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  • Subject area(s): Science
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  • Published on: 15th October 2019
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Introduction (What background information would someone need to know, what research have you found, why are you doing this lab in particular?)

On Friday, September 28, 2018, a group of students from the class of SBI3U0A had conducted a genetics inquiry lab experiment to extract, isolate, then observe the DNA of a strawberry and banana. The experiment consisted of making a solute using household items, which included, water, salt, dish soap, and isopropyl (rubbing) alcohol. By mashing up the strawberry or banana and adding it to the beaker with the solvent you can separate the DNA from the strawberry or banana- but how and why did this phenomenon occur?

When a source of DNA is needed by a scientist, it must be extracted from the cells that are being used or studied. The fibers of DNA store the information for the functioning chemistry of life. DNA is present in every cell of animals and plants. The DNA that is found in, for example, strawberry cells can be extracted by using common, everyday household materials. By using salt, it will break up the protein chains that bind around nucleic acids. The dish soap will play a role of dissolving the lipid (fat) part of the strawberry-cell wall and the nuclear membrane of the strawberry. This extraction solvent will help us access the DNA inside the cells. Since strawberries are soft, it will be easy to pulverize, while other fruits are just as soft and easy to pulverize, for example, a banana, strawberries are a perfect choice. There are two reasons behind that, the first being, strawberries yield way more DNA than any other fruit, and secondly, they are octoploid, which means that they have eight copies of each type of DNA chromosome, which no other fruit has. Human cells have two sets of chromosomes, meaning they are diploid, and because of these special circumstances, it makes extracting DNA from a strawberry easy to see and extract. To extract the DNA as effective as possible, it is important to remember to keep the isopropyl alcohol in the freezer because DNA is not soluble in the IPA. The colder the IPA, the less soluble the DNA, which is of course what you want. After the lab is completed, the long thick fibers you pull out of the extraction mixture would be the real strands of strawberry DNA which determines all its genetic traits of the individual organism.

To fully understand how the DNA is broken down, we need to understand what role each of these items plays in it breaking down.

Why do we use detergent?

 Detergent dissolves the lipids in the cell membranes and nuclear envelope, releasing the DNA into the solution

Soap molecules and the lipids (fats) in cell membranes are made of two parts: hydrophilic heads and hydrophobic tails

When detergent comes close to the cell, it captures the lipids and proteins (due to their similar structures) and breaks them apart

Why do we use salt?

DNA has a negative electrical charge which is due to the fact that the phosphate groups are on the DNA backbone and the electrical charge is what makes it soluble.

When salt is added to the mixture of the fruit, the salts (NaCl) positively charged sodium ions (Na+) get attracted to the negative charges of the DNA, which neutralizes the electrical charge of the DNA. This allows the DNA molecules, instead of repelling each other, to come together.  

Why do we use alcohol?

When molecules are soluble, they are dissolved in the solution and are thus, not visible. When molecules are insoluble, they start to clump together and precipitate, then becoming visual. DNA is, in fact, soluble in water, which is why it is not visible in the filtered solution.  

DNA is not soluble in high salt and alcohol solutions, so the addition of the alcohol makes the DNA precipitate.  

Molecules are less soluble at lower temperatures, so we chill the alcohol to get more of the DNA to precipitate

Our experiment focuses on extracting DNA from strawberries and bananas using common household items and how the DNA is extracted from that. We chose to use strawberries and bananas because they are both easy to manipulate and strawberries contain a large genome. To extract the DNA we chose the alcohol extraction method to help isolate the DNA and make it visible to the naked eye. According to what was stated in the introduction, strawberries have an octoploid genome, which basically means that the nucleus has eight complete copies of each chromosome, unlike the banana which has three. By extracting the strawberries DNA and comparing it to the banana’s DNA, it will help us understand the difference of an octoploid genome versus a triploid genomes DNA. Our prediction is that the strawberries DNA strands will rise to the top of the solution after the alcohol is added quicker than the banana’s will. We predict that because according to what was stated in the introduction, strawberries break the cell walls much quicker than any other fruit and strawberries also contain cellulase enzymes and pectinase, which assist in the degradation of cell walls.

