DNA Replication Overview and Models

DNA Replication Overview

• DNA replication is crucial for living organisms.
• It ensures each daughter cell receives a complete copy of genetic information.
• This process occurs during the S phase of interphase.
• Accurate replication is vital for cell growth, repair, and normal function.

Replication Models

• Three models for DNA replication were proposed :
  • Semiconservative model (correct): Each new DNA molecule consists of one original strand and one new strand.
  • Conservative model: Both original strands stay together, forming a new DNA molecule entirely.
  • Dispersive model: Fragments of the original strands are mixed with new strands in each new DNA molecule.
• The Meselson-Stahl experiment confirmed the semiconservative model.

Semi-Conservative Model Details

• Base pairing allows each strand to serve as a template for a new strand.
• Each new strand is half-parent template DNA and half-new.

Anti-parallel Strands

• DNA's backbone is formed by bonding phosphates to sugars.
• The bond occurs between the 3' and 5' carbons.
• DNA has a direction, and complementary strands run in opposite directions.
• This anti-parallel structure poses a problem for replication.

Bonding in DNA

• DNA strands are held together by hydrogen bonds between bases.
• Covalent phosphodiester bonds link nucleotides along each strand.
• These hydrogen bonds are relatively weak.

Enzyme Coordination

• Several enzymes coordinate DNA replication in living cells.
  • Leading strand: synthesized continuously in the 5' to 3' direction.
  • Lagging strand: synthesized discontinuously in short segments (Okazaki fragments) in the 5' to 3' direction away from the replication fork, requiring multiple RNA primers.

Enzymes Involved in Replication

• Helicase: Unwinds the DNA double helix (unzips).
• Topoisomerase: Prevents the unwinding DNA from tangling.
• Primase: Adds short RNA primers to the DNA to start replication.
• DNA Polymerase III: Builds new DNA strands.
• DNA Polymerase I: Replaces RNA primers with DNA.
• Ligase: Joins Okazaki fragments in the lagging strand.
• Single-stranded binding proteins (SSBs): Stabilize the single strands of DNA.

Step-by-Step Replication

• Step 1: DNA unwinds: Helicase unwinds the DNA double helix. Topoisomerase prevents tangling.
• Step 2: Create a primer: RNA Primase adds RNA primers to the template strand.
• Step 3: Build the new strand: DNA Polymerase III adds nucleotides to the RNA primer in the 5' to 3' direction.
• Step 4: Replace primer with DNA: DNA Polymerase I removes the RNA primers and replaces them with DNA nucleotides. Ligase joins Okazaki fragments in the lagging strand.

Leading and Lagging Strands Summary

• Leading strand: Synthesized continuously in the 5' to 3' direction, toward the replication fork.
• Lagging strand: Synthesized discontinuously, in Okazaki fragments, away from the replication fork; Requires RNA primers for each Okazaki fragment.
• Okazaki fragments are joined by DNA ligase.