Cell Cycle

Cell Cycle

Introduction to Cell Division

  • Purpose of Cell Division:

    • The continuity of life is based on the reproduction of cells through cell division.

    • Enables organisms to reproduce more of their own.

    • In unicellular organisms, division of one cell leads to reproduction of the entire organism.

    • In multicellular eukaryotes, cell division is crucial for:

    • Development from a fertilized cell.

    • Growth.

    • Repair.

Cellular Organization of Genetic Material

  • Genetic Information:

    • Most cell division results in daughter cells that possess identical genetic information (DNA).

    • Exception: Meiosis - a special type of division that produces sperm and egg cells.

  • Definitions:

    • Genome: The complete set of DNA in a cell, which may consist of:

    • A single DNA molecule (common in prokaryotes).

    • Multiple DNA molecules (common in eukaryotes).

    • Chromosomes: Packaged forms of DNA molecules.

Chromatin and Chromosomes

  • Chromatin: A complex of DNA and protein that condenses during cell division.

    • Somatic cells (non-reproductive cells) contain two sets of chromosomes:

    • Maternal chromosomes inherited from the mother.

    • Paternal chromosomes inherited from the father.

    • Gametes: Reproductive cells (sperm and eggs) with half as many chromosomes as somatic cells due to meiosis.

Chromosome Structure During Cell Division

  • Preparation for Cell Division:

    • DNA replication occurs, producing duplicated chromosomes.

    • Each duplicated chromosome comprises two sister chromatids, which are joined copies of the original chromosome:

    • Centromere: The narrow part of the duplicated chromosome where sister chromatids are attached.

    • Chromosome Components:

    • Chromosomal DNA molecules.

    • Chromosome arms.

    • Sister chromatids, forming after DNA duplication.

The Cell Cycle

  • The cell cycle encompasses:

    • Interphase: Cell growth and chromosome copying (90% of the cycle).

    • Subphases of Interphase:

      • G1 phase: First gap; metabolic activity and growth.

      • S phase: Synthesis phase; metabolic activity, growth, and DNA synthesis (chromosome duplication occurs).

      • G2 phase: Second gap; metabolic activity, growth, and preparation for cell division.

    • Mitotic (M) phase: Includes mitosis and cytokinesis.

Mitosis Stages

  • Mitosis can be subdivided into:

    • Prophase.

    • Prometaphase.

    • Metaphase.

    • Anaphase.

    • Telophase.

Mitosis: Detailed Stages

  • Prophase & Prometaphase:

    • Chromosomes condense and become visible.

    • Centrosomes begin to move apart.

    • Spindle fibers form from microtubules.

  • Metaphase:

    • Chromosomes align at the metaphase plate at the cell's equatorial center.

  • Anaphase:

    • Sister chromatids separate and move towards opposite poles of the cell.

  • Telophase:

    • Two genetically identical daughter nuclei form at opposite ends of the cell.

    • Cytokinesis: The cytoplasm divides; in animal cells, this occurs via cleavage furrow formation.

    • In plant cells, a cell plate is formed during cytokinesis.

Mitosis and the Mitotic Spindle

  • The Mitotic Spindle:

    • Composed of microtubules controlling chromosome movement.

    • Incorporates:

    • Centrosomes (microtubule organizing centers).

    • Spindle microtubules.

    • Asters (radial arrays of microtubules extending from centrosomes).

  • Kinetochore: Protein complexes that form at the centromeres of chromosomes, facilitating attachment to spindle microtubules and chromosome movement.

Bacterial Cell Division

  • Binary Fission in Prokaryotes:

    • Prokaryotes reproduce through binary fission.

    • The chromosome replicates starting at the origin of replication.

    • Daughter chromosomes move apart.

    • The plasma membrane pinches inward, resulting in two daughter cells.

Cell Cycle Control System

  • The cell cycle is regulated by a control system with checkpoints:

    • Key Checkpoints: G1, G2, and M phase checkpoints ensure proper progression.

    • The control system is influenced by internal and external factors.

  • Regulatory Proteins:

    • Cyclins and Cyclin-Dependent Kinases (Cdks) play crucial roles in cell cycle regulation.

    • MPF (Maturation-Promoting Factor): A cyclin-Cdk complex that moves the cell past the G2 checkpoint into the M phase.

Checkpoints in the Cell Cycle

  • M phase checkpoint: Ensures chromosomes are properly aligned before anaphase.

  • G1 checkpoint: Most significant; a go-ahead signal here usually leads the cell to proceed through S, G2, and M phases.

    • If no go-ahead signal is received, the cell enters G0 state (nondividing).

  • External Factors: Growth factors stimulate cell division; density-dependent inhibition stops crowded cells from dividing.

Cancer Cell Characteristics

  • Cancer cells divide uncontrollably due to disregarding normal control mechanisms.

  • Transformation: Normal cells can become cancerous through mutations in genes regulating the cell cycle.

  • Cancer cells may not require growth factors to grow and divide.

  • Unremoved cancer cells can form tumors:

    • Benign tumors: Remain localized.

    • Malignant tumors: Invade surrounding tissues and can metastasize, spreading to distant sites.

    • Treatment options include high-energy radiation or chemotherapies targeting the cell cycle, especially for metastatic cancers.