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Bring Science Home

Find the DNA in a Banana

Bring Science Home: Activity 9



Kagen McLeod

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Key concepts
Cells
DNA
Genes

From National Science Education Standards: Reproduction and heredity

Introduction
What do you have in common with a banana? Even though we might not look alike, all living things—bananas and people included—are made up of the same basic material.

Just like houses are made up of smaller units such as bricks, all living things are made up trillions of microscopic building blocks called cells. Within an organism, each cell contains a complete set of "blueprints". These directions determine the organism's characteristics.

Background
If we could zoom in on a single, tiny cell, we could see an even teenier "container" inside called a nucleus. It holds a stringy substance called DNA, which is like a set of blueprints, or instructions. DNA contains a code for how to build a life-form and put together the features that make that organism unique. Segments, or pieces, of DNA are called "genes". In living things, such as us, each gene determines something about our bodies—a trait. In our DNA there are genes that are responsible for hair color, eye color, earlobe shape and so on. We get our DNA from our parents. Some characteristics, like eye color, are pretty much entirely determined by DNA. Some are determined both by DNA and by your environment as you grow up, like how tall you will be as an adult. And some traits are not very directly tied to DNA at all, like the kind of books you like to read.

Just like us, banana plants have genes and DNA in their cells, and just like us, their DNA determines their traits. Using only our eyes, we couldn't see a single cell or the DNA inside of it. If we remove DNA from millions of cells, however, we will be able to view it without a microscope. That is what we will do today!

Materials
•     Ripe banana
•     Half cup of water
•     Teaspoon of salt
•     Resealable zip-top bag
•     Dishwashing soap or detergent
•     Rubbing alcohol
•     Coffee filter
•     Narrow glass
•     Narrow wooden stirrer

Preparation
•     Place your bottle of rubbing alcohol into the refrigerator or freezer and let it chill for the duration of this experiment.
•     Peel a banana.
•     Put the peeled banana in a resealable zip-top bag and close the bag.
•     On a hard surface like a tabletop or kitchen counter, mush the banana in the bag for about a minute until it has a fine, puddinglike consistency and until all lumps are gone. Do not slap the bag or mash the banana too close to the bag's zip seal. (This could cause the seal to open and the banana to squirt out and make a mess.)

Procedure
•     Fill a measuring cup with a half cup of hot water and a teaspoon of salt.
•     Pour this saltwater into the bag, and close the bag. Gently mix and slosh the saltwater and mashed banana together for 30 to 45 seconds.
•     Add a half of a teaspoon of dishwashing detergent or dish soap into the bag. Again, mix around the contents gently. You do not want the mixture to become too foamy.
•     Place the bottom half of a coffee filter in a clear glass cup. The top part of the filter should be folded over the rim of the glass to keep it in place.
•     Carefully pour the contents of the bag into the filter and let it sit for several minutes until all of the liquid has dripped down into the cup. (You can now throw out the coffee filter and its contents.)
•     Take the rubbing alcohol from the refrigerator. Tilt the glass and slowly pour the alcohol down the side of the cup until there is a layer that is 2.5 to five centimeters (one to two inches) thick. You want to keep the alcohol and the liquefied banana as separate as possible, so complete this step slowly.
•     Let this two-layered mixture sit for eight minutes. During this time, what do you see happening between the alcohol and the banana liquid layer? It looks cloudy and may have some tiny bubbles in it. The longer you wait, the more defined this layer becomes. This is the DNA pieces clumping together.
•     Stick the wooden stirrer into the cup. Spin it in place so that cloudy layer spools around it. Remove the stirrer. Can you capture some of the stringy middle layer on your stirrer and remove it from the cup? The substance that you see on the stirrer is DNA!

Read on for observations, results and more resources.

Observations and results
The stringy substance that you see is DNA! It has been removed from the millions and millions of cells that make up the banana. All living things have DNA. The more similar and closely related two living things are, the more similar their DNA is. Every human shares 99 percent of his or her DNA with every other person. Furthermore, human DNA is very similar to that of other species. We share most of our genes, which make up DNA, with fellow primates such as chimpanzees and with other mammals such as mice. We even have genes in common with the banana plant!

In this activity each material plays a specific role in helping to extract the DNA from the cells. For instance, the detergent or soap helps to break down the cell's outer membrane, and the salt helps to separate the DNA from other materials in the cell. And because the DNA doesn't dissolve in alcohol, this substance helps the DNA clump together in a separate layer.

Share your banana DNA observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Cleanup
You can wash the bag and reuse it. Pour the banana liquid and alcohol down the drain and wash out the cup.

More to explore
"Can Science Save the Banana?" from Scientific American
"Bar Code of Life: DNA tags help classify animals" from Scientific American
DNA model activity from CSIRO's Double Helix Science Club
DNA Interactive site from Cold Spring Harbor Laboratory
My First Book about DNA by Katie Woodard, ages 9–12
Have a Nice DNA by Frances R. Balkwill, ages 9–12

Up next…
For the Birds: Best-Adapted Beaks

What you'll need
•     Tweezers
•     Cotton swab
•     Binder clip
•     Several different kinds of seeds, grains or nuts that differ in size and shape. It is best if you have a wide range: some that are tiny (for instance, grass seeds or couscous), some that are medium-sized (black-eyed peas or lentils), and some that are larger (almonds, cashews, walnuts or hazelnuts).
•     Timer with a second hand or clock
•     Dish
•     Paper
•     Pen or pencil

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