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

Extracting DNA From Strawberries

Introduction

DNA (deoxyribonucleic acid) is the molecule that contains all the instructions an organism needs to function, develop, and survive. Because DNA is so important, it's found in nearly every cell in living organisms. Each cell contains a complete copy of the organism's genome, which is the entire set of genetic instructions.

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

• Solving forensic investigations
• Developing new medical therapies
• Genetically modifying crops to improve food production
• Establishing the evolutionary relationships between different species

Why strawberries are ideal for DNA extraction

Strawberries are particularly good for DNA extraction experiments for several reasons:

Octoploid cells: Strawberries possess eight copies of each chromosome (they are octoploid), which means they yield more DNA than most other fruits. Having eight sets of chromosomes instead of the usual two means there's simply more genetic material to extract.

Helpful enzymes: Strawberries naturally contain pectinases and cellulases. These are enzymes that help break down cell walls when you apply mechanical force (like mashing). This makes it easier to release the DNA from inside the cells.

Easy to process: Strawberries are soft and easy to mash, which helps break open the cells without requiring complicated equipment.

Aim

To extract, isolate, and observe DNA from strawberries.

Materials required

Chemicals and solutions:

• 5 g of table salt (NaCl)
• 45 mL water
• 5 mL of liquid detergent
• 25 mL of isopropyl alcohol (also called rubbing alcohol)
• 1 ripe strawberry

Equipment:

• 1 × 50 mL lysis buffer falcon tube
• 1 × 25 mL measuring cylinder
• 1 × 5 mL disposable pipette
• 1 × sealable plastic bag
• 1 × glass beaker
• 1 × funnel
• Filter paper
• 1 × electronic balance
• 1 × plastic weighing tray

Safety equipment:

• Gloves
• Lab coat
• Safety goggles

Method

The DNA extraction procedure has four main parts: making the lysis buffer, making the strawberry lysate, filtering the lysate, and precipitating the DNA.

Part A: Making the lysis buffer

The lysis buffer is a special solution that will help break down cell membranes and release the DNA.

1. Use the electronic balance to weigh out 5 g of table salt.
2. Add the 5 g of table salt to the lysis buffer falcon tube.
3. Add 45 mL of water and 5 mL of liquid detergent to the same tube.
4. Place the cap securely on the tube and mix the solution by gently turning the tube upside down several times.

This mixture is your lysis buffer, which you'll use in the next part.

Part B: Making the strawberry lysate

A lysate is a mixture containing broken-down cells. You'll create this by breaking open the strawberry cells.

1. Place the ripe strawberry in a sealable plastic bag and remove all the air before sealing it.
2. Mash the strawberry through the bag using your fingers. Be careful not to break the bag. This mechanical force helps break open the cell walls.
3. Open the bag and add your lysis buffer to the mashed strawberry.
4. Remove the air again, seal the bag, and continue mashing the strawberry with your fingers. The mixture at this stage is called strawberry lysate.

Part C: Filtering the lysate

Filtering separates the liquid containing DNA from the solid cellular debris.

1. Place the funnel over the glass beaker and insert the filter paper into the funnel.
2. Carefully pour the strawberry lysate from the plastic bag into the funnel. Wait until all the liquid has dripped through the filter paper into the glass beaker.

The liquid collected in the beaker is called the filtrate. It contains the dissolved DNA along with other cellular components.

Part D: Precipitating and observing DNA

Precipitation means making dissolved DNA become solid and visible.

1. Remove the funnel from the glass beaker.
2. Measure out 25 mL of isopropyl alcohol and slowly add it to the glass beaker.
3. Carefully observe the boundary between the isopropyl alcohol layer and the strawberry filtrate layer.

You should see white, stringy DNA precipitating (becoming solid) at the boundary between the two layers. This happens because DNA is not soluble in alcohol, so it clumps together and becomes visible.

Understanding the procedure

Each step in this investigation serves a specific purpose in extracting DNA:

Mashing the strawberry: The mechanical force of mashing breaks open the cell walls and cell membranes, releasing the cellular contents including DNA. The enzymes naturally present in strawberries (pectinases and cellulases) help break down the cell wall material.

Filtering the lysate: The filter paper traps large pieces of cellular debris (broken cell walls, proteins, and other solid materials) while allowing the liquid filtrate containing dissolved DNA to pass through. This step is crucial because it removes unwanted material that would interfere with DNA precipitation.

Adding isopropyl alcohol: DNA is soluble in water but not in alcohol. When you add isopropyl alcohol, the DNA becomes insoluble and precipitates out of solution. Because you're extracting DNA from thousands of cells at once, enough DNA strands clump together to become visible as white, stringy material.

Safety considerations

Key takeaways