Unzipping Life: DNA Replication - Leading vs. Lagging Strands

Biology Grades High School 10:18 Video
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Lesson Description

Explore the fascinating process of DNA replication, focusing on the roles of key enzymes, the difference between leading and lagging strands, and the importance of 3' and 5' ends.

Video Resource

Leading and lagging strands in DNA replication | MCAT | Khan Academy

Khan Academy

Duration: 10:18
Watch on YouTube

Key Concepts

  • DNA structure (3' and 5' ends)
  • Role of enzymes in DNA replication (helicase, topoisomerase, primase, DNA polymerase, ligase)
  • Leading vs. lagging strands
  • Okazaki fragments
  • RNA Primers

Learning Objectives

  • Students will be able to describe the roles of key enzymes involved in DNA replication.
  • Students will be able to explain the difference between the leading and lagging strands and why they replicate differently.
  • Students will be able to identify the 3' and 5' ends of a DNA strand and explain their significance in DNA replication.
  • Students will be able to explain the role of RNA Primers in DNA replication.

Educator Instructions

  • Introduction (5 mins)
    Briefly review the structure of DNA, including the 3' and 5' ends, and the importance of complementary base pairing (A-T, C-G). Introduce the central question: How does DNA, with its antiparallel structure, replicate itself efficiently?
  • Video Viewing (10 mins)
    Watch the Khan Academy video 'Leading and lagging strands in DNA replication'. Students should take notes on the roles of each enzyme and the differences between the leading and lagging strands.
  • Enzyme Breakdown (10 mins)
    Discuss each enzyme mentioned in the video: topoisomerase, helicase, primase, DNA polymerase, and ligase. What is each enzyme's specific function? How do they contribute to the overall process of replication? Write their functions on the board.
  • Leading vs. Lagging (10 mins)
    Explain in detail the difference between the leading and lagging strands. Why can the leading strand be replicated continuously, while the lagging strand is replicated in fragments (Okazaki fragments)? Use diagrams on the board to illustrate the direction of replication on each strand. Emphasize the importance of the 3' end for DNA polymerase activity.
  • Synthesis and Wrap-up (5 mins)
    Summarize the key steps of DNA replication, emphasizing the coordinated action of the enzymes and the distinct processes on the leading and lagging strands. Address any remaining questions.

Interactive Exercises

  • DNA Replication Diagram Labeling
    Provide students with a simplified diagram of DNA replication. Have them label the leading strand, lagging strand, 3' and 5' ends, Okazaki fragments, and the key enzymes involved. Provide a word bank for assistance.

Discussion Questions

  • Why is it important that DNA replication is a very accurate process?
  • What would happen if DNA polymerase could add nucleotides in both the 5' to 3' and 3' to 5' directions?
  • How might errors in DNA replication lead to mutations and potentially diseases?

Skills Developed

  • Critical thinking
  • Visual literacy
  • Note-taking
  • Scientific communication

Multiple Choice Questions

Question 1:

Which enzyme is responsible for unwinding the DNA double helix?

Correct Answer: Helicase

Question 2:

In which direction does DNA polymerase add nucleotides?

Correct Answer: 5' to 3'

Question 3:

Which strand is synthesized continuously during DNA replication?

Correct Answer: Leading strand

Question 4:

What are the short fragments of DNA synthesized on the lagging strand called?

Correct Answer: Okazaki fragments

Question 5:

Which enzyme joins Okazaki fragments together?

Correct Answer: Ligase

Question 6:

What is the function of topoisomerase in DNA replication?

Correct Answer: Relieves tension caused by unwinding

Question 7:

Which enzyme synthesizes RNA primers?

Correct Answer: Primase

Question 8:

The 3' and 5' refer to the position of what molecule on the deoxyribose sugar?

Correct Answer: Phosphate group

Question 9:

Why is the lagging strand synthesized discontinuously?

Correct Answer: DNA polymerase can only add nucleotides to the 3' end

Question 10:

What happens to the RNA primers after they have initiated DNA synthesis?

Correct Answer: They are degraded and replaced with DNA

Fill in the Blank Questions

Question 1:

The enzyme that unwinds the DNA double helix is called ________.

Correct Answer: Helicase

Question 2:

DNA polymerase can only add nucleotides to the _______ end of a DNA strand.

Correct Answer: 3'

Question 3:

The strand of DNA that is synthesized continuously is called the ________ strand.

Correct Answer: leading

Question 4:

The short fragments of DNA synthesized on the lagging strand are called _______ fragments.

Correct Answer: Okazaki

Question 5:

The enzyme that joins Okazaki fragments together is called _______.

Correct Answer: ligase

Question 6:

_______ relieves the tension caused by the unwinding of DNA.

Correct Answer: Topoisomerase

Question 7:

_______ synthesizes RNA primers to initiate DNA synthesis.

Correct Answer: Primase

Question 8:

DNA replication proceeds from the _______ end to the _______ end.

Correct Answer: 5', 3'

Question 9:

The lagging strand is synthesized ________ because DNA polymerase can only add nucleotides to the 3' end.

Correct Answer: discontinuously

Question 10:

RNA primers are eventually replaced with _______ to complete DNA replication.

Correct Answer: DNA

Educational Standards

B.LS1.1:Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins, which carry out the essential functions of life through systems of specialized cells. B.LS3.1:Ask questions to clarify relationships about the role of DNA and chromosomes in coding the instructions for characteristic traits passed from parents to offspring. B.LS1.4:Use a model to illustrate the role of cellular division (mitosis) and differentiation in producing and maintaining complex organisms.

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