DNA Replication Overview

Overview

This lecture explains the structure of DNA nucleotides, the directionality of DNA replication, the roles of key enzymes, and the mechanisms ensuring accurate DNA duplication.

Nucleotide Structure and DNA Directionality

• A DNA nucleotide consists of deoxyribose (a five-carbon sugar), a nitrogenous base, and a phosphate group.
• The sugar carbons are numbered 1’ to 5’; new nucleotides connect only to the 3’ (three prime) end.
• DNA strands have directionality: one runs 5’→3’, the other 3’→5’ (anti-parallel).
• New nucleotides can only be added to the 3’ end; replication occurs in the 5’→3’ direction.

DNA Replication Fork and Strand Synthesis

• DNA helicase separates the two parent strands, creating a replication fork.
• The leading strand is synthesized continuously in the 5’→3’ direction toward the replication fork.
• The lagging strand is synthesized discontinuously in short segments called Okazaki fragments, also in the 5’→3’ direction, but away from the fork.

Enzymes Involved in DNA Replication

• DNA helicase breaks hydrogen bonds to separate DNA strands.
• DNA primase lays down a short RNA primer to start DNA synthesis.
• DNA polymerase III adds new nucleotides to the 3’ end using complementary base pairing and forms phosphodiester bonds.
• DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.
• DNA ligase connects Okazaki fragments and seals nicks by forming phosphodiester bonds.

Proofreading and Error Correction

• DNA polymerase III proofreads new DNA, removing and replacing mismatched nucleotides to prevent mutations.
• Accurate replication ensures two identical DNA molecules after the process.

Key Terms & Definitions

• Deoxyribose — the five-carbon sugar in DNA nucleotides.
• 3’ (three prime) end — the end of a DNA strand where new nucleotides can be added.
• Replication fork — the Y-shaped region where DNA is split into two strands for replication.
• Leading strand — the new DNA strand synthesized continuously toward the replication fork.
• Lagging strand — the strand synthesized in short fragments away from the fork.
• Okazaki fragments — short DNA segments made on the lagging strand.
• Primase — enzyme that adds an RNA primer to start DNA synthesis.
• Ligase — enzyme that connects DNA fragments by forming bonds.

Action Items / Next Steps

• Review the steps and enzymes involved in DNA replication.
• Ensure you understand the directionality of DNA synthesis and its significance.
• Prepare for assessment on enzyme functions and replication fidelity.