Low Plasmid Quality Due to Degradation or Denaturation (DNA Cleanup & Plasmid Purification)

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Plasmid DNA shows smearing on gel, poor A260/280 ratio, or fails in downstream applications despite adequate concentration. Multiple bands or reduced supercoiled form may be visible. Common causes: 1) Strains with high endogenous endonuclease activity used (e.g., HB101, JM100 series) 2) Plasmid denatured by excessive incubation with Plasmid Lysis Buffer (B2) containing NaOH (>2 minutes) 3) Genomic DNA contamination from vortexing during lysis causing chromosomal DNA shearing 4) RNA contamination due to insufficient neutralization buffer incubation time (<2 minutes) 5) Improper DNA storage in solutions containing magnesium or at incorrect temperature Solutions: 1) Avoid strains with high endonuclease levels (e.g., HB101, JM100 series); use DH5α, TOP10, or similar 2) Limit incubation with Plasmid Lysis Buffer (B2) to maximum 2 minutes to prevent NaOH-mediated denaturation 3) Use gentle inversion mixing after cell lysis; never vortex to avoid chromosomal DNA shearing 4) Incubate in neutralization buffer for full 2 minutes; for cultures >3 ml, centrifuge 5 minutes after neutralization 5) Elute DNA in DNA Elution Buffer or nuclease-free water; store at -20°C, never in magnesium-containing solutions