DNA Extraction
5th Grade Physical Science Standard 1.1
Concepts and Skills: Mixtures of matter can be separated regardless of how they
were created.
Evidence Outcomes: Develop, communicate, and justify a procedure to separate
simple mixtures based on physical properties.
7th Grade Life Science standard 1.1
Concepts and Skills: Mixtures of substances can be separated based on their
properties such as solubility, boiling points, magnetic properties, and densities
Evidence Outcomes: Identify properties of substances in a mixture that could be
used to separate those substances from each other.
7th grade Life science standard 2.3
Concepts and Skills: Cells are the smallest unit of life that can function
independently and perform all the necessary functions of life.
Evidence Outcomes: Develop, communicate, and justify an evidence-based
scientific explanation regarding cell structures, components, and their specific
functions
8th grade Life Science standard 2.2
Concepts and Skills: Organisms reproduce and transmit genetic information
(genes) to offspring, which influences individuals’ traits in the next generation
Evidence Outcomes: Use direct and indirect observations, evidence, and data to
support claims about genetic reproduction and traits of individuals.
Objectives
Demonstrate how the principles of dissolution, filtration, and chemical
precipitation are used in a real world application.
Materials
- strawberry
- Ziploc bag
- extraction buffer (90mL of water, 50mL of soap, 1.5g of rock salt)
- 10 mL syringe
- plastic funnel
- coffee filters
- test tube
- Ice cold rubbing alcohol (the colder the better)
- stirring rod
- transfer pipette
Procedure
1. Place a strawberry in the Ziploc, and mash up well.
2. Add 10mL of extraction solution with syringe and mix.
3. Filter the solution into the test tube using the coffee filter and funnel, do
not fill more than half full.
4. Carefully add cold isopropanol on top of the solution with the pipette.
Make sure the layers don't mix.
5. Gently pull DNA strands from the bottom solution into the isopropanol with
a stir rod. DNA should precipitate into fine white strands that look like cotton
candy.
What is happening?
Mashing up the strawberries breaks up cells and releases their content into the
bag. The soap in the extraction buffer further helps to break apart the nucleus of
the cell that contains the DNA, which then subsequently dissolves in the water.
The mixture is then filtered to get a solution containing the DNA (as well as
proteins, sugars, and other stuff). When the DNA is pulled out of the water into
the rubbing alcohol it becomes insoluble and precipitates out as visible strands.
In more detail:
DNA contains a "backbone" that is made up of a sugars (pentose) and
phosphate molecules which are negatively charged. Water is a very polar solvent
and the interaction between the water molecules and the negatively charged
backbone keeps the DNA in solution.
Isopropanol, however, is less polar and if the DNA is physically pulled out of the
water phase, it will also grab positively charged sodium atoms (from the salt)
along, which will attach to the negative backbone. This, in turn, will cause the
DNA to become neutral, once it enters the Isopropanol. As a result, the DNA
chains will start coiling up, forming the visible strands that look like cotton candy.
The extracted DNA is not very pure and scientists in a laboratory would add
further steps to the extraction (centrifugation and dissolution) to remove
contaminants such as proteins and sugars.
References
Britos, L.; Goyenola, G.; Orono, S.U. "Simple Protocol for Secondary School
Hands-on Activity". Biochemistry and Molecular Biology Education, 2004, 32 (5),
341-347.