Extracting DNA From Strawberries (v2) (VCE SSCE Biology): Revision Notes

Extracting DNA From Strawberries (v2)

Introduction to DNA extraction

DNA (deoxyribonucleic acid) is found in nearly every cell of all living organisms. This remarkable molecule contains the instructions for how living things work, develop, and survive. Because DNA is so essential, each cell carries a complete copy of an organism's genome - the entire set of genetic instructions.

Scientists need to extract and study DNA for many important reasons, including:

• Solving forensic investigations and identifying suspects
• Developing new medical treatments and therapies
• Creating genetically modified crops with improved characteristics
• Understanding the evolutionary relationships between different species

Why strawberries are perfect for DNA extraction

Strawberries are particularly excellent for DNA extraction experiments for three main reasons:

Aim

To extract, isolate, and observe DNA from strawberry cells.

Safety and materials

Safety equipment required

Materials needed

• 1 × 50 mL lysis buffer falcon tube
• 1 × 45 mL measuring cylinder
• 1 × 5 mL disposable pipette
• 1 × sealable plastic bag
• 1 × glass beaker
• Filter paper
• 1 × funnel
• 5 g of table salt (NaCl)
• 45 mL water
• 5 mL of liquid detergent
• 1 × ripe strawberry
• 25 mL of isopropyl alcohol (also called rubbing alcohol)

Method

Part A: Making the lysis buffer

The lysis buffer is a special solution that helps break open cell membranes. Here's how to prepare it:

1. Add 5 g of table salt to the lysis buffer falcon tube.
2. Add 45 mL of water and 5 mL of liquid detergent to the same tube.
3. Secure the cap tightly and mix by gently turning the tube upside down several times. You now have your lysis buffer ready for Part B.

Part B: Making the strawberry lysate

This stage breaks down the strawberry cells to release the DNA:

1. Place one ripe strawberry inside a sealable plastic bag. Remove all air from the bag before sealing it completely.
2. Mash the strawberry thoroughly through the bag using your fingers. Take care not to tear or break the bag. Record your observations about the strawberry's appearance.
3. Open the bag and add the lysis buffer you prepared in Part A.
4. Remove the air again, seal the bag, and continue mashing the strawberry with your fingers. The mixture you now have is called strawberry lysate - a combination of broken strawberry cells and lysis buffer.
5. Record your observations about how the mixture looks after adding the lysis buffer.

Part C: Filtering the lysate

This stage separates the liquid containing DNA from the solid strawberry pieces:

1. Position the funnel over the glass beaker and place the filter paper inside the funnel.
2. Carefully pour the strawberry lysate from the plastic bag into the funnel. Wait patiently until all the liquid has dripped through into the beaker below. The liquid that collects in the beaker is called the filtrate.

Part D: Precipitating and observing DNA

This final stage makes the DNA visible:

1. Remove the funnel from the glass beaker.
2. Measure exactly 25 mL of isopropyl alcohol and slowly pour it into the glass beaker.
3. Observe carefully where the isopropyl alcohol layer meets the strawberry filtrate layer. You should see white, stringy material appearing at this boundary - this is the DNA! Record your observations in your results table.

Recording your results

Use tables to record what you observe at each stage:

Table 1: Appearance of strawberry lysate

Lysate	Before lysis buffer	After lysis buffer
Observations

Table 2: Appearance of DNA

DNA	Before isopropyl alcohol	After isopropyl alcohol
Observations

Understanding the process

Why each step matters

The DNA extraction process involves four main stages, each with a specific purpose:

Procedure	Purpose
A. Mash the strawberry in the sealable plastic bag	Breaking open the strawberry tissue to start releasing cells
B. Add the DNA lysis buffer to the mashed strawberry	Breaking down cell membranes and nuclear membranes to release DNA
C. Pour the strawberry lysate through the funnel with filter paper	Separating the liquid (containing DNA) from solid cellular debris
D. Add isopropyl alcohol to the strawberry filtrate	Making DNA precipitate (become solid and visible) at the boundary between layers

How the chemicals work

Each chemical in this experiment plays a specific and essential role in extracting DNA:

Salt (sodium chloride): The salt helps DNA stick together and makes it easier to see. It also helps separate DNA from proteins by neutralising charges.

Detergent (liquid soap): Detergent breaks down the fatty membranes that surround cells and nuclei, just like it breaks down grease on dirty dishes. This releases the DNA from inside the cells.

Isopropyl alcohol: DNA dissolves in water but not in alcohol. When you add cold alcohol to the filtrate, the DNA cannot stay dissolved, so it precipitates (becomes solid) and appears as white, stringy material. This makes the normally invisible DNA visible to your eyes.

Key vocabulary

Genome: The entire set of genetic instructions contained in an organism's DNA.

Octoploid: Having eight copies of each chromosome. Strawberries are octoploid, which means they contain eight times more DNA than organisms with just one copy of each chromosome.

Lysis buffer: A solution designed to break down cell membranes and release cellular contents, including DNA.

Lysate: The mixture created when cells are broken down. In this experiment, the strawberry lysate is the mashed strawberry combined with lysis buffer.

Filtrate: The liquid that passes through filter paper. In this experiment, the filtrate contains DNA and other dissolved cellular components.

Precipitation: The process of making a dissolved substance become solid. DNA precipitates when isopropyl alcohol is added to the filtrate.

Pectinases and cellulases: Enzymes naturally present in strawberries that help break down cell walls.

Exam tips

Remember!