DNA Replication and Polymerase Quiz

DNA Replication

• DNA replication is the process where DNA makes a copy of itself.

• Three models for DNA replication were proposed:

  • Semiconservative: Each new DNA molecule is made of one original strand and one new strand.
  • Conservative: One new DNA molecule retains both original strands, and one new DNA molecule contains two new strands.
  • Dispersive: Original and new DNA segments are interspersed in both new strands.

• Meselson and Stahl's experiment provided evidence that the semiconservative model is correct.

  • Bacteria were grown in a medium containing a heavy isotope of nitrogen (15N).
  • The DNA was extracted and its density was measured.
  • The results of successive generations matched the semiconservative model's predictions.

DNA Polymerase

• Enzymes that catalyze the polymerization of nucleotide precursors (deoxyribonucleotides)

• In 1955, Arthur Kornberg identified these enzymes.

• Found in prokaryotes and eukaryotes.

• DNA polymerase III is the enzyme involved in replication.

• DNA is copied in a 5' to 3' direction on the template strand

• Leading strand is synthesized continuously.

• Lagging strand is synthesized discontinuously (Okazaki fragments)

Mechanism of DNA Elongation

• At the growing end of the DNA chain, DNA polymerase forms a phosphodiester bond between the 3'-OH of the last nucleotide and the 5' phosphate of a dNTP

• In each step, polymerase finds the correct dNTP

• Nucleotide addition is rapid in prokaryotes ( ~ 850 per second).

• High accuracy, low error frequency.

Mechanism of DNA Replication

• Initiation is directed by a DNA sequence called the replicator.
• The initiation of replication in a circular chromosome has a specific region call oriC
• E.coli initiator protein DnaA
• E.coli helicase (DnaB) and helicase loader (DnaC) are recruited.
• Once the DNA is unwound, a replication bubble forms.
• Replication forks move in opposite directions.

Termination of Replication

• Termination occurs at a specific region of the DNA termed the Ter region.
• Tus proteins (terminus utilization substances) bind to the Ter region.
• The Ter-Tus complex halts the replication fork progression.

Replication Bubble

• The segment of unwound DNA resulting from the separation of DNA stands during initiation of replication.
• The replication forks move in opposite directions away from the origin of replication.

Rolling Circle Replication

• Replication that produces linear DNA from a circular template.
• A nick is created in one strand of the double helix.
• The displaced strand is used as a template to initiate the synthesis of a complementary strand.

DNA Replication in Eukaryotes

• Eukaryotes have linear chromosomes & multiple origins of replication
• Relatively slower than prokaryotes, due to multiple origins of replication.
• The end replication problem: The inability to completely replicate the ends of linear chromosomes.
• Telomerase: An enzyme that adds telomeric repeats to the ends of chromosomes, solving the end-replication problem. Telomerase RNA provides a template for synthesis.

Okazaki Fragments

• Short DNA fragments made during lagging strand synthesis.
• These fragments are later joined together by DNA ligase.

Eukaryotic DNA Replication

• Multiple origins of replication are found in eukaryotic chromosomes to speed up the process.
• Different DNA polymerases participate.
• Elongation uses leading and lagging strands like prokaryotes.
• Post-replication processing includes removal of RNA primers and joining of Okazaki fragments by special enzymes.
• There are specific mechanisms to cope with the end-replication problem in linear chromosomes.

Test your understanding of DNA replication processes, including the semiconservative, conservative, and dispersive models. Explore the significance of Meselson and Stahl's experiment, and learn about DNA polymerases and their functions. This quiz covers key concepts relevant for biology students.