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During the purification process for recombinant DNA technology, addition of chilled ethanol precipitates out
After enzymatic digestion of RNA (RNase) and proteins (proteases), adding ice-cold (chilled) ethanol precipitates pure DNA as a collection of fine threads that can be spooled out!
To obtain pure genetic material, bacterial/plant cells are digested with lysozyme/cellulase, followed by selective enzyme treatments:
Because DNA is a polyanionic macromolecule surrounded by a hydration shell of water molecules, adding cold ethanol lowers the dielectric constant, allowing monovalent cations () to neutralize the phosphate backbone, precipitating DNA out of solution:
The precipitated DNA is visible as a tangle of fine white threads and is collected onto a glass rod by spooling.
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Polysaccharides are degraded by enzymes (cellulase/pectinase) or removed in organic phases during initial lysis.
RNA is digested by ribonuclease (RNase) prior to ethanol addition so it does not precipitate as the intact target nucleic acid.
Matches NCERT verbatim: addition of chilled ethanol ultimately precipitates out purified DNA.
Histones are basic proteins digested by proteases into soluble peptides, not precipitated intact.
Students confuse the role of enzymes (RNase, proteases) with the role of chilled ethanol. Enzymes degrade contaminants, whereas chilled ethanol precipitates the DNA.
"Other molecules can be removed by appropriate treatments and purified DNA ultimately precipitates out after the addition of chilled ethanol. This can be seen as collection of fine threads in the suspension."
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