Principles of Nucleic Acid Extraction

Three steps, every time

However different two methods look, every extraction does the same three things ¹:

1. Lyse: break open cells (and walls, for bacteria, fungi, and plants) and strip proteins off the nucleic acid.
2. Separate: get proteins, lipids, polysaccharides, heme, and salts away from the nucleic acid.
3. Recover: collect the purified nucleic acid in a defined volume of water or buffer.

Lysis reagents

The goal is complete release without shearing the DNA into uselessly short pieces. Guanidinium is also why lysis buffers are dangerous: mixed with bleach it releases toxic gas, so keep guanidinium waste away from bleach (see the lab operations lesson on contamination).

Three chemistries

Organic (phenol-chloroform). Mix lysate with phenol-chloroform and spin. Nucleic acid stays in the upper aqueous phase; denatured protein collects at the interphase; lipids go to the lower organic phase. Draw off the aqueous layer, clean up with chloroform, and recover by alcohol precipitation (salt plus ethanol or isopropanol, spin, wash, redissolve) ³. High quality, but toxic, corrosive, and slow.

Salting-out. After detergent and proteinase K lysis, a saturated salt precipitates proteins, a spin pellets them, and the DNA in the supernatant is alcohol-precipitated ¹. Cheap and solvent-free, slightly less pure.

Solid phase (silica). The basis of nearly every kit and automated platform. In high chaotropic salt, nucleic acid binds silica - a column membrane or magnetic beads - while contaminants don’t ¹:

1. Bind lysate in chaotropic buffer to the silica.
2. Wash with ethanol-containing buffers to remove protein and salt; the nucleic acid stays bound.
3. Dry briefly so no ethanol carries over (ethanol inhibits PCR).
4. Elute in low-salt buffer or water ¹.

DNA versus RNA

RNA needs more care: the 2’-OH makes it chemically fragile, and RNases are everywhere - on skin, dust, and reagents - and survive autoclaving ^{2 3}. RNA work means RNase-free tubes and tips, gloves changed often, quick work on ice, and lysis straight into a chaotropic or RNA-stabilizing buffer.

Many RNA kits do the DNase step on the column. Specimens for RNA testing are often collected in stabilizing tubes (PAXgene for blood) or frozen quickly.

Input sets the ceiling

What comes out of extraction caps every assay after it ¹:

• Carryover inhibits: phenol, guanidinium, ethanol, heme, and heparin inhibit polymerases and give falsely low or negative results.
• Damage is permanent: sheared or degraded nucleic acid can’t be repaired later ³.
• Wrong nucleic acid: genomic DNA in an RNA prep inflates RT-qPCR signal.

That’s why the next steps are measuring quantity, purity, and integrity, and why most labs move the silica chemistry onto instruments.

What to take from this

Every extraction lyses, separates, and recovers. Detergent opens membranes, proteinase K clears proteins and nucleases, and chaotropic salt denatures and drives silica binding. Organic extraction keeps nucleic acid in the upper aqueous phase; salting-out precipitates protein; silica binds in chaotrope, washes in ethanol, and elutes in low salt. RNA needs RNase-free technique and a DNase step. A poor extract can’t be rescued downstream.