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

DNA Replication

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

DNA Helicase

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

DNA Polymerase

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

Leading Strand

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

Lagging Strand

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

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.

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.

Common Misconceptions

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.