After this activity, students should be able to:

• Explain that certain DNA sequences code for specific characteristics.
• List several types of engineers and engineering technologies that rely on DNA sequences.
• Investigate basic gene sequences to determine the genotype and phenotype of an individual.

We have all heard about DNA, but what exactly is DNA and why is it important to us? DNA stands for deoxyribonucleic acid and is made up of billions of biochemicals. DNA is the genetic material for all living things — this means that you, me, flowers, dogs, elephants and even viruses contain DNA. You can think of DNA as the "recipe" for living things — it provides the instructions for every part of the organism. In humans, 99.99% of our DNA is exactly the same as every other person's. Why is there a 0.01% difference? This small amount of DNA is what determines our physical differences such as eye color, hair color, height, etc. Even though our DNA is almost all the same, every single person (except for identical twins) has a unique DNA "recipe."

Where in our bodies is DNA located? DNA is stored in the nucleus of each cell where it is best protected from damage. Each nucleus contains 23 pairs (23 from your mom and 23 from your dad) of DNA, called chromosomes. This DNA is folded over and over into VERY small bundles — much too small for the human eye to see.

DNA is organized into shorter segments called genes. Think about a gummy candy worm as the entire strand of DNA, but each colored segment is a different gene. Genes are specific sequences of DNA that code for certain characteristics. The DNA sequence is called the genotype — this is the recipe — and the characteristics are called the phenotype — this is the cake!

DNA is made of four biochemicals called nucleotide bases (or just "bases"). Think of these as the ingredients in the recipe. They are: adenine, thymine, guanine and cytosine. To make things easier, people usually abbreviate these as A, T, G and C. These four bases pair with each other in a very specific way: A always pairs with T and G always pairs with C. One gene usually contains 10,000 to 15,000 base pairs!

DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone. click for copyright

Why is it important to understand genes and base pair sequences? Have you ever heard of color blindness, Down syndrome, cystic fibrosis or hemophilia? Well, biomedical engineers work with others in the scientific and medical fields to help improve health care and quality of life. They study DNA to help us understand genetic disorders like these. As engineers develop technologies to recognize certain DNA mutations and where they are located, they work with geneticists to diagnose, treat and prevent these disorders.

Genetic engineers study genes and DNA to understand things like DNA replication, cloning and genetically-modified organisms such as food and crops. Genetic engineers have helped us advance our crop technologies and make synthetic (artificial) insulin for people with diabetes.

DNA can also identify people — even better than fingerprints. DNA is found in all of our cells: hair, teeth, bones, blood and saliva. We can leave our DNA behind when we drink from a cup, use a toothbrush, shed hair or cut ourselves on something sharp. Because of this, DNA is used for "DNA fingerprinting" — or describing the unique DNA recipe for a person. Even 0.01% difference is enough to distinguish one person from another when it comes to collecting evidence from a crime scene.

Using DNA in a crime investigation does have its limitations. The probability of laboratory error or contamination — errors made when collecting and running the DNA samples — must be factored into the results. It is always best to consider DNA fingerprinting along with other evidence. Biomedical engineers create the tools, equipment and processes to accurately collect and examine DNA evidence for crime and paternity cases. They are always working to make the laboratory errors fewer and the machines for identifying the gene sequences more accurate.

Today, we are going to practice determining the phenotypes (physical characteristics) of persons from their DNA. We are going to work together to make models of human DNA and swap them with each other to decode. Like biomedical engineers, let's break down DNA gene sequences into individual traits to describe the people to which the DNA belongs.

Background

Before the Activity

With the Students: Part 1

1. Divide the class into groups of two students each.
2. Hand out supplies to each pair of students: 1 plate, ~25 toothpicks ~30 gumdrops, 1 DNA identity card and 1 color key.
3. Explain that the color key contains the three-base genotypes that code for certain phenotypes (physical characteristics).
4. Explain that the DNA identity cards contain the names and physical characteristics of various people, different for every team. This is the person's DNA that the team will construct. Remind students to keep this person's identity a secret from the other groups (for now).
5. For each physical characteristic on the identity cards (phenotype), refer to the color key and have groups write down in columns the sequences of letters (genotypes, using A, T, G and C) for their persons. Then, have students write the corresponding base pairs in second columns.
6. Allow enough time (~15-20 minutes) for teams to build the strand of DNA for their persons. For DNA building tips, see Figures 1 and 2 and the next section.

Figure 1a-b-c. Students construct a gumdrop DNA strand. click for copyright

7. Once all groups have completed building, have them trade DNA strands, and by working backwards from the strand only (no peeking at the identity cards), each group should determine whose DNA they have (by referring to the possible identities shown with the overhead projector). Students really enjoy this "decoding" part!
8. Have students check with the original creator teams of the DNA strands to see if they determined the right DNA identities. Discuss with the class: How many groups were able to name the right identity for their DNA strands? What made decoding difficult?

Figure 2. Students show off their completed gumdrop DNA double helix model. click for copyright

Suggested DNA Building Steps

With the Students: Part 2

Groups may need to trade with other groups from their given gumdrop supply, as the gumdrop colors are different on each color key.

When connecting all the genes together, be sure to keep the genes in the correct order and orientation, or else they won't be able to be decoded by another team.

Have students research a specific genetic disorder and write a one-page summary about it, including a description of which chromosome is affected and the associated mutation. Examples include Down syndrome, color blindness, hemophilia and cystic fibrosis.

About R&D - Research & Development. Updated November 27, 2008. GlaxoSmithKline plc. Accessed March 2, 2009. http://www.genetics.gsk.com/kids/dna01.htm

Bachor, Kevin. Fun Facts (about DNA). The Best Detergent for Plentiful DNA Extraction, Cirque du Soleil, Ecole Nationale de Cirque, 2006 Canada Wide Virtual Science Fair. Accessed March 2, 2009. http://www.virtualsciencefair.org/2006/bach6k2/Funfacts.htm