After the strawberry was mixed in the solvent, the DNA started to form and rise on the top in a clear, kind of fizzy/bubbly layer on top, it arised 10 ml, which is 5 ml more than the DNA that was formed on the banana

Once the banana was mixed in the solvent, the DNA started to form quickly but stopped after rising 5 ml

the solvent with the bananas colors changed drastically in a short period of time, it turned a dark yellow on the bottom and a blue/green color on the top

the banana solvent fizzed and bubbled a lot more than the strawberry solvent did when the alcohol was added

The strawberry solvent took a while for the DNA to build up, but overall had more than the banana

The bananas fruit mixture was a lot thicker than the strawberries fruit mixture

his double bar graph portrays the difference of the amount of solution before the DNA was formed for each fruit and after 15 minutes, how much DNA was formed. We performed the strawberry extraction first, the DNA didn’t form right away, but after around 5 minutes, the process of the DNA forming accelerated. The mixture before the alcohol was added, was in fact, foggy, and we could tell where the DNA was forming because at the top of the mixture it had started to clear up. The fruit mixture was also fizzy and bubbly, so once the alcohol was added in, that also started to clear up. Comparing the extraction of the strawberry to the banana was actually quite different. The bananas extraction was much thicker and harder to mash. Also, when the alcohol was added in, the color of the solution changed drastically. The bottom changed to a dark yellow, and the top was a light blue or green. In about 5 minutes the DNA was formed, but after that, it didn’t form much more DNA. The solution of the banana was also a lot fizzier than the strawberries and didn’t fully clear up as the strawberries did. The strawberry formed more DNA due to the fact that the strawberry holds more DNA than the banana, as we learned that a strawberry is octoploid and a banana is triploid. Therefore, that is why the strawberry yield more DNA in the same amount of time the banana was given to build DNA.

Discussion: (connect your observations to what you mentioned in the intro; what possible scientific errors might be present in how you performed the lab)

In this experiment, the DNA was successfully extracted from the strawberry and banana, we were able to observe the visible buildup and strands of DNA. The DNA extraction was further facilitated by the eight copies presence of each chromosome, or you can say, an octoploid genome for the strawberry; and a triploid genome for the banana which had three copies of each chromosome. In contradistinction, humans are diploid, having a diploid genome, meaning they have only two copies of each chromosome. In the lab experiment, we degraded the strawberry-cell walls, nuclear membranes, and the cell membranes by mashing the fruits and mixing it in a solute of saltwater and dish detergent. The mashing of the fruit physically interrupted the cell walls, the salt provided optimum osmolarity, and the dish soap dissolved the membrane of phospholipid bilayers. Natural occurring of pectinase and cellulase enzymes promoted the cell wall degradation as well. By filtration, using a strainer, the large solids were separated, and the resulting solution was collected in a beaker. At the time, when the fruit DNA was dissolved in the water, it was a polar solvent as we learned in our past inquiry lab; and the sodium ions bonded with the DNA that was negatively charged via an ionic bond. After adding in the Isopropyl alcohol, the DNA led into the nonpolar layer, which then made the DNA visible to the naked eye. A possible error we may have performed in the lab would be, not giving the DNA enough time to lead into the nonpolar layer, affecting the results of how much DNA we collected.

Conclusions(Do you accept or refute your hypothesis and why; is there anything else we need to consider, other information we need to understand?):

We accept our hypothesis. In our hypothesis, Our prediction was that the strawberries DNA strands will rise to the top of the solution after the alcohol is added quicker than the banana’s will. We predicted that because according to what was stated in the introduction, strawberries break the cell walls much quicker than any other fruit and strawberries also contain cellulase enzymes and pectinase, which assist in the degradation of cell walls. By reffering to our results, the strawberries DNA was formed much quicker and produced more than the bananas DNA, just as we predicted.

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