DNA Replication

DNA Replication

Middle School

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Learning Objectives

• Explain the purpose and timing of DNA replication in the cell cycle.

• Describe the roles of enzymes like helicase, polymerase, and primase.

• Differentiate between the synthesis of the leading and lagging strands.

• Explain why DNA replication is considered a semi-conservative process.

Key Vocabulary

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DNA Replication

The process where a cell’s entire DNA is copied before the cell division process occurs.

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DNA Helicase

The enzyme that breaks hydrogen bonds to unwind the two DNA strands during replication.

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DNA Polymerase

The main enzyme that builds a new DNA strand by adding complementary nucleotides.

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Leading Strand

The new DNA strand that is synthesized continuously in the 5' to 3' direction.

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Lagging Strand

The strand synthesized discontinuously in short fragments, known as the Okazaki fragments.

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How DNA Makes a Copy of Itself

• DNA copies itself during the S phase of the cell cycle.

• An enzyme, DNA helicase, unwinds the double helix by breaking hydrogen bonds.

• DNA polymerase adds new complementary bases, pairing A with T and C with G.

• Each new DNA molecule has one original strand and one new strand.

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Multiple Choice

Which enzyme is responsible for unwinding the DNA double helix by breaking the hydrogen bonds between bases?

• DNA Polymerase

• DNA Helicase

• Primase

• Topoisomerase

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Leading vs. Lagging Strand

• DNA polymerase builds new strands only in the 5' to 3' direction.

• The leading strand is built continuously, following the replication fork's direction.

• The lagging strand is built in short pieces called Okazaki fragments.

• An enzyme called DNA ligase joins these Okazaki fragments together.

Multiple Choice

On the lagging strand, DNA is synthesized in short, discontinuous pieces called:

• Primers

• Leading fragments

• Okazaki fragments

• Polymerase fragments

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Common Misconceptions

Misconception	Correction
DNA replication creates a completely new DNA molecule from scratch.	Each new DNA molecule contains one original and one new strand.
DNA Polymerase can build the new strand in any direction.	DNA Polymerase can only synthesize DNA in the 5' to 3' direction.
Only one of the two DNA strands gets copied.	Both original DNA strands serve as templates to be copied.

Multiple Choice

Why is DNA replication on the lagging strand more complex than on the leading strand?

• Because the lagging strand is longer than the leading strand.

• Because DNA polymerase can only build in the 5' to 3' direction.

• Because the lagging strand is more prone to mutations.

• Because the lagging strand uses different base pairing rules.

Multiple Choice

What would be the most likely result if the enzyme DNA ligase were defective and non-functional?

• The DNA double helix would fail to unwind at the start of replication.

• The leading strand would not be synthesized.

• The Okazaki fragments on the lagging strand would not be joined together into a continuous strand.

• The RNA primers required for the lagging strand would not be created.

Multiple Choice

If a mutation occurred that allowed DNA Polymerase to build in both the 5'->3' and 3'->5' directions, how would this affect replication?

• The DNA would not unwind.

• Replication would stop completely.

• There would no longer be a need for a lagging strand or Okazaki fragments.

• The cell would not need DNA Helicase anymore.

Multiple Choice

A single DNA molecule undergoes two complete rounds of replication. What is the composition of the four resulting DNA molecules?

• Four molecules, each containing one of the original strands.

• Two molecules with one original and one new strand, and two molecules with only new strands.

• All four molecules are made entirely of new strands.

• One molecule contains both original strands, and three molecules are made of all new strands.

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Summary

• DNA replication occurs in the S phase and is semi-conservative.

• Helicase unwinds the DNA, and DNA Polymerase builds new strands.

• The leading strand is built continuously, and the lagging strand in fragments.

• All new DNA strands are synthesized in the 5' to 3' direction.

Poll

On a scale of 1-4, how confident are you about the process of DNA replication?

• 1

• 2

• 3

• 4