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
Introduction
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 from generation to generation 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. Sometimes the change can have a deleterious (bad) affect.
1
If you look at a sequence of DNA ATCGTTCAAA etc. how do you read it and make sense of it?
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.
ACTG----genes ---traits/phenotype
CAHT----words-----sentences---book
It turns out that there is a genetic code that is used to "read" the DNA. You will learn about this code in
high school. Knowing the code, you will know what protein can be made from a gene.
Pass out two strips of paper/student for the activity (before split in groups)
1.
Who Ate the Cookies Activity
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 person took a bite out of piece of cookie remaining and there was some saliva with cells left
on the cookie. We were able to extract DNA using a similar procedure that we just used for isolating DNA from
cells inside your cheek. Using the DNA we were able to obtain DNA sequence from the DNA of that person.
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)
SPLIT in Groups
2.
Isolation of DNA



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.
We will give them a cup with about 6 ml of water. Swish water around their mouth making sure not to
swallow and pour back into cup. Give each student a test tube that is labeled with a number that has 1
ml of detergent/salt solution. Have them 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. During the 3
minutes explain that detergent is used to break apart the cell membrane and release the DNA. The salt
neutralizes the negative charge that the DNA has allowing it to aggregate and precipitate so it can be
seen. Show students a tube that is showing the white strands of DNA.
2
Let students start loading agarose gels and later go back to pick up DNA with transfer pipette and transfer to
eppindorf tubes with the same numbers so students get their own DNA. Make DNA bracelet by attaching a
piece of hemp or string before closing cap of tube (teacher and one TA could be doing this while students
loading gel)
3