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Lesson Plans
The Ladder of Life
OverviewProcedure for teachersStudent Resources and Materials

Prep for Teachers

Prior to the teaching, bookmark all of the Web sites used in the lesson. CUE the videotape to the section of the tape entitled, "Mechanism of Replication." There will not be an audio cue at this point of the tape.


Cut elastic into two-foot strips and mark every inch in a different color on one half of the elastic strips. Duplicate Lab Activity Sheet and set up lab materials before students begin lab activity.

When using media, provide students with a FOCUS FOR MEDIA INTERACTION, a specific task to complete or information to identify during the viewing of the video segments, Web sites, or multimedia elements.

Introductory Activities: Setting the Stage

Beginning the Building

All the information needed to sustain life is contained in the molecule deoxyribonucleic acid, or DNA. All living creatures – from bacteria, viruses and fungi, to plants and animals and humans – use this universal instruction language. In humans, sperm and an egg join forces to assemble the unique instruction booklet for the newly forming embryo. DNA directs the formation of every single cell in the body from conception to death, and coordinates all functions of every organ system, every tissue, every cell and every molecule.

From the moment of conception, DNA spells out many physical traits, such as eye and hair color. It is therefore the molecule by which all life forms inherit their specific traits.

For all the complexity of life, this compound is deceptively simple in structure. DNA is composed of four chemical bases. These bases line up to form slender strands. These long strands associate together in pairs that coil around each other to resemble a twisted ladder. Within the tiny cell nucleus, each long twisted ladder is folded compactly around other molecules.

Each person's DNA sequence contains their genetic instructions, the information that directs all their biological processes. Ninety-nine percent of the DNA between two individuals is identical. But certain variations in a person's DNA many cause him or her to develop a genetic illness or to be susceptible to a disease. By deciphering the sequence of human DNA, scientists can gain new insights into how DNA misspellings cause these illnesses, thereby allowing them to develop better treatments and even cures to a disease.

Distribute (or have set up on tables) the tongue depressors, glue gun, elastic, and 4 colors of markers listed in Materials for groups of two students. As students enter ask them to sit in groups of two.

Let's begin building the DNA...

Step 1:

Explain to your students that in front of them are all of the materials needed to create their own model of a strand of DNA.

Step 2:

Tell you students that one of the pieces of elastic has already been encoded with half of the DNA information to create a specific DNA molecule. The colors on the elastic represent the nitrogen bases that make up DNA. Each nitrogen base (color on the elastic) will "match" with only one other nitrogen base (another color).

Step 3:

Explain that each colored block on the one strand represents four specific nitrogen bases that make up a strand of DNA. These molecules can only be combined in specific sequences.

Step 4:

Explain to the students that with the information that is given on one strand, they are to try and figure out how they can create the second half of the DNA strand on the other strand of elastic. Ask your students to think about how they are going to do this task and do it correctly.

Step 5:

Let your students discuss this task for a few minutes. They should all come up with the answer that is impossible to complete the task without more information.

Step 6:

Explain to your students that they are going to be given an opportunity to try their hand at this task again once they think they have the necessary information to do it.

Step 7:

Insert the videotape in the VCR that has been cued to the beginning of the video where the narrator says, "Nucleotides, the building blocks of DNA," and the visual on the tape is showing five small molecules against a black screen. Provide students with a FOCUS FOR MEDIA INTERACTION, asking them to explain the three main components of a nucleotide. PLAY the video until your hear the narrator say, "It is the base unit that differentiates the four base units of DNA." PAUSE the tape. (Note: You will be stopping the video several times and refocusing the video each time with a new question.) Ask the students to raise their hands and state the answer to the question (three components – phosphate group, sugar group, and a base). Ask the students to take a moment and identify these parts on the existing strand in front of them. (Students will be able to identify the outer part of the elastic as the sugar and phosphate group and the colored marks as the nitrogen base.)

Provide the students with a FOCUS FOR MEDIA INTERACTION, asking them to name the four nitrogen bases and the pairs they form. RESUME PLAY and STOP when the narrator says, "…is called the base pairing rule." The visual on the screen will flash the words BASE PAIRING RULE. Ask the students the names of the bases and their respective pairs (adenine, guanine, thymine and cytosine; the pairings are thymine with adenine and guanine with cytosine).

Provide the students with a FOCUS FOR MEDIA INTERACTION, asking them how each nitrogen base can fit so well with the other nitrogen base. RESUME PLAY and STOP when the narrator says, "…a whirl of molecular activity." Ask the students what was unique about each nitrogen base coming together. (Each nitrogen base has a specific opening allowing only its complementary base to pair with it, like a puzzle piece.)

Say to your students, "Do you think you have enough information to complete your model?" Student responses will vary. Some will say they have a basic idea of what to do but they don't know what each colored section represents.
Tell them that the Blue sections on the elastic strip represent Adenine. The Red sections represent Thymine. The Green sections represent Guanine. The Purple sections represent Cytosine. Say to your students, "Now do we have enough information?" When they respond yes, instruct them to return to their groups and work stations and give them about 15 minutes to create the second section/half of the DNA strand.

When they have completed the other half of the DNA strand, instruct them to place a bead of hot glue on one side of a tongue depressor and place one end of each colored strip on the bead of glue side by side evenly spaced. Then instruct them to place hot glue on the side of the second tongue depressor and sandwich the elastic strip between the two tongue depressors. Have them repeat the process on the opposite end. (It will look like a ladder.)

When all the groups have completed the construction of the DNA model have each member of the team take one end of the model and stretch it out until it is tight, but not too tight. Have each member of the group start twisting/turning the tongue depressor in the opposite direction of his or her partner until the elastic coils itself up into a knotted ball. When this happens explain to the class that this is really what DNA looks like in its natural form.

Next ask your students, "Why do you suppose examining DNA is helpful in the field of forensics or medicine?" Students may give response such as it may help solve crimes or identify diseases. Respond to the students by saying, "Let's look at specific ways that DNA extraction and analysis have been helpful in science."

Learning Activities

Web Based

Step 1:

Assemble the students into groups of four. Instruct the students that after a given amount of time on the computer they will be getting into their groups to share answers. Distribute the Learning Activity Sheet to each student. Now ask the students to log on to the Web site that you have already bookmarked. Provide the students with a FOCUS FOR MEDIA INTERACTION by asking them to answer the questions on the Learning Activity Sheet. Students will use the areas entitled, "Molecules of Genetics" and "Genetic Organization and Control" on the Web site.

Step 2:

Have students work for about 20 minutes and then stop them. Have students come back into their group of four and compare answers.

Step 3:

Ask all the students to log on to the site and explain how they got their answers. Answers should be similar to the following.

Slide #15
Another name for a polymer is "polypeptide."

Slide #16
This describes there are 20 different amino acids. The conclusion scientists made from mutating bread mold was that the mutated mold could be restored by giving it specific supplements. The students should have that one gene creates the production of a protein.

Slide #17
The enzyme responsible is DNA polymerase.

Slide #18
This reveals the answer to RNA converting to DNA as reverse transcriptase. Those viruses having this to occur would be called retroviruses.

Slide #19
RNA is found to not be as stable as DNA because it is easily damaged by enzymes.

Slide #27
When you click on the "Problem" activities, it reveals the answers. You should have all the answers to these questions already.

Slides #29-31
The answer to the question would be that hydrogen bonds are broken and DNA dissociates into single strands. The bonds can be restored by reducing the temperature below 25°Celsius.

Slide #32
This reveals that DNA jumping into another position on a different chromosome would be called a "transposon."

Slide #34
The main function for splicing human genes into E. Coli would be for the manufacturing of human insulin and growth hormone.

Cross-Curricilar Extensions

Step 1:

Explain to your students that we have discussed what DNA looks like with all of its components now it is time to actually apply extraction techniques to view DNA. DNA is found in many forms in nature. We are going to view DNA from wheat germ.

Step 2:

Before students arrive for this activity, prepare lab materials and set up at tables as they are described in the Materials section. Have students resume the groups of four used for the Internet activity.

Step 3:

Distribute Lab Activity Worksheet to each of the students (see Student Materials). Instruct the students to follow the steps on the worksheet. This lab is designed to use simple ingredients and techniques to ensure results. If students have difficulties seeing the DNA, there should be sufficient quantities to repeat the experiment. All students should view DNA on their own slides. When all have successfully completed the activity, have each one show you and their peers their slide.

Cross-Curricilar Extensions

Take students to the library or research relevant Internet sites on breakthroughs in the Human Genome project. Scientists are mapping genes for such diseases as cancer, multiple sclerosis, Parkinson's disease, Alzheimer's, heart disease, and many other genetically inherited disorders internationally. After reading the articles, students could predict the outcomes of such technology for the year 2100. Relate this to vaccines and medicines that were not around 100 years ago.

Students can research the following scientists and write a one-page report on their life and contributions to the concepts discussed in this lesson.

Barbara McClintock
Christiane Nusslein-Volhard
Mario Capecchi
Michael Wigler
Harold Varmus
Jim Darnell
James Watson
Stanley Norman Cohen
John Craig Venter
Jacques Lucien Monod
Francis Collins
Eric Wieschaus

Community Connections

  • Have students predict the impact of technology on their future. Have them place their ideas in a time capsule to be opened in ten years by another class to see if their predictions have come true.
  • Invite a toxicologist/coroner to speak to your class about the use of DNA profiling to assist in solving murders, origins of death, and to identify possible assailants in criminal cases.
  • Invite a genetic engineer into your classroom to share recent research being performed. If possible have them bring in equipment to show how the process of electrophoresis gel is used to analyze DNA.
  • Write your county prosecutor's office to find out how DNA evidence is being presented to defend or prosecute a defendant in a criminal case.
  • Visit your state police forensics lab to learn how DNA extraction is used in solving crimes.