Chapter 24: The Cell Cycle & Mitosis

Chapter 24: The Cell Cycle & Mitosis

The Cell Cycle Overview

  • Definition: The cell cycle is the "life cycle" of a cell, which includes normal growth, division, and indirectly, its death.

  • Purpose of the Cell Cycle:

    • Growth and development in tissues

    • Replacement of damaged or dead cells

    • Other potential roles (not explicitly stated)

Phases of the Cell Cycle

The cell cycle is divided into two primary phases:

  1. Interphase

    • Consists of three sub-phases:

      • G1 (Gap 1)

      • S (Synthesis)

      • G2 (Gap 2)

  2. M Phase

    • Includes Mitosis and Cytokinesis

Interphase Explained

G1 Phase (Gap 1)

  • Function: The cell grows in size and functions metabolically as normal.

  • G1 Checkpoint (also known as the "restriction point" or "start checkpoint"):

    • Evaluates the following conditions:

      • DNA damage

      • Availability of proper nutrients

      • Adequate cell size

    • If conditions are not optimal, the cell may enter the G0 state (resting state).

S Phase (Synthesis)

  • Function: DNA replication occurs, resulting in the formation of sister chromatids that are joined at the centromere.

  • Centrosomes also replicate in preparation for spindle formation during M phase.

  • Purpose: To double the genetic material ensuring each daughter cell receives a full set of DNA.

G2 Phase (Gap 2)

  • Function: Primarily preparation for cell division.

    • Organelles duplicate

    • Formation of microtubules for the mitotic spindle

  • G2 Checkpoint:

    • DNA is proofread and repaired if necessary.

Mitosis: An Overview

Definition and Stages

  • The M phase follows interphase and includes Mitosis, which is subdivided into the following stages:

    1. Prophase

    2. Metaphase

    3. Anaphase

    4. Telophase

    5. Cytokinesis

Mitosis Process Breakdown:

  • Prophase:

    • Chromatin condenses into visible chromosomes.

    • The mitotic spindle begins to form from centrosomes.

    • The nuclear envelope (nucleolis disappears.

  • Prometaphase (often included in Prophase):

    • Nuclear envelope fully disintegrates. 

    • Spindle microtubules attach to kinetochores (protein complexes at centromeres).

    • Chromosomes move towards the cell's equator.

  • Metaphase:

    • Chromosomes align at the "metaphase plate" (the cell's equator).

    • Spindle fibers connect each kinetochore of the sister chromatids to opposite poles of the cell.

  • Anaphase:

    • Cohesin proteins holding sister chromatids are cleaved, allowing them to separate and move towards opposite poles.

    • The cell elongates as microtubules push against each other, aiding in chromatid separation.

  • Telophase:

    • Individual chromosomes arrive at opposite poles.

    • Nuclear envelopes re-form around each set of chromosomes.

    • Chromosomes begin to decondense back into chromatin.

    • Nucleoli reappear within each nucleus.

  • Cytokinesis:

    • Following M phase, cytokinesis involves the division of the cytoplasm, resulting in two daughter cells.

    • In animal cells: The actin-myosin contractile ring pinches the cell, forming a cleavage furrow.

    • In plant cells: Vesicles form a cell plate that develops into the new cell wall.

Duration of the Cell Cycle

  • Typical Duration: Most mammalian cells complete the cell cycle in approximately 24 hours.

  • Specialized Cells: Certain cell types (e.g., certain mammalian cells) may take months to years before they divide.

  • Terminally Differentiated Cells: Cells like neurons exit the cell cycle and remain in G0 phase permanently, indicating they do not divide further.

Apoptosis: Programmed Cell Death

  • Definition: Apoptosis is an intentional form of cell death that benefits the organism by maintaining tissue health and preventing cancer.

Pathways of Apoptosis

  1. Intrinsic Pathway:

    • Triggered by internal stressors (e.g., DNA damage, radiation, oxidative stress, lack of growth factors).

  2. Extrinsic Pathway:

    • Triggered by external signals, including death ligands binding to receptors.

Mechanism of Apoptosis

  • Extrinsic Pathway Process:

    1. Death ligands produced by immune and inflammatory cells bind to receptors on the target cell.

    2. These receptors cluster and recruit adaptor proteins such as FADD.

    3. A death-inducing signaling complex (DISC) forms.

    4. Initiator caspase-8 (or caspase-10) is activated, leading to the downstream activation of executioner caspases (caspase-3, -6, -7).

    5. Functions of executioner caspases include:

    • DNA fragmentation

    • Breakdown of structural proteins

    • Cell shrinkage and membrane blebbing (formation of bubble-like protrusions)

    • Formation of apoptotic bodies, resulting in membrane-bound fragments.

  • Intrinsic Pathway Process (also referred to as the “Mitochondrial Pathway”):

    1. Initiated by cellular stress detected (often involves activation of p53).

    2. Bax/Bak proteins create pores in the mitochondrial outer membrane leading to cytochrome c leakage.

    3. Cytochrome c binds to Apaf-1, forming the apoptosome, which activates initiator caspase-9.

    4. Caspase-9 triggers executioner caspases resulting in apoptotic events.

Flow Cytometry

  • Definition: A lab technique used to analyze physical and chemical characteristics of cells in a sample.

  • Purposes:

    • Counting cells

    • Immunophenotyping (identifying cellular markers)

    • Measuring DNA content to assess cell cycle phases

    • Detecting apoptosis

    • Additional applications

Flow Cytometry Process

  1. Sample Preparation:

    • Target cells isolated and suspended in a fluid.

    • Fluorescent antibodies or dyes added to tag specific proteins.

  2. Scanning:

    • Cells pass through a laser beam one at a time.

    • The flow cytometer detects scattered light and fluorescence emitted by cells.

  3. Data Analysis:

    • Cell size measured by forward scatter (amount of light scattered forward).

    • Side scatter reflects the shape and complexity of the cell.

    • Comparison of forward and side scatter data assists in identifying the characteristics of cells in the sample.

    • Fluorescent markers help differentiate between distinct cell types in the sample.