Unit 4: Cell Division and its regulation

The Cell Cycle and its Regulation

The cell cycle is the ordered series of events that a cell passes through, from its formation by the division of a parent cell to its own division into two daughter cells.

The cell cycle has two main phases:

1. Interphase (Growth Phase): The longest phase, where the cell grows and prepares for division.
   • G1 Phase (Gap 1): The cell grows, carries out normal metabolic functions, and synthesizes proteins.
   • S Phase (Synthesis): The cell's DNA is replicated. The centrosome is also duplicated.
   • G2 Phase (Gap 2): The cell continues to grow and synthesizes proteins and organelles needed for division.
2. M Phase (Mitotic Phase): The phase of actual cell division.
   • Mitosis: Division of the nucleus.
   • Cytokinesis: Division of the cytoplasm.

Regulation of the Cell Cycle

The cell cycle is tightly controlled by checkpoints, which act as "stop/go" signals. Key regulatory proteins are Cyclins and Cyclin-Dependent Kinases (CDKs). A CDK is an enzyme that is only active when bound to a specific cyclin, and this complex drives the cell from one phase to the next.

Cell-Cell Interaction and Cell Locomotion

Cell-Cell Interaction

This refers to how cells communicate and attach to each other. In tissues, cells are connected by cell junctions.

• In Animal Cells: Tight junctions (seal), Desmosomes (anchor), Gap junctions (communicate).
• In Plant Cells: Plasmodesmata are channels that pass through cell walls, connecting the cytoplasm of adjacent cells and allowing for communication.

Cell Locomotion

This is the active movement of a cell from one place to another. It relies on the cytoskeleton.

• Amoeboid Movement: "Crawling" motion used by cells like Amoeba and white blood cells. It involves the extension of pseudopods (false feet), driven by actin microfilaments.
• Flagellar Movement: A long, whip-like tail (e.g., sperm) that propels the cell in a propeller-like motion. Made of microtubules.
• Cilliary Movement: Short, hair-like projections (cilia) that beat in a coordinated way to move the cell or move fluid over the cell surface (e.g., Paramecium). Also made of microtubules.

Muscle and Nerve Cells: These are highly specialized cells. Muscle cells are specialized for contraction (using actin and myosin). Nerve cells (neurons) are specialized for transmitting electrical signals (locomotion of signals, not the cell itself).

Cell Senescence and Programmed Cell Death (Apoptosis)

Cell Senescence

Senescence is a state of permanent cell cycle arrest. The cell is still alive and metabolically active, but it no longer divides. It is a protective mechanism against cancer (preventing damaged cells from dividing) and is also linked to the aging process.

Programmed Cell Death (PCD)

PCD is the an organized, genetically controlled process of "cell suicide." It is a normal and essential part of an organism's life.

Apoptosis

Apoptosis is the main and most well-studied form of PCD in animals.

What is Apoptosis?

It is a neat, orderly process of cell dismantlement that avoids inflammation. The cell is "chopped up" and packaged into vesicles for disposal.

Process of Apoptosis:

1. The cell shrinks and its chromatin condenses.
2. The membrane begins to "bleb" (form irregular bulges).
3. The cell breaks apart into small, membrane-enclosed vesicles called apoptotic bodies.
4. These bodies are quickly cleaned up by phagocytic cells (like macrophages) before they can leak their contents.

Cell Differentiation

Cell differentiation is the process by which a less specialized cell (like a stem cell) becomes a more specialized cell type (like a muscle, skin, or nerve cell).

This process is the key to how a single fertilized egg (a zygote) can develop into a complex multicellular organism. It involves differential gene expression—meaning, all cells have the same DNA, but differentiation involves turning on the specific genes for that cell type (e.g., the gene for hemoglobin in a red blood cell) and turning off all other genes.

Biology of Cancer

Cancer is essentially a disease of uncontrolled cell division. It is a failure of cell cycle regulation.

Key Characteristics of Cancer Cells:

• They divide indefinitely, ignoring normal "stop" signals.
• They have mutations in genes that control the cell cycle:
  • Proto-oncogenes: Normal genes that act as "gas pedals" (e.g., "divide now"). When mutated, they become oncogenes, which are like a "stuck gas pedal."
  • Tumor Suppressor Genes: Normal genes that act as "brakes" (e.g., "stop dividing," or "trigger apoptosis"). When mutated, the "brakes" fail (e.g., the p53 gene).
• Metastasis: Cancer cells lose their normal adhesion and can spread (metastasize) through the blood or lymph to new parts of the body, forming new tumors.

Mechanisms of Cell Division: Mitosis

Mitosis is the process of nuclear division used by eukaryotic cells to produce two genetically identical daughter cells. It is used for growth, repair, and asexual reproduction.

Result: 1 Diploid Cell (2n) → 2 Diploid Cells (2n)

Stages of Mitosis:

1. Prophase:
   • Chromatin condenses into visible chromosomes.
   • The mitotic spindle begins to form from the centrosomes.
   • The nuclear envelope breaks down.
2. Metaphase:
   • The chromosomes (each with two sister chromatids) align at the metaphase plate (the cell's equator).
3. Anaphase:
   • The sister chromatids are pulled apart by the spindle fibers and move to opposite poles of the cell. Each chromatid is now considered a full chromosome.
4. Telophase:
   • The chromosomes arrive at the poles and begin to decondense back into chromatin.
   • New nuclear envelopes form around the two sets of chromosomes.
   • The spindle apparatus breaks down.

Cytokinesis (division of cytoplasm) usually begins during late anaphase or telophase, forming two separate cells.

Mechanisms of Cell Division: Meiosis

Meiosis is a special type of "reductional" division used to produce gametes (sperm and eggs) for sexual reproduction. It ensures genetic diversity.

Result: 1 Diploid Cell (2n) → 4 Haploid Cells (1n)

Meiosis involves two rounds of division: Meiosis I and Meiosis II.

Meiosis I (Reductional Division)

This is where homologous chromosomes are separated.

• Prophase I: The most complex phase. Homologous chromosomes pair up (synapsis) to form a bivalent. Crossing Over (exchange of genetic material) occurs between non-sister chromatids, creating new gene combinations.
• Metaphase I: Homologous pairs (bivalents) line up at the metaphase plate.