OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(ii) practical investigations into the purification of DNA by precipitation
Full specification wording
Nucleotides and nucleic acids: (ii) practical investigations into the purification of DNA by precipitation
DNA precipitation separates DNA from other cell components by exploiting its chemistry.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
DNA precipitation separates DNA from other cell components by exploiting its chemistry. In a practical investigation you would break open cells, remove protein and other material, then add cold ethanol so DNA comes out of solution as a visible white precipitate that can be spooled out. The method depends on DNA being soluble in water but insoluble in alcohol, and on salt helping DNA clump. Questions test why each step is done: why the sample is chilled, why ethanol is layered carefully, why salt is added, and how the precipitate is collected. You must link each manipulation to the property of DNA it exploits, and recognise that precipitation purifies DNA rather than simply extracting a crude mixture.
What earns the marks
- DNA is soluble in water but insoluble in cold ethanol, so adding cold ethanol causes DNA to precipitate out of solution.
- Chilling the ethanol and sample reduces DNA solubility and slows enzyme activity, encouraging precipitation and protecting DNA.
- Adding salt (for example sodium chloride) neutralises the negative charges on the phosphate backbone so DNA molecules can aggregate and precipitate.
- The white precipitate can be spooled or collected, separating purified DNA from soluble contaminants left in the supernatant.
- Precipitation is a purification step: contaminants that stay dissolved are removed when the DNA precipitate is separated from the liquid.
Common mistakes and how to put them right
Thinking DNA dissolves in ethanol.
Put it right by stating DNA is insoluble in cold ethanol and precipitates, while it dissolves in water.
Believing salt breaks the DNA.
Put it right by explaining salt neutralises phosphate charges to allow aggregation, not strand breakage.
Assuming precipitation destroys DNA.
Put it right by noting the DNA remains intact and can be collected and redissolved for further use.
Exam tips
- For each step, name the property of DNA it exploits rather than just describing the action.
- Use the words soluble and insoluble precisely when explaining the role of ethanol and water.
- Link the visible white precipitate to purification, explaining what remains dissolved and is therefore removed.
Try the lesson
The start of the Nucleotides and nucleic acids: (ii) practical investigations into the purification of DNA by precipitation lesson, made from checked teaching notes for this specification point.
Have a go first
After DNA has been precipitated with ice-cold ethanol, which piece of apparatus is used to collect the fibrous DNA?
Listen
DNA precipitation separates DNA from other cell components by exploiting its chemistry. In a practical investigation you would break open cells, remove protein and other material, then add cold ethanol so DNA comes out of solution as a visible white precipitate that can be spooled out. The method depends on DNA being soluble in water but insoluble in alcohol, and on salt helping DNA clump. Questions test why each step is done: why the sample is chilled, why ethanol is layered carefully, why salt is added, and how the precipitate is collected. You must link each manipulation to the property of DNA it exploits, and recognise that precipitation purifies DNA rather than simply extracting a crude mixture. A strong answer earns marks for these points. DNA is soluble in water but insoluble in cold ethanol, so adding cold ethanol causes DNA to precipitate out of solution. Chilling the ethanol and sample reduces DNA solubility and slows enzyme activity, encouraging precipitation and protecting DNA. Adding salt (for example sodium chloride) neutralises the negative charges on the phosphate backbone so DNA molecules can aggregate and precipitate. The white precipitate can be spooled or collected, separating purified DNA from soluble contaminants left in the supernatant. A common slip: Thinking DNA dissolves in ethanol. Instead, stating DNA is insoluble in cold ethanol and precipitates, while it dissolves in water. Exam tip: For each step, name the property of DNA it exploits rather than just describing the action.
Key words
Say what each word means before you turn the card.
Solubility differences between DNA in water and in alcohol
Try answering aloud or in your head before turning the card.
Role of salt in neutralising phosphate charge
Try answering aloud or in your head before turning the card.
Cold conditions protecting DNA and aiding precipitation
Try answering aloud or in your head before turning the card.
Separation of purified DNA from soluble contaminants
Try answering aloud or in your head before turning the card.
See every definition
- Solubility differences between DNA in water and in alcohol
- DNA is soluble in water but insoluble in cold ethanol, so adding cold ethanol causes DNA to precipitate out of solution.
- Role of salt in neutralising phosphate charge
- Salt neutralises the negative charges on the phosphate backbone so DNA molecules can aggregate and precipitate.
- Cold conditions protecting DNA and aiding precipitation
- Chilling the ethanol and sample reduces DNA solubility and slows enzyme activity, encouraging precipitation and protecting DNA.
- Separation of purified DNA from soluble contaminants
- The white precipitate can be spooled or collected, separating purified DNA from soluble contaminants left in the supernatant.
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