Extracting DNA from a banana or an onion · Concept Studio

The idea

DNA is a physical substance present in every cell, and it can be released by breaking membranes and made visible by precipitating it in cold alcohol.

Safety card

Low riskAn adult supervises

Hazards

Ethanol and isopropanol are flammable
Detergent in eyes
Hot tap water

Controls

No naked flames; alcohol in small volumes from a labelled bottle
Safety glasses; wash splashes
Nothing is tasted
Hot water from the tap only, never boiling water; hold the bag by its top

Note

Chemicals: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package (section 1, general information for all staff) and complete a RiskAssess risk assessment before the lesson.

What you need

Half a ripe banana, or 20 g of onion (the SAPS onion resource notes onion extracts DNA more reliably than kiwi fruit)
Zip-lock bag; extraction liquid of 100 mL of water, 5 g of dishwashing liquid and 2 g of table salt, the quantities in the CSIRO Double Helix DNA detective activity
Coffee filter or muslin in a funnel over a 250 mL beaker
Rubbing alcohol (isopropanol) or ethanol, 50 mL, kept in the freezer for at least 30 minutes before use
A tall test tube or clear glass, a wooden skewer or glass rod

How to do it

Mash the banana in the bag with the extraction liquid for two minutes until smooth (the detergent breaks open the membranes; the salt helps the DNA clump).
Stand the sealed bag in a container half filled with hot tap water for 15 minutes, as the CSIRO DNA detective activity does.
Filter the mash into the beaker and pour about 20 mL of the filtrate into the tall tube.
Tilt the tube and pour an equal volume of ice-cold alcohol slowly down the side so it floats as a layer on top.
Wait about five minutes without stirring, as the CSIRO activity does, and watch the boundary between the layers.
Spool the white strands onto the skewer by turning it slowly at the boundary.
Repeat with onion and compare the amount of DNA obtained.

What you should see

A white, cloudy film forms at the boundary between the filtrate and the alcohol within about five minutes and can be wound onto the skewer as a stringy mass; SAPS reports that onion extracts more reliably than kiwi fruit, and the class compares its sources by the amount spooled. The learner knows it worked when white strands can be lifted out of the alcohol layer on the skewer.

What changes

What you change: Source tissue (banana, onion, kiwi fruit, strawberry)
What you measure: Amount of spooled DNA (qualitative, or wet mass on the skewer)
What you keep the same: Tissue mass
Detergent and salt quantities
Mashing time
Time in hot water (15 minutes)
Alcohol volume and temperature

Common misconceptions

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

Only animals have DNA.
DNA is too small to see; individual molecules are, but millions clumped together are visible.
The white strands are the double helix at its true scale.

Curriculum references

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

Science 7–10 Syllabus (2023)SC5-GEV-02 SC5-WS-01 SC5-WS-04
Australian Curriculum v9AC9S10U01AC9S10I03

Sources

The pages the author read to write this activity.