Dna key Concepts

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Lesson III: DNA

Key Concepts:

Double-stranded DNA

Base pairing

Base sequence

Genetic Code

Materials needed:

DNA model

Pieces of paper for gene guessing

  • Bottles of water

  • Small Dixie cups

  • Test tubes

  • Detergent/Salt Solution (200 ml distilled water, 3 tsp salt, 3 tsp clear liquid soap---stir until well mixed)

  • Transfer Pipettes

  • Microcentrifuge Tubes

  • Hemp/string for bracelets

  • Coffee sticks (8” best)

  • Markers to label tubes

  • Tube Rack

  • Isoproponal/rubbing alchohol (>90%, not 70%) on ice (could put dry ice and then wet ice) to keep cold

Today we are going to discuss the genetic material or DNA, deoxyribonucleic acid (write on the board and then have children say this). DNA is referred to as the blueprint for life. Show model of DNA and ask what kind of structure is this? Helix. The building blocks of DNA are deoxyribonucleotides. Deoxyribonucleotides are sugar molecules (deoxyribose) connected to a phosphate group and a base. Thousands of these are joined end-to-end to make up the backbone of the DNA molecule. Two backbones coil around each other in a double helix, with the sugar phosphate part on the outside of the helix and the bases on the inside.

There are 4 different bases (see appendix, page 24). They are adenine (A), guanine (G), thymine (T) and cytosine (C). Write on the board. Show model and discuss how bases are paired. Base-pairing has two rules: guanine always pairs with cytosine and adenine always pairs with thymine.

Activity: Ask volunteer student to read the sequence on one strand of the model and you write it out on the board. Ask children in class to give letter for opposite strand. Then have another volunteer read the complementary strand to check if it is correct. Discuss base pairing rule.

Activity: To illustrate how differences in a DNA sequence can alter what a gene encodes, write out a hypothetical sequence for a tongue rolling gene, big R. Write out a similar sequence but change one of the bases as an example of the alternative allele, r. Tell students how the change in the DNA sequence between the two forms of the tongue rolling gene lead to a difference in genotype which then affects the phenotype, i.e. ability to roll the tongue. The sequence of DNA is usually constant being duplicated and passed to new daughter cells after mitosis or meiosis. However, sometimes mutations can occur which cause a change in the DNA sequence.

DNA has information. But if you look at a sequence of DNA ATCGTTCAAA etc. how do you make sense of it? When DNA was found to be the genetic material in 1953, it took scientists another 9 years to figure out the genetic code. You will learn about this code in high school. Knowing the code, allows people to know the protein that eventually will be made with the genetic information from each gene.

Make analogy with reading and the alphabet. First learn letters, then put together for words. Then write 4 letters and ask them to compose words with letters (act,cat,chat, hat). With words we can make sentences, then paragraphs, then stories and a book. The genetic material uses the bases like letters and these are put together in a way that has information that can be read by the cell. This information determines what traits you will have.
1. Who Ate the Cookies Activity

Pass out two strips of paper for each student and a pencil if they need it. Tell a story.

There were a plate of cookies on the table and an hour later they were gone with only a part of a cooking remaining. Let’s pretend that someone at this table ate the cookies. We want to determine who ate the cookies. It turns out that the piece of cookie remaining had a little saliva with cells from the person eating the cookie. Let’s pretend that we can isolate DNA from those cells and determine the sequence of the DNA.

Ask the children to write on their strip of paper a random sequence of 15 bases and write the complementary sequence below it. Then ask them to write the exact same thing on the second piece of paper. Collect one of the papers, fold, and put in bag. Ask students to pass other piece of paper to their neighbor on the right or left. Pick one sequence out of the bag, read sequence and ask students to see if their sequence matches what is being read. Identify who ate the cookie in a joking manner.
Activity students: How do we use DNA sequence information? (helped people get out of jail that were wrongfully convicted, can tell story of Sally Hennings and Thomas Jefferson-Y chromosome)

[with 15 bases, have a billion possible combinations 4x4x4…. (4)15]

2. Isolation of DNA and Creation of DNA Bracelet

Pass out to each students: cup with 6 ml of water, string for bracelet, and wooden scrapper.

Tell children we will be isolating DNA from their cheek cells.

  • First we will scrape the inside of their cheek with a stick to dislodge the cells on the surface. Scrape vigorously to get more DNA. Then Swish water around their mouth making sure not to swallow and pour back into cup. Demonstrate yourself first and then have students do the same. Then pass trash can for sticks.

  • Next pass out test tube with 1 ml of soap/salt solution and explain that soap breaks open the cells and salt allows the DNA to come together so it can be seen as we proceed. (salt neutralizes the negative charge that the DNA has allowing it to aggregate).

  • Have students pour contents of cup into the tube. Have student put the cap tightly on the tube and mix the tube.

  • Then have students remove the cap and slightly tilt the tube. You will slowly pour down the side of the test tube the ice cold 95% rubbing alcohol (fill transfer pipette). Have students hold their tube upright and after 3 minutes should start seeing the DNA in the upper layer which is the water layer.

  • Students should be looking for the white strands of DNA.

  • Taking rack with eppindorf tubes, use the same transfer pipette, to remove students DNA and place in eppindorf tube. Place string on tube and cap so string is attached. This is their bracelet.

  • MOVE quickly and work with 2 people if possible.

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