial genomic DNA is a high-molecular-weight circular chromosome ($~4.6,text{Mb}$ in E. coli) packaged within the bacterial nucleoid. Recovery of intact, unfragmented DNA requires four sequential biochemical steps:
- Enzymatic Cell Wall Lysis: Lysozyme (muramidase) endo-hydrolyzes the $beta(1to 4)$-glycosidic bonds between $N$-acetylmuramic acid (NAM) and $N$-acetylglucosamine (NAG) in the peptidoglycan wall. EDTA chelates divalent magnesium ($text{Mg}^{2+}$) and calcium ($text{Ca}^{2+}$) ions, destabilizing outer membrane lipopolysaccharides (LPS) and inactivating endogenous cellular deoxyribonucleases (DNases).
- Membrane Solubilization & Proteolysis: The ionic detergent sodium dodecyl sulfate (SDS, $10%$) dissolves phospholipid bilayers and denatures cellular proteins. Proteinase K, a broad-spectrum serine protease active in SDS and EDTA, digests nucleoid-associated histonelike proteins (HU, H-NS) and cellular nucleases.
- Organic Phase Extraction: Phenol:Chloroform:Isoamyl Alcohol ($25:24:1 text{v/v/v}$) at $text{pH } 8.0$ denatures proteins completely. Hydrophobic protein cores unfold and partition into the organic bottom layer or precipitate as a dense white disc at the interphase. Highly charged polar DNA molecules remain exclusively in the upper aqueous phase. Isoamyl alcohol prevents foaming.
- Ethanol Precipitation: Addition of sodium acetate ($0.3,text{M}, text{pH } 5.2$) neutralizes the negative charges on the phosphodiester backbone. Two volumes of ice-cold absolute ethanol drastically lower the dielectric constant of the solvent, causing high-molecular-weight DNA to collapse into visible white fibers that can be spooled onto a glass rod.
Figure 1.1: Bacterial Genomic DNA Isolation & Phenol-Chloroform Phase Separation Mechanics
🧪 Reagents, Materials & Solution Recipes
Reagents & Solutions
- TE Buffer: $10,text{mM}$ Tris-HCl, $1,text{mM}$ EDTA, $text{pH } 8.0$.
- Lysozyme Solution: $10,text{mg/mL}$ in $10,text{mM}$ Tris-HCl, $text{pH } 8.0$ (fresh).
- 10% (w/v) Sodium Dodecyl Sulfate (SDS) stock.
- Proteinase K: $20,text{mg/mL}$ dissolved in sterile deionized water.
- PCI (25:24:1): Phenol:Chloroform:Isoamyl alcohol equilibrated to $text{pH } 8.0$.
- 3 M Sodium Acetate (pH 5.2), chilled absolute ethanol ($100%$), and $70%$ ethanol.
Equipment & Consumables
- Overnight culture of E. coli DH5α in Luria-Bertani (LB) broth
- Refrigerated microcentrifuge ($4^circtext{C}$, up to $14,000,text{rpm}$)
- Water bath set to $37^circtext{C}$ and $55^circtext{C}$
- $1.5,text{mL}$ microcentrifuge tubes, sterile aerosol micropipette tips, glass spooling hooks
📋 Step-by-Step Experimental Protocol
- Cell Harvesting: Transfer $1.5,text{mL}$ of overnight E. coli culture into a microcentrifuge tube. Centrifuge at $8,000,text{rpm}$ for 5 minutes at $4^circtext{C}$. Discard the culture supernatant and drain the pellet.
- Enzymatic Digestion: Resuspend the bacterial pellet in $500,mutext{L}$ of TE buffer by gentle pipetting. Add $50,mutext{L}$ of lysozyme ($10,text{mg/mL}$) and mix gently by inversion. Incubate at $37^circtext{C}$ for 30 minutes.
- Lysis & Proteolysis: Add $30,mutext{L}$ of $10%$ SDS followed by $5,mutext{L}$ of Proteinase K ($20,text{mg/mL}$). Mix thoroughly by gentle inversion until the solution clears and becomes viscous. Incubate at $55^circtext{C}$ for 45 minutes.
- Phenol Extraction: In a fume hood, add an equal volume ($500,mutext{L}$) of PCI ($25:24:1, text{pH } 8.0$). Gently rock the tube for 10 minutes to form an emulsion. Centrifuge at $12,000,text{rpm}$ for 10 minutes at $4^circtext{C}$.
- Aqueous Recovery & Precipitation: Carefully transfer the upper clear aqueous phase ($~450,mutext{L}$) to a fresh sterile tube using a wide-bore pipette tip. Add $45,mutext{L}$ of $3,text{M}$ sodium acetate ($text{pH } 5.2$) and 2 volumes ($900,mutext{L}$) of ice-cold absolute ethanol. Mix gently; white stringy fibers of DNA will precipitate immediately.
- Washing & Rehydration: Spool the genomic DNA fibers onto a sterile hooked glass rod (or pellet at $10,000,text{rpm}$ for 5 min). Wash with $500,mutext{L}$ of $70%$ ethanol, air-dry for 10 minutes, and dissolve in $100,mutext{L}$ of sterile TE buffer ($text{pH } 8.0$). Store at $4^circtext{C}$.
💬 Viva-Voce Questions & Model Answers
Q1: Why must the phenol used for DNA extraction be equilibrated to pH 8.0?
At acidic pH (~4.5–5.0), the phosphate groups of DNA become partially protonated and non-ionized, causing DNA to partition into the organic phenol layer alongside proteins. At alkaline pH (8.0), DNA phosphates are fully deprotonated and carry a strong negative charge, ensuring 100% partitioning into the upper aqueous layer while denatured proteins remain at the interphase.
Q2: Why is vortexing strictly avoided during genomic DNA extraction?
Bacterial genomic DNA is an exceptionally long molecule ($>4.6times 10^6,text{bp}$). High shear forces generated by mechanical vortexing snap the long double-helical strands into small heterogeneous fragments, destroying its high-molecular-weight integrity. Gentle inversion is mandatory.
Q3: Why is 70% ethanol used instead of 100% ethanol for washing the DNA pellet?
70% ethanol contains 30% water, which effectively dissolves and washes away co-precipitated sodium acetate salts without dissolving the insoluble DNA pellet. Absolute (100%) ethanol cannot solubilize sodium acetate.