6.2 - DNA Replication

DNA Replication

DNA replication is the process by which DNA makes a copy of itself, ensuring that each new cell receives a complete set of genetic information. This process occurs during the S phase of interphase in both mitosis and meiosis.

Importance of DNA Replication

• Genetic Continuity: Ensures that each daughter cell receives an identical copy of the DNA.

• Error Correction: Although mistakes can occur during replication, eukaryotes have repair mechanisms to correct most errors. Some mutations can be beneficial, contributing to evolutionary adaptation, such as lactose tolerance in humans.

Models of DNA Replication

• Semi-Conservative Model: Proposed by Watson and Crick, suggesting that each new DNA molecule consists of one original strand and one newly synthesized strand.

• Conservative Model: The original DNA molecule remains intact, and an entirely new molecule is synthesized.

• Dispersive Model: Segments of the original DNA molecule are interspersed with newly synthesized segments.

Meselson and Stahl Experiment

To determine the correct model of DNA replication, Meselson and Stahl conducted an experiment using E. coli bacteria and isotopes of nitrogen.

• Method:

  • Grew bacteria in a medium containing the "heavy" isotope of nitrogen (N15), which incorporated into their DNA.

  • Transferred the bacteria to a medium containing the "light" isotope of nitrogen (N14) and allowed them to replicate.

  • Used density gradient centrifugation to analyze the DNA after replication.

• Findings:

  • After one round of replication, the DNA had a hybrid density, indicating a mix of N15 and N14.

  • After two rounds of replication, there were two bands: one with hybrid density and one with light density.

• Conclusion: The experiment supported the semi-conservative model of DNA replication. Each DNA molecule consists of one original (heavy) strand and one newly synthesized (light) strand.

Process of Semi-Conservative DNA Replication

1. Initiation:

   • The DNA double helix unwinds and separates into two strands.

   • Each parental strand serves as a template for the new strand.

2. Elongation:

   • Free nucleotides in the cell pair with the complementary bases on each template strand (A with T, and C with G).

   • DNA polymerase enzymes synthesize the new strands by forming phosphodiester bonds between nucleotides.

3. Termination:

   • The replication process continues until the entire molecule is copied.

   • Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

Major Steps of DNA Replication

1. Unzipping and Unwinding DNA:

   • Hydrogen Bonds: DNA strands are held together by hydrogen bonds, which are relatively easy to break, allowing the strands to separate.

   • Helicase: This enzyme unwinds and separates the DNA strands at the origins of replication. Helicase breaks the hydrogen bonds between the base pairs, creating two single strands of DNA.

2. Formation of Replication Bubble and Forks:

   • Single-Stranded Binding (SSB) Proteins: These proteins bind to the separated single strands of DNA to keep them apart and stabilize the unwound DNA, forming a replication bubble.

   • Origin of Replication: The specific location where replication begins. In eukaryotes, there are multiple origins of replication along the linear DNA, whereas prokaryotes typically have a single origin of replication.

3. Creation of Replication Forks:

   • Replication Forks: The Y-shaped regions where the two DNA strands separate and new DNA strands are synthesized. Each replication bubble has two replication forks, moving in opposite directions.

4. Relieving Tension Ahead of the Replication Fork:

   • Topoisomerase: This enzyme alleviates the overwinding strain ahead of the replication forks by making temporary cuts in the DNA, allowing it to unwind more easily. It then reseals the cuts to maintain the integrity of the DNA molecule.

Key Points

• Eukaryotes vs. Prokaryotes:

  • Eukaryotes: Have multiple origins of replication on their linear DNA, allowing for simultaneous replication at several points.

  • Prokaryotes: Typically have a single origin of replication on their circular DNA, resulting in one replication bubble.

Importance of Enzymes in DNA Replication

• Helicase: Unwinds and separates the DNA strands.

• SSB Proteins: Stabilize the single strands and keep them separated.

• Topoisomerase: Prevents overwinding and relieves tension ahead of the replication forks.