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- Students will know that genetic material is contained in DNA.
- Students will demonstrate that all organisms contain DNA.
- Students will demonstrate that DNA is similar among all organisms.
- Students will demonstrate that DNA is contained within cells.
- Students will extract the DNA from a banana and from their own cheek cells. This gives them a chance to see a physical representation of the basics of the DNA that they have been studying.
- Students will compare and contrast the DNA that they extract from the banana and from the cheek cells.
- Banana mixture (1 large banana blended into 1 to 2 cups of water until the banana is no longer chunky), one mixture per 30 students.
- Salt (several tablespoons per 30 students).
- Clear dish detergent (approximately 1 cup per 30 students).
- Chilled isopropyl ethanol.
- Plastic spoons (1 per group).
- Plastic pipettes (2 to 3 per group).
- Plastic or paper cups (2 per group).
- Coffee filters (1 per group).
- Test tubes (2 per group).
- Stirring rod (1 per group).
- Beakers (1 per group).
- Copies of handouts (1 per group).
- This activity went fairly smoothly. If the students do not keep their tubes still, the DNA layer can be difficult to find (or it may become mixed into the other layers so that it no longer exists). During the “waiting” period while the DNA was forming a layer, some of the students became distracted and bored. Overall, the students liked that they could actually see the DNA. It made it more real to them and less of an abstract concept. Also, I think that they liked the idea that science can be done with simple household items, and does not have to be overly complicated or technical.
- Engage: Ask students what DNA is and what it does. Make a list of the responses. Discuss the responses as a class.
- Explore: Put students into groups of 2 or 3. Give each student a picture of a strand of DNA. Have the students describe the strands. Also, have students make a list of the elements that they think are part of DNA and a list of their body traits and functions that are controlled by DNA.
- Explain: Give the students a short lecture on the structure and purpose of DNA. Discuss their lists of elements and traits at the end of the lecture.
- Elaborate: In their small groups, have students complete the DNA extraction activity, and complete the associated worksheet.
- Evaluate: Have students write a paragraph about what life would be like if DNA replication was a more complicated process or if fundamental processes were not conserved.
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- Students need to be familiar with the following concepts before completing the activity (this may be accomplished through a short lecture prior to beginning the activity): 1) DNA, 2) Cell membranes (information not included in lecture), 3) Solubility (information not included in lecture).
- DNA is the basis of genetics and heredity. However, for most of us, it is more of an abstract concept than a physical molecule. This activity gives students the opportunity to actually see some DNA and to compare the appearance of DNA between bananas and their own cheek cells. It uses common household items, which will hopefully help to make science seem more accessible to the students.
- DNA and RNA belong to a class of organic macromolecules known as nucleic acids. DNA and RNA are the only naturally occurring nucleic acids. Nucleic acids are generally very large molecules. DNA stands for deoxyribonucleic acid. RNA stands for ribonucleic acid. DNA and RNA are very similar, but DNA has one less hydroxyl group (hence the deoxy part of the name).
- Nucleic acids are composed of smaller parts known as nucleotides. Each nucleotide is composed of a backbone made up of a pentose sugar group (containing 5 sugars) and a phosphate group. The phosphate group and the sugar group are connected by phosphodiester linkages. Attached to each backbone is a nitrogenous base. There are 4 bases in DNA: the purines, adenine (A) and guanine (G); and the pyrimidines, thymine (T) and cytosine (C). Pyrimidines are heterocyclic aromatic compounds containing two nitrogen atoms in the six member ring. Purines are also heterocyclic aromatic compounds with a pyrimidine ring fused to an imidazole ring. Purines contain four nitrogen atoms, two in the pyrimidine ring and two in the imidazole ring. In RNA, the purines are the same. However, uracil is used instead of thymine.
- These bases are the only four bases used in DNA, and they are the same for all organisms. These four bases contain the information that cells need in order to develop, grow, and function. These bases are important because they determine how DNA forms. denine always bonds with thymine (or uracil in RNA) and guanine always bonds with cytosine. The predictably of the bonds allows the DNA to replicate itself accurately many, many times. When the DNA strands unravel, a complementary strand will match up to the existing strand. This is a very simple mechanism, but the simplicity is critical because it allows the DNA to replicate while making very few mistakes. This allows information to be passed from one generation of cells to the next.
- DNA generally exists as a double helix. This means that two strands of DNA are bound together at the nitrogenous bases (the bases bond via hydrogen bonds). The phosphate/sugar backbones run down both sides of the double helix and the nitrogenous bases are in the center joining the two strands together. The bases join predictably (A to T/U and G to C).
- DNA is not actually a very complicated molecule. However, it is very important. It contains all of the instructions necessary for the development and functioning of all living organisms. It also allows that information to be passed from one generation of cells to the next, within an organism and from parent to offspring. The DNA in all organisms from bacteria to plants to people is the same and contains only 4 bases. DNA controls the day to day workings of the cell. It also controls things that we see like hair color, sex, skin color, eye color, and height (although that is also influenced by nutrition). DNA is also a part of many diseases. Some diseases are passed directly from parent to offspring (like sickle cell anemia). In other cases, the parents pass along a genetic susceptibility to a disease (like high blood pressure or breast cancer). In these cases, a person who is susceptible may or may not develop the disease, and this will probably have quite a bit to do with lifestyle choices and environmental factors. This is why it is important for doctors to know about a patient’s family history when trying to understand the diseases that a patient has or is likely to develop (or is likely to pass on to his/her children).
- DNA is important and similar in all organisms, but it is still capable of creating the huge diversity of life that we see. All DNA uses only four bases and has the same double helix shape. The diversity of life that we see is caused by changes in the order of the base pairs in small sections of the DNA. In some ways, this is very important and makes a lot of sense. Some processes, like cellular respiration and division are necessary for all cells in order to survive and reproduce. These processes are absolutely fundamental to life, so having a lot of variability in those processes would not be desirable. These processes are complicated and essential, so evolution has conserved them through millions of generations.
- This activity was modified from the NOVA science NOW “Extracting DNA from Bananas” activity (found at PBS.org) and the “DNA Extraction from Human Cheek Cells” written by Jessica McLemore and Krista Botton (found on the website of California State University in Northridge).
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The Shape of DNA
DNA is a Nucleic Acid
DNA Controls Skin Color
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- Time Frame: Designed to take up 55 minutes of a 90 minute period.
- Biology Standard 1a: "Students know that cells are enclosed in semi-permeable membranes that regulate their interactions with their surroundings."
- Biology Standard 4d: "Students know that specialization of cells in multi-cellular organisms is usually due to different patterns of gene expression rater than to differences of the genes themselves."
- Biology Standard 5a: "Students know the general structures and functions of DNA, RNA, and proteins."
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