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BIO 120 Chapter Eleven V2 F2024

Chapter 11: DNA Replication and Cell Division

1. Overview of Cell Division

  • Definition: The process by which cells produce new cells.

  • Functions:

    • Cell growth.

    • Cell replacement.

    • Cell healing.

    • Asexual reproduction.

2. Binary Fission in Bacteria

  • Step-by-step process of binary fission:

    1. DNA attachment: Circular DNA molecule attaches to the inner membrane.

    2. DNA replication: Starts at a specific site, proceeds bidirectionally.

    3. Elongation: Newly synthesized DNA is attached to the membrane.

    4. Cell elongation: DNA attachment sites separate as the cell elongates.

    5. Constriction: Forms at the cell midpoint, leading to daughter cell formation.

    6. Completion: Cell membrane and wall synthesis separates two daughter cells.

3. Comparison of Eukaryotic and Prokaryotic Cell Division

  • Eukaryotes: Large linear genome, DNA in the nucleus.

  • Prokaryotes: Small circular genome, DNA in the cytoplasm.

  • Similarities: DNA replication processes are analogous.

  • Differences: Presence of nucleus and genome size affect division processes.

4. The Eukaryotic Cell Cycle

  • Phases:

    1. M phase: The parent cell divides into two.

    2. Interphase: Between M phases, consists of:

      • G1 phase: cell growth and regulatory protein expression.

      • S phase: DNA synthesis.

      • G2 phase: preparation for mitosis.

      • G0 phase: resting phase with no division preparation.

5. DNA Replication Hypotheses

  • Semiconservative model: Each new DNA duplex contains one original strand and one new strand.

  • Conservative model: New DNA consists of two newly synthesized strands.

6. Research on DNA Replication

  • Meselson and Stahl experiment: Utilized isotopes of nitrogen to validate the semiconservative model.

  • After replication, if semiconservative, all DNA should show a mix of densities: one strand containing 15N and the other 14N.

7. Mechanisms of DNA Replication

  • Proteins involved:

    • DNA polymerase: extends RNA primers.

    • Helicase: unwinds DNA strands.

    • Topoisomerase II: alleviates strain during unwinding.

    • Single-strand binding protein: stabilizes unwound DNA.

  • Directionality: DNA strands replicated from 5′ to 3′, with mechanisms for proofreading.

  • Leading vs. Lagging Strand:

    • Leading strand is synthesized continuously.

    • Lagging strand synthesized discontinuously in Okazaki fragments.

8. Lagging Strand Synthesis

  • Okazaki fragments: Short pieces on the lagging strand formed due to the need for RNA primers and the 5′ – 3′ synthesis direction.

  • RNA primers: Necessary for initiation of DNA synthesis.

  • DNA ligase: Joins Okazaki fragments by completing the sugar-phosphate backbone.

9. Features of DNA Structure

  • Antiparallel strands: Necessitates the leading and lagging strand synthesis.

10. Telomeres and Telomerase in Eukaryotes

  • Telomeres: Ends of linear chromosomes containing repeated sequences.

  • Telomerase: Enzyme that extends telomeres, mitigating loss during replication.

11. Checkpoints and Regulation of Cell Division

  • CDK/Cyclin complexes: Regulate progression through the cell cycle.

  • Cell Cycle Checkpoints:

    • DNA damage checkpoint.

    • DNA replication checkpoint.

    • Spindle assembly checkpoint.

  • p53 protein: Involved in DNA damage response, can lead to apoptosis if damage is irreparable.

12. Cancer Biology

  • Cancer: Uncontrolled cell division often due to mutations in proto-oncogenes or tumor suppressor genes.

    • Oncogenes: Mutated genes that promote excessive cell division.

    • Tumor suppressors: Proteins that normally inhibit cell division.

13. Summary of Cell Division Steps

  1. Prophase: Chromosomes condense, mitotic spindle forms.

  2. Prometaphase: Microtubules interact with chromosomes.

  3. Metaphase: Chromosomes align at the cell equator.

  4. Anaphase: Sister chromatids are pulled apart.

  5. Telophase: Nuclear membrane re-forms, chromosomes decondense; cytokinesis occurs.

14. Importance of Cell Signaling

  • Cells communicate using signaling molecules, which activate responses through cellular junctions and pathways.

  • Cell adhesion: Vital for tissue integrity, involving various junction types.