DNA replication

Overview of DNA Synthesis and Replication

Polymerase Function

  • Definition: Polymerases are enzymes responsible for the synthesis of nucleic acids.

  • Template Example: 3'ATCGTA is used as a template for synthesis.

  • Base Pairing and Hydrogen Bonds:

    • Thymine (T) is specifically defined as a type of base (pyrimidine/purine).

    • The number of hydrogen bonds involved in base pairing is crucial for stability but is not detailed here.

  • Limitations of DNA Polymerase: Cannot bond two nucleotides together without a 3'OH group to add a new DNA nucleotide.

Key Issues in DNA Replication

  • Seven key issues must be resolved during the DNA replication process:

    • Unwinding of the Helix: The double helix structure needs to be unwound for replication.

    • Reduce Increased Coiling: The unwinding creates tension and coiling that must be managed.

    • Synthesis of Primer for Initiation: Primers are needed to begin the replication process.

    • Discontinuous Synthesis of the Second Strand: This leads to the formation of Okazaki fragments on the lagging strand.

    • Removal of RNA Primers: After replication, the RNA primers must be removed and replaced with DNA.

    • Joining of Gap-Filling DNA: DNA fragments need to be joined together after the removal of primers.

    • Proofreading: To ensure fidelity in replication, proofreading mechanisms must correct errors.

Prokaryotic Chromosome Structure

  • RNA Polymerase: Capable of bonding two nucleotides together, unlike DNA polymerase in eukaryotes.

  • Origin of Replication (ORI): The initial site at which replication begins.

  • Breakage of Hydrogen Bonds: This occurs at the ORI to initiate the formation of replication bubbles.

  • Role of Helicase: Unwinds the DNA strands further creating replication forks where synthesis occurs.

Mechanisms of DNA Replication in Prokaryotes

  • Supercoiling:

    • As DNA unwinds, it can become supercoiled, leading to additional tension that must be countered by enzymes such as gyrase (a type of DNA topoisomerase).

  • RNA Primer Synthesis: RNA primers are synthesized by RNA primase to initiate DNA synthesis.

  • Covalent Bond Formation: DNA ligase catalyzes phosphodiester bonds to join Okazaki fragments.

  • Core Enzymes: The operation of DNA polymerase I and DNA polymerase III both play crucial roles in DNA replication during the lagging and leading strands, respectively.

Continuous and Discontinuous DNA Synthesis

  • Antiparallel Strands: Two strands of the double helix are antiparallel: 5'-3' and 3'-5'.

  • Leading Strand vs. Lagging Strand:

    • Leading Strand: Synthesized continuously in the direction of the replication fork.

    • Lagging Strand: Discontinuous synthesis happens in segments (Okazaki fragments) due to orientation constraints.

    • DNA Polymerase I: Responsible for removing RNA primers on lagging strands and replacing them with DNA.

Eukaryotic vs. Prokaryotic Replication

  • Complexity of Eukaryotic Cells:

    1. Eukaryotic cells contain more DNA than prokaryotic cells.

    2. DNA is complexed with histones, forming chromatin, which affects replication.

    3. Eukaryotic DNA is organized in linear chromosomes, as opposed to the circular structure found in prokaryotes.

  • Multiple Origins of Replication (ORI): To manage the larger amount of DNA present in eukaryotes, there are multiple origins of replication.

Challenges and Solutions in Eukaryotic Replication

  • Chromatin Structure: Eukaryotic DNA is complexed with histones, requiring specific mechanisms for replication.

  • Enzyme Assistance: Enzymes are required to remove histones as replication proceeds. Two modes of histone inheritance during replication are explained:

    • Semiconservative Mode: Each daughter strand contains one original and one newly synthesized strand.

    • Conservative Mode: The original strands remain together in one double helix, while new strands form a separate double helix.

Telomeres and Linear Chromosomes

  • Protection of Chromosomes:

    • Telomeres are the protective ends of linear chromosomes, preventing loss of DNA during replication.

  • Mechanism of Telomerase:

    • Telomerase extends the ends of chromosomes. Most somatic cells do not have active telomerase, leading to telomere shortening with each cell division.

  • Role of Telomeres in Aging and Cancer:

    • Cancer cells often maintain telomerase activity, allowing them to become immortalized.

Telomere Functionality During Replication

  • Steps of Telomerase Activity:

    • Binding: Telomerase binds to the 3' G-rich tail of the chromosome.

    • Synthesis: Telomeric DNA is synthesized using the G-rich tail as a template.

    • Translocation and Repetition: Telomerase moves along the chromosome, repeating the synthesis process.

    • Filling Gaps: After telomerase activity, primase and DNA polymerase fill any resultant gaps.

    • Sealing Gaps: Finally, DNA ligase seals any remaining gaps, completing the replication process effectively.