SBI 3U
Independent Variable
The independent variable is the variable that is manipulated by the researcher. The independent variable is a factor that is hypothesized to influence the dependent variable.
The researcher determines what level or condition of the independent variable will be used in the experiment. The values for the independent variable are determined before the experiment starts.
Dependent Variable
The dependent variable is the variable that is simply measured by the researcher. It is the variable that reflects the influence of the independent variable. For example, the dependent variable would be the variable that is influenced by being randomly assigned to either an experimental condition or a control condition.
Controlled Variables
All variables that are not the independent or dependent variable must be controlled variables. Controlled variables must be kept constant throughout the experiment. This ensures the validity and repeatability of the results.
The Control
This usually ensures that the procedure works before any variables are altered. The results of the control procedure are compared to the results of the modified procedure.
All well designed experiments must have a control procedure or control sample.

SBI 3U
DNA Extraction Lab
A complete copy of DNA is found in every cell in any organism. In order to release the DNA to analyze it, scientists must break open the cells and remove structural proteins and enzymes that interfere with the DNA structure. This simplified procedure releases a great deal of DNA so that DNA can be seen. DNA is extracted to allow observation of DNA’s physical and chemical properties. This extraction does not, however, purify the sample enough for the strict standards of a research or forensics lab.
Strawberry DNA Extraction Lesson Plan
This lab is for the extraction of DNA from strawberries. Strawberries are an exceptional fruit to use for this lesson because each individual student is able to complete the process by themselves and strawberries yield more DNA than any other fruit (i.e. banana, kiwi, etc.).
Strawberries are decaploid (ten sets; 10x) meaning that they have 10 copies of each type of chromosome.
In this lab, you will work with a partner and compare your findings. One partner will conduct the lab as described below, while the other partner will manipulate one of the following variables:
1.
Double the amount of DNA extraction buffer
2.
Omit DNA extraction buffer
3.
Double the amount of ethanol
4.
Omit ethanol
5.
Double the mass of strawberries
6.
Omit meat tenderizer
Materials
- 1 heavy duty Ziploc bag
- Meat tenderizer
- DNA extraction buffer (soapy, salty water)
- Cheesecloth
- 1 test tube
- pipette
- rubber stopper
- cold ethanol
- funnel
- strawberry
Set up Instructions for Teacher:
To make 1 L of the DNA extraction buffer, mix 100 mL of shampoo and 15 g of table salt.
Add water to make a final volume of 1 L. Stir slowly, to dissolve the salt, and avoid foaming.
Control Procedure:
1. Place one strawberry in a Ziploc bag.
2. Smash/grind up the strawberry using your fist and fingers for 2 minutes. Be careful not to break the bag!!

3. Add 10 mL of DNA extraction buffer to the bag.
4. Mush the strawberry in the bag again for 1 minute.
5. Assemble the filtration apparatus as shown in Fig 1.
6. Pour the strawberry solution into the filtration apparatus and let it drip
directly into your test tube.
7. Add 2 toothpick ends of meat tenderizer into the test tube. Use the wide end of a wooden toothpick. Cover the test tube with a rubber
SBI 3U stopper and gently invert 2-3 times to mix.
8. Holding the test tube at an angle, gently dribble cold ethanol down the side of the
Fig 1 test tube. The DNA will precipitate out of the mixture into the ethanol. Look for a semitransparent, hazy substance at the interface between the 2 solutions.
9. Dip the pipette into the test tube where the strawberry extract and ethanol layers
come into contact with each other and spool out the DNA.
Pre Lab Assignment: Each partner will hand in their own copy of their work
1.
Read the lab introduction and procedure carefully.
2.
Make a well labelled sketch of the control procedure. Use 4-6 boxes to sketch the key steps.
3.
Make a list of safety precautions for this lab.
4.
Both partners must rewrite the procedure to accommodate the independent variable.
5.
Answer the following questions: a) What part of the cell did the DNA come from? b) What was the purpose of mashing up the strawberry? c) One way to purify a molecule is to get rid of everything but that molecule. If we want to isolate DNA from kiwifruit, what do we have to get rid of? d) What is a precipitate?
6.
a) What is the independent variable? b) What is the dependent variable for this lab? c) What are two controlled variables in the lab?
7. Write a hypothesis that predict discuss how the modified procedure might affect the appearance and yield of DNA.

SBI 3U
8 . Match the procedure step with its function
A. Filter strawberry slurry through cheesecloth ___ To precipitate DNA from solution
B. Mush strawberry with salty/soapy solution
D. Addition of ethanol to filtered extract
C. Initial smashing and grinding of strawberry
___ Separate components of the cell
___ Break open the cells
___ Break up proteins
Observations: Record in the space below
Analysis Questions: Complete AFTER the lab [1 mark each]
Write all answers neatly on a separate page and attach to the lab.
1.
Why was it necessary to have one partner follow the experimental procedure as outlined?
2.
a.)What variable did the team manipulate?
b.) How did the appearance of your strawberry’s DNA compare to that of your partners?
3.
DNA molecules are not visible under the microscope unless they are tightly coiled into a chromosome. Why can the DNA be seen after it is put into the ethanol?
4.
What was the purpose of the filter?
5.
Why is DNA extraction important in research and / or industry?
6.
How would the amount of DNA yield by a strawberry compare to that of a triploid fruit
(three sets; 3x), for example seedless watermelons?
7.
If this experiment was conducted on both plant and animal cells, how would their DNA compare?

DNA EXTRACTION LAB PERFORMANCE RUBRIC
Name:
Criteria
Pre-Lab [3 marks]
- all answers complete
- both partners have a copy of both procedures
- sketches complete
Level 1
0
Partner:
Level 2
1
Level 3
2
0 0.5 1 Safety [2 marks]
- uses all safety gear
- handles material correctly
- keeps equipment in middle of bench
Organization [3 marks]
- knows what to do
- understands all instructions
- never asks teacher questions about material already in written lab
- works well with partner
0 0.5 1
Observations [2 mark]
- records required data for self and partner
Clean up [2 marks]
- all equipment cleaned
- all materials put back in original place
- hands washed
- Enviroclean bench
Analysis [7 marks]
- answers all the questions accurately and succinctly
0
0
0 1
0.5
0.5
2 3
1
1
4 5
SBI 3U
Level 4
3
2
2
2
2
6 7
TOTAL / 18