This document provides instructions for extracting DNA from onion cells through a multi-step process. It begins by having students finely chop and crush an onion in a mortar and pestle and adding it to a beaker with salt, washing up liquid, and water to break open the cell walls and membranes. The mixture is then filtered and the filtrate collected. Ethanol is slowly added to the filtrate to precipitate out any extracted DNA, appearing as a white layer between the two liquids after settling. Students then use forceps to remove strands of extracted DNA from the mixture.

2. Extracting DNA
WALT: To carry out a practical to extract DNA from
onion cells.
WILF:
~ Recall where DNA is found inside plant and animal cells.
(D)
~ List the steps taken during the practical which helps to
extract the DNA. (C)
~ Describe some implications that the HGP has had in the
world of science and medicine. (B)
~ Evaluate the impact of the HGP. (A/A*)

3. Method
1. Finely chop an onion, crush in the mortar with the
pestle and add them to a large beaker.
2. Add 8 teaspoons of salt, 70cm3 of washing up liquid
and 200cm3 of warm water to the beaker.
3. Leave for 5mins but stir intermittently.
4.Place some filter paper into a funnel and place over a
conical flask.
5. Gently pour some of the mixture into the funnel and
let it drain through.
6.Ensure you write you and your partners name onto
the conical flask ready for next lesson.

4. What’s happening?
DNA is found in the nucleus of
the cell. To extract this we need
to break through the cell wall
and cell membrane.
It attracts fats
therefore causing
the cell membrane
to collapse.
The salt binds
to the DNA
allowing us to
see it.

5. Method
1. Take a boiling tube and fill a third of the way with
some of filtrate. Take another boiling tube and fill a
quarter of the way up with ethanol.
2. Tilt the boiling tube and very slowly pour the
ethanol down one side. This should take a few
minutes and the surgical spirit will form a layer.
3. Place your boiling tube in a rack and leave to settle
for about 10mins. (See board for ‘To do’ list!)
4. You should now see a white layer forming between
the filtrate and ethanol, this is DNA.
5. Use a pair of forceps to extract some of the DNA.

6. Hopefully what you should of
saw is…
Strands of DNA