Mr. Nelson     Biology I         50 Minutes

Objectives:

The student will identify the key components of DNA replication (Biology,5.a)

Materials:

Whiteboard, whiteboard markers, paper, writing utensils.

Bell work:

Review previous days lesson

Set: 

At one point we were all small children or babies. How did we get the way we are today? We grew. How do we grow? What is happening in our bodies? How did we turn out the way we did? Have them think and write down ideas for a few minutes. Explain that like most things that have to do with our bodies, growth begins at the cellular level. But do our cells just grow bigger and bigger or do we grow more cells? Maybe draw a person on the board to show this. Right, today we will be talking about DNA replication. Why might DNA replication be important? Explain its importance as it relates to the students...state objectives.

Procedure:

1. Draw a DNA molecule on the board (or have one for an overhead)

2. So here we have our DNA molecule, (A-T, C-G). Quickly review the structure (deoxyribose sugar phosphate backbone (covalent bond), nucleotides in the middle (hydrogen bond))

3. Explain that the parent molecule has two complementary strands of DNA. Ask the students to think about how replication might occur for a strip of DNA and have them write down their ideas. (5 minutes or so, really make them think about it) The first step is the separation of the two DNA strands. Each separate strand now serves as a template or “key” for the order of new nucleotides that come in. To complete replication, the nucleotides are connected to form a sugar phosphate back bone.

4. Briefly explain that the process of body cell replication is named mitosis and that in mitosis one cell divides into two cells that have identical genetic information.

5. Ask how much of the original genetic information of the parent cell is transferred on to the daughter cells. (1/2, each gets one strand). Explain that this theory is called the semi-conservative model (write on overhead) which states that each of the two daughter cells will emerge with one old strand and one newly made strand. Contrast this model to the conservative (The parent cell keeps both original strands) and dispersive models (all strands have a mixture of original and duplicate DNA). The dispersive and conservative models were disproved in the 1950s, so the semi-conservative model is the accepted model.

6. All right, so lets take a closer look at the process of DNA replication. Ask the students to turn to page 188 and follow along. Remember that the first step of DNA replication is the separation of the two nucleotide chains. The spot where the two chains separate is named the replication fork. The separation of the two DNA chains is initiated by enzymes called helicases. Explain that the helicases break the weak hydrogen bonds that were talked about earlier. This separates the two chains.

7. The next step involves enzymes called DNA polymerases. These enzymes move along the separated chains and make a new chain using the original as a key. The nucleotides for this process are found in the surrounding environment. (sugar-phosphate =covalent, nucleotide attached using hydrogen bonding).

8. Stress that it is essential that replication be correct. (hear through your nose and smell through your ears). Also talk about how it occurs all over the cell to speed things up. (show them what it might look like on the board).

9. Review the process. Ask the students to take out a blank sheet of paper and tell them to explain the process of replication using replication fork, helicases, hydrogen bond, DNA polmerases and semi-conservative model.  Collect

Closure:

Great job today. Quick Q and A. Lead into protein synthesis. What does DNA code for? How does it get expressed? Tomorrow we will discuss this as we begin to think about protein synthesis.

Assessment/Evaluation:
Objective: The student will identify the key components of DNA replication (Biology, 5.a )

Formal: The student will complete a written explanation (M) of DNA replication that will highlight the main concepts (C). This assignment will be graded and recorded in the grade book (D).

Informal: The teacher will observe student progress (M) in concern to the process of DNA replication (C) throughout the class period.