• Collect DNA is Like a Zipper worksheet for individual assessment.
• Assess students’ journal responses for understanding of replication.
• Check models to assess whether they include at least these three steps of replication: 1. DNA strands unwind, 2. Complementary base pairing occurs on each strand, and 3. Bases bond to form the backbone of each new strand. Models should label the steps and show the steps in the correct order.
• Assess students’ answers to the questions at the end of Part 2.

Background Knowledge: Students must understand the cell cycle and meiosis in order to grasp the connection between replication and heredity. In order to grasp the content in this lesson, students must also have an understanding of the structure of the DNA molecule and base pairing rules. Review DNA structure and base pairing rules with students before this lesson if needed.

Part 1

Have students draw two circles and label one “prokaryotic cell” and the other “eukaryotic cell.” In each cell, have students sketch where they think DNA is found.

Show students the transparency of DNA in Prokaryotic and Eukaryotic Cells, or use a document camera to project it (S-B-5-2_Prokaryotic and Eukaryotic Cells.doc). Tell students, “Most prokaryotes have a single circular DNA in the cytoplasm that carries all of the cell’s genetic information (they don’t have a nucleus). Eukaryotes have up to 1000 times as much DNA as prokaryotes, and the DNA is found inside the nucleus in the form of chromosomes. Chromosomes are made up of DNA and proteins packed together to form a substance called chromatin.”

To give students an idea of how much DNA fits into each cell, show students the 1-m string. Draw a circle on the board with a diameter of 1 mm. Explain that the string represents a DNA molecule that needs to fit into the “cell” on the board. Ask students, “How do you think the DNA molecule is able to fit into the cell? Why is it so long?” (It is wound tightly into coils that allow it to fit. It is so long because it carries all of the genetic information for organisms.)

Explain that the goal of the lesson is to describe the process of DNA replication, which is a crucial process without which cells could not grow or reproduce to pass on genetic information. Define replication. Tell students that replication is able to make an exact copy of all the information on an entire long strand of DNA.

Have students read about the process of replication and answer the questions on the DNA Is Like a Zipper handout (S-B-5-2_DNA Zipper and KEY.doc).

Review the steps of replication using colored yarn. Have a student hold up two strands of the same color together to represent the original DNA. Then, the student should hold up the ball and pull the strands apart next to the ball. Have another student help by holding up two strands of a different color, one next to each original strand. Remove the ball, and show the class the two new DNA molecules. Connect them with a paper clip to represent the chromosome pairs until they separate during cell division. Ask students to describe how this demonstration is similar to and different than real replication.

Assign students to small groups. Tell students that they are going to create a paper model of DNA replication in class tomorrow. Explain that today they will be planning the model and creating a sketch of their design on a blank sheet of paper. Make sure students include a list of the materials they will need.

As a journal prompt or homework assignment, have students record their plan for creating a model of replication. Sample journal response:

The first step in DNA replication is the separation of complementary strands. We can represent this in a model by having a portion of the DNA molecule contain strands that are physically disconnected. The next step in replication involves the pairing of free nucleotides to their complementary bases in the separated strands. This could be modeled by showing free nucleotides with complementary ends approaching base pairs on one of the separated strands. The third step in replication involves the bonding of the free nucleotide bases to each other. The sugar and phosphate groups bond to form the backbone of the new DNA strand. This could be shown in the model by the formation of a double helix strand.

Part 2

Have students reconvene in their groups from the previous class period. Make sure that students understand that their models need to represent all of the steps involved in DNA replication. Have them refer to their sketches from the previous class period.

In their groups, have students create paper models of DNA replication and label the steps represented in their models using paper, colored paper, glue, and markers.

To conclude the lesson, have students write answers to the questions, “How does replication result in transmitting genetic information?” and “How can replication conserve (save) genetic information?” Discuss how replication results in the transmission and conservation of genetic information.

Extension:

• For students who need extra assistance, provide students with leading questions that will help them focus on the main ideas of the reading. If possible, use a ball-and-stick model of a DNA molecule to illustrate the steps in DNA replication.
• For students who need extra assistance, give students copies of the DNA Pieces handout to cut out and paste together in their replication model (S-B-5-2_DNA Pieces.pdf). It may be helpful to have them color-code each of the four nitrogenous bases, the sugars, and the phosphates before assembling the DNA model. Guide the students step-by-step as they get started to assist them with how to construct the model.
• Encourage students who have trouble with handwriting to contribute to the design of the DNA model verbally, having other team members sketch out their ideas.
• For students who are working beyond the standards, assign an alternate modeling activity:

o   Materials: modeling clay, pipe cleaners, paper clips, push pins, blank paper, markers, scissors, and glue.

o   Assign students to small groups, and then have them plan to create a three-dimensional model that represents the steps of DNA replication. Show students the materials they have available, and then allow them about 15 minutes to plan their model or poster. Check student plans, making sure that the models can be created during the class period. Then, have students create their models.