Biology 11–12 · Year 12

Extracting DNA from split peas or onion

Module 5: Heredity

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Breaking cells with detergent, stripping proteins with a protease and adding cold ethanol pulls the long DNA molecules out of solution as a visible mass.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Ethanol is highly flammable
Protease powder is harmful if inhaled and an irritant in solution
Blender blades

Controls

No flames in the room; keep ethanol capped and chilled in a sealed container
Eye protection; add protease as a pinch, not poured
Adult operates the blender

Note

Ethanol and enzyme handling: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf) and RiskAssess (https://www.riskassess.com.au/).

What you need

Dried split peas 100 mL (about half a cup) or one onion (the sources recommend peas or onion)
Table salt, 1/8 teaspoon (less than 1 mL); cold water 200 mL; blender or mortar; sieve or coffee filter
Liquid detergent, 2 tablespoons (about 30 mL)
Protease: a pinch of meat tenderiser (or fresh pineapple juice)
Ethanol 95 percent (industrial denatured alcohol) chilled for at least 2 hours, about the same volume as the filtered extract; test tubes; wooden skewer or glass rod

How to do it

Blend the peas with the salt and cold water for 15 seconds and strain into a beaker.
Add the washing-up liquid, swirl gently and leave for 5 to 10 minutes to break the membranes.
Pour into test tubes to one-third full and add a pinch of protease to each; stir gently.
Tilt the tube and pour ice-cold ethanol slowly down the side so it forms a layer on top.
Watch the interface; after a few minutes twirl the skewer at the interface to collect the white strands.
Compare a tube prepared without detergent and one without protease.

What you should see

Clumps of white, stringy DNA appear where the ethanol meets the pea mixture and can be wound onto the skewer, and more may appear if the tube stands for 30 to 60 minutes, as the Utah guide notes; tubes without detergent give little or none because the membranes stay intact and the DNA stays in the lower layer. The protease-free tube is compared and any difference in the amount or look of the precipitate recorded. The learner knows it worked when long threads wind onto the skewer rather than a diffuse haze.

What changes

What you change: presence of detergent, protease, or the temperature of the ethanol
What you measure: amount and appearance of DNA precipitated
What you keep the same: tissue mass
volumes
timing
ethanol concentration

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The white strands are single DNA molecules; they are millions of molecules tangled together.
DNA is only found in the nucleus of animals; plants, fungi and bacteria carry it as well.
The detergent dissolves the DNA; it dissolves the membranes so the DNA is released.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-12 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-07
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities