PPT 7

Introduction to Cell Division and Mitosis

• Overview of Mitosis as a crucial part of cell division.

• Importance of understanding the molecular basis of inheritance and its relationship with DNA.

Mitosis vs. Meiosis

• Mitosis:

  • Type of cell division associated with somatic (body) cells, such as skin cells.

• Meiosis:

  • Type of cell division that produces gametes (eggs or sperm).

• DNA Replication:

  • Always the first step in both mitosis and meiosis.

  • Involves unzipping and copying DNA in the nucleus, resulting in two identical DNA copies.

  • Semi-Conservative Replication:

    • Each new DNA molecule contains one original (parental) strand and one new strand.

Historical Context of DNA Discovery

• Watson, Crick, and Franklin were pivotal in discovering the structure of DNA in 1953.

DNA Structure and Function

• Base Pairing in DNA:

  • Nitrogenous bases pair through hydrogen bonds (A-T, G-C).

• Nucleotide Composition:

  • Comprised of a sugar (deoxyribose), phosphate group, and nitrogenous base.

• Sugar-Phosphate Backbone:

  • Structure supporting the DNA molecule.

DNA Replication Process

• Origins of Replication:

  • Replication begins at specific points where bubbles of DNA form.

  • Starts with an RNA primer and proceeds in a 5’ to 3’ direction due to enzyme action (DNA polymerase).

  • Involves mismatch repair mechanisms to correct errors.

Telomeres and Chromosome Erosion

• Telomeres:

  • Noncoding DNA sequences at the chromosome ends that protect against erosion during replication.

• Consequences of Telomere Shortening:

  • Associated with aging and limit the number of times a cell can divide.

  • Germ line cells possess telomerase that maintains telomere length, preventing erosion in gametes.

Cell Cycle Overview

• Phases of the Cell Cycle:

  • G1: Initial growth phase with cytoplasmic division (cytokinesis) commencing.

  • S: DNA replication occurs.

  • G2: Preparation for mitosis, with centrosome integrity being crucial.

• Checkpoints:

  • G1 checkpoint: critical regulation point ensuring conditions are right for division.

• Apoptosis:

  • Mechanism for programmed cell death to eliminate damaged cells.

Phases of Mitosis

• 1. Prophase:

  • Chromatin condenses into chromosomes; nucleoli disappear; spindle formation begins.

• 2. Prometaphase:

  • Nuclear envelope dissolves; kinetochores appear on chromosomes.

• 3. Metaphase:

  • Chromosomes line up at the metaphase plate; spindle fibers attach to kinetochores.

• 4. Anaphase:

  • Sister chromatids are pulled to opposite poles; spindle fibers shorten.

• 5. Telophase:

  • Daughter nuclei are formed; chromatin begins to decondense; cytoplasmic division (cytokinesis) starts.

Cytokinesis in Plant and Animal Cells

• In Animal Cells:

  • Cleavage furrow forms due to actin filament contraction, resulting in two daughter cells.

• In Plant Cells:

  • Formation of a cell plate that separates the two daughter cells.

Conclusion

• Mitosis is essential for growth, development, and tissue repair in organisms.

• Understanding its mechanisms aids in comprehending larger biological processes, including cancer development.