Biology · Biotechnology: Principles and Processes · NEET
During DNA isolation, after RNA and proteins are removed, adding chilled ethanol makes the purified DNA come out of the mixture as fine white threads that can be spooled onto a glass rod. Ethanol works because DNA does not dissolve in alcohol, so it clumps together and separates from the water. Memory hook: "Cold alcohol pulls DNA out as white threads."
DNA is soluble in water but not in ethanol. After RNase and protease remove RNA and protein, adding chilled ethanol makes the pure DNA come out of solution as fine white threads that can be spooled on a glass rod.
Your doubts, answered
Why does DNA precipitate when ethanol is added?
DNA is soluble in water but not in ethanol. Ethanol removes the water shell around DNA and lets the negative charges on DNA clump together, so the DNA falls out of the solution as fine, thread-like solid. Only DNA remains because RNA and proteins were already removed before this step.
Why must the ethanol be chilled (cold)?
Cold slows down molecular movement and lowers the solubility of DNA even more, so DNA comes out more completely and as neat threads instead of staying dissolved. NCERT specifically says chilled ethanol, so in NEET the answer is chilled ethanol, not room-temperature alcohol.
Does ethanol dissolve DNA?
No. This is the common trap. Ethanol does the opposite of dissolving. DNA dissolves in water; ethanol makes DNA come out of the water as a solid. So ethanol is a precipitating agent, not a solvent for DNA.
What is removed before the chilled ethanol step?
First the cell is broken open (lysozyme for bacteria, cellulase for plants, chitinase for fungi). Then RNA is removed by ribonuclease (RNase) and proteins by protease. Only after this purification is chilled ethanol added, so what precipitates is pure DNA, seen as fine threads in suspension.
Why do we see DNA as fine white threads?
When DNA precipitates it collects into visible thread-like strands (fine threads in suspension) that can be spooled out on a glass rod or spindle. This is the visible sign that pure DNA has separated from the mixture.
⚠️ The NEET trap ✗ Chilled ethanol dissolves DNA so it can be collected, or ethanol precipitates RNA and proteins. ✓ Chilled ethanol precipitates DNA (makes it come out of solution as fine threads). RNA and proteins were already removed earlier by ribonuclease and protease. 🧠 Ethanol PULLS DNA out, it does not DISSOLVE it. And it precipitates DNA, not RNA or histones.
Real NEET questions
NEET 2019
DNA precipitation out of a mixture of biomolecules can be achieved by treatment with
A · Isopropanol
B · Chilled ethanol ✓
C · Methanol at room temperature
D · Chilled chloroform
Solution: During DNA isolation, after the cell is broken open and RNA and proteins are removed (by ribonuclease and protease), the purified DNA precipitates out on adding chilled ethanol, seen as fine threads in suspension. NCERT specifies chilled ethanol as the reagent.
NEET 2021 / 2023
During the purification process for recombinant DNA technology, addition of chilled ethanol precipitates out
A · RNA
B · DNA ✓
C · Histones
D · Polysaccharides
Solution: RNA is removed by ribonuclease and proteins (histones) by protease before this step. So the molecule that finally precipitates with chilled ethanol is the purified DNA, seen as fine threads. Not RNA, histones or polysaccharides.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the exact NCERT line about DNA and ethanol?
NCERT (Ch 9) states that the purified DNA ultimately precipitates out after the addition of chilled ethanol and can be seen as collection of fine threads in the suspension.
Can isopropanol also precipitate DNA?
In real labs isopropanol can precipitate DNA, but for NEET the NCERT-specified and correct answer is chilled ethanol. Do not pick isopropanol or room-temperature methanol in the exam.
Which enzymes remove RNA and protein before this step?
Ribonuclease (RNase) removes RNA and protease removes proteins. Other molecules are removed by appropriate treatments, leaving pure DNA to precipitate with chilled ethanol.
Why is this concept important for NEET?
It is a direct one-line NCERT fact that has been asked repeatedly (2019, 2021, 2023). Knowing that chilled ethanol precipitates DNA (not RNA, not proteins) and does not dissolve it guarantees an easy mark.
