DNA Replication Read the title aloud to students.

Learning Objectives Explain the role of DNA polymerase in copying DNA. Compare DNA replication in prokaryotic cells and in eukaryotic cells. Click to reveal each of the learning objectives in turn. Read the objectives aloud or have a volunteer do so. Ask: What are some ways to make a copy of a page of the text? Sample Answers: by hand, by using a copier Ask: Why is it important to make an exact copy? Answer: so the information doesn’t change Explain that cells copy DNA in a process called DNA replication. Then lead a discussion in which you challenge students to apply this same thinking about copying pages to the process of DNA replication. Before a cell divides, its DNA must first be copied. Ask them to consider the following: How might the double-helix structure of DNA make that possible? What might happen if one of the nucleotides were damaged or chemically altered just before the copying process? How might this affect the DNA inherited by each daughter cell after cell division?

Write a statement about each word and how it relates to DNA replication Complementary S-phase Base pairing Template Semi-conservative Replication fork DNA polymerase Telomeres Prokaryote DNA vs. Eukaryote DNA Prokaryote replication vs. Eukaryote replication Histones vs. nucleosomes

Review of DNA Structure nitrogenous bases sugar-phosphate backbone Ask for volunteers to identify the different features of the DNA diagram. Click to reveal the correct answers. Ask: How are the two strands joined together? Answer: by hydrogen bonding at paired bases double helix

Eukaryotic Chromosome Structure

Copying DNA replication fork DNA polymerase Direction Direction of replication Direction of replication Walk students through the diagram to describe the process of DNA replication. Emphasize that DNA polymerase joins nucleotides to synthesize a new, complementary strand of DNA. Also emphasize the role of DNA polymerase: Besides producing the sugar-phosphate bonds that join nucleotides together, DNA polymerase also “proofreads” each new DNA strand so that each molecule is a near-perfect copy of the original. Ask: How is DNA unzipped at the replication forks? Answer: Hydrogen bonds are broken. One replication fork is labeled. Ask for a volunteer to go to the screen to point out the other replication fork. Click to reveal the correct answer. Ask students how the base-pairing rules play a role in building a strand of DNA that is complementary to the original, template strand. One DNA polymerase molecule is labeled. Ask for a volunteer to go to the screen to point out another region where nucleotides are being added to build a new strand. Click to reveal the label. Ask: What are the two roles of DNA polymerase in replication? Answer: DNA polymerase joins individual nucleotides to produce a new strand of DNA and proofreads the new strand. new nucleotides being added

DNA Replication The blue strand represents the DNA strand. original The orange strand represents the original new Ask: Is a new strand identical to its original template strand? Answer: No, they are complementary, not identical. Ask for a volunteer to read the statements, filling in the appropriate terms. Click to reveal the correct answers.

Telomeres Telomeres: the tips of eukaryotic chromosomes The enzyme telomerase adds short, repeated DNA sequences to telomeres as the chromosomes are replicated. Telomeres Explain that the tips, or telomeres, of a DNA molecule are difficult to replicate. Cells use a special enzyme called telomerase to add short repeated DNA sequences to telomeres as chromosomes are replicated. Tell students that, in the micrograph, the red structures are human chromosomes and the telomeres are stained white, and appear in photograph as yellowish orange.

Prokaryotic DNA Replication new DNA replication fork replication fork Remind students that, unlike eukaryotic DNA, the DNA of prokaryotic organisms exists as a single loop. Explain that replication in most prokaryotic cells begins at a single starting point and proceeds in two directions until the entire chromosome is copied. Have volunteers go to the board to label the parts of the diagram based on what they have already learned about DNA replication. Use the labels: replication fork (twice), new DNA, and unreplicated DNA. Click to reveal each correct term. unreplicated DNA

Eukaryotic DNA Replication Unreplicated DNA Replication forks Point out that the process students looked at earlier in the presentation was for replication eukaryotic DNA replication. Remind students that eukaryotic chromosomes are linear. Ask for a volunteer to point out the replication forks in the diagram. Click to reveal the labels. Ask: Where is the origin of replication? Answer: in the center of each newly forming strand Emphasize that in eukaryotic cells replication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copied. New DNA