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How can you get the DNA out of living cells and actually see it, using everyday materials?

Describe how DNA can be extracted from cells in the laboratory and explain the purpose of each step

A focused answer to the O-Level outcome on DNA extraction. Breaking open cells, removing proteins, precipitating DNA with cold alcohol, and the reason for each step.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

This outcome asks you to describe a simple laboratory method for extracting DNA from cells and to explain the purpose of each step. It is a practical favourite because it can be done with everyday materials, and because every step has a clear scientific reason that examiners test.

The answer

The goal

To extract DNA you must break open the cells, release the DNA, separate it from other cell contents, and then make it come out of solution so it can be seen and collected.

The steps and their purposes

A typical extraction from plant material such as fruit follows these steps:

  1. Mash or grind the tissue. This breaks up the tissue and separates the cells, increasing the surface area.
  2. Add detergent. The detergent breaks down the cell membranes and nuclear membranes by dissolving their lipids, releasing the DNA into solution.
  3. Add salt. The salt helps the DNA molecules clump together by neutralising their charged groups, so the DNA can come out of solution more easily later.
  4. Filter the mixture. Filtering removes large pieces of debris, leaving a solution containing the DNA.
  5. Add cold alcohol gently. DNA is insoluble in cold alcohol, so when cold alcohol is layered on top, the DNA precipitates at the boundary as white, stringy strands.
  6. Spool out the DNA. The visible strands can be lifted out with a glass rod or loop.

Why each reagent is chosen

  • Detergent for breaking membranes (lipids dissolve in detergent).
  • Salt for making the DNA clump.
  • Cold alcohol for precipitating the DNA, because DNA does not dissolve in cold alcohol and the cold also slows enzymes that could break the DNA.

Examples in context

Example 1. A classroom extraction. A common practical extracts DNA from strawberries, kiwi or onion using washing-up liquid, table salt and cold alcohol. It demonstrates the whole principle, breaking membranes and precipitating DNA, with safe, everyday materials.

Example 2. The first step of real laboratory work. Before any genetic engineering, sequencing or PCR can happen, the DNA must first be extracted from cells. Professional kits use the same logic, breaking cells open, removing proteins, and precipitating the DNA, just with purer reagents and tighter control.

Try this

Q1. State the purpose of adding detergent during DNA extraction. [1 mark]

  • Cue. It breaks down the cell and nuclear membranes by dissolving their lipids, releasing the DNA.

Q2. Explain why cold alcohol makes the DNA appear. [2 marks]

  • Cue. DNA is insoluble in cold alcohol, so it comes out of solution (precipitates) at the boundary as visible white strands.

Q3. Give one reason the mixture is filtered before the alcohol is added. [1 mark]

  • Cue. To remove large pieces of cell debris, leaving a clearer solution containing the DNA.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe how you would extract DNA from a sample of fruit, explaining the purpose of the detergent, the salt and the cold alcohol.
Show worked answer →

Examiners want an ordered method with the purpose of the three key reagents.

First, mash the fruit to break up the tissue and separate the cells. Add a mixture containing detergent and salt. The detergent breaks down the cell membranes and nuclear membranes by dissolving their lipids, releasing the DNA into solution. The salt makes the DNA clump together by helping its charged groups stick, which helps it come out of solution later.

Filter the mixture to remove large debris. Then gently pour cold alcohol (such as cold ethanol) down the side of the tube so it forms a layer on top. DNA is insoluble in cold alcohol, so it comes out of solution (precipitates) at the boundary as white, stringy strands that can be spooled out.

What markers reward: mashing to break up tissue, detergent to break down membranes, salt to help the DNA clump, filtering, and cold alcohol to precipitate the DNA because it is insoluble in alcohol, with the DNA appearing as white strands.

Original3 marksExplain why the alcohol used to precipitate DNA should be cold, and why it is poured gently to form a layer.
Show worked answer →

The answer should give the reasons for cold alcohol and gentle layering.

The alcohol is cold because DNA is even less soluble in cold alcohol than in warm alcohol, so cold alcohol makes more DNA come out of solution and precipitate. Cold also slows down enzymes that might break up the DNA.

It is poured gently to form a separate layer so that the DNA precipitates cleanly at the boundary between the watery solution and the alcohol, where it can be seen and spooled out without mixing and breaking up the strands.

What markers reward: cold alcohol because DNA is less soluble in the cold so more precipitates (and enzymes are slowed), and gentle layering so the DNA forms cleanly at the boundary and can be collected.

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