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?
Distribute the lesson worksheet. Instruct students to create combination notes during the presentation that describe the processes and structures involved in DNA replication.
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
Encourage students to recall the relationship between DNA replication and mitosis.
Ask for a volunteer to go to the board to fill in the blank.
Ask: When in the cell cycle is DNA replicated?
Answer: in the S phase, before mitosis can start
Click to reveal the correct answer.
Ask: Why is DNA replicated before mitosis?
Answer: so that each of the daughter cells resulting from mitosis contains the full amount of genetic material
Explain that the micrograph shown here shows a human DNA molecule in the process of replication. Have students describe what the are seeing. Guide students to realize that the DNA molecule has opened up.
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.
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.
Emphasize that DNA replication is described as semiconservative because half of the “old” DNA is conserved in a new strand.
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 blue structures are human chromosomes and the telomeres are stained white.
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.
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.
Click to reveal each correct term.
Following the presentation, allow students time to complete the lesson worksheet. As a reference, project Slide 5 “Copying DNA” while students work.
Remind students that combination notes use words and pictures to help convey concepts.
Worksheet Answers:
Exact notes and diagrams will vary from student to student, but be sure students have covered the following concepts in their notes:
Role of DNA polymerase
Function of replication forks
Addition of nucleotides to a new strand using an old DNA strand as a template
Combination of a new strand and an original strand in newly replicated DNA
Replication occurs in two directions
Function of telomeres
Difference between DNA replication in eukaryotes and in prokaryotes
