Home DNA extraction kits

Extraction is the oldest demonstration in molecular biology, and the kit version works because the chemistry is real: every reagent in the box does a defined job on real cellular structures. What the kit does not produce is a purified, measured sample — and the difference between a visible thread and a usable preparation is the lesson.

Lysis: getting through the membranes#

A cheek cell or a strawberry cell is two barriers deep: the plasma membrane and, in animals and plants alike, the nuclear envelope around the DNA. Both are lipid bilayers, and detergent molecules — the same amphiphiles as in dish soap — insert into them and tear them into micelles, spilling the contents into solution. Salt then does two jobs: sodium ions neutralise part of the negative charge along the DNA backbone and screen the remaining charges from each other, while the high ionic strength strips histone proteins off the DNA and salts many other proteins out of solution, exactly as in the precipitation experiments of a chemistry class. Some protocols add a protease or a heating step to digest the nucleases that would otherwise begin cutting the liberated DNA.

Precipitation: why alcohol makes DNA visible#

DNA dissolves readily in water: its backbone is charged and dressed in a hydration shell, and water’s high dielectric constant keeps the negative charges from collapsing onto each other. Cold ethanol or isopropanol changes both conditions at once — it lowers the dielectric constant of the mixed solvent and strips away the hydration shell — so the charges attract, molecules aggregate, and the polymer leaves the solution. It leaves as fibres because DNA is enormously long: a single human cell holds on the order of two metres of it, and entangled strands of that length cannot redissolve quickly, so they wind onto a glass rod as a visible mass. Spooling works for the same reason a long cooked noodle can be lifted out whole and a short one cannot.

What the thread is and is not#

The extract is real DNA and also a mess. It is sheared — every stroke of a pipette breaks the molecules shorter — and contaminated with bacterial DNA from the mouth or the fruit, with RNA fragments, proteins and polysaccharides that precipitated along with it. Its concentration is unknown, and the detergent and salt it carries inhibit the enzymatic reactions a laboratory would run next; cleaned up and amplified, such a sample could feed the verification workflow described on the CRISPR kit page, but as collected it answers no question. That is not a defect of the kit but its point: the visible thread demonstrates that DNA is a chemical object with solubility, charge and length — the three properties every purification method downstream manipulates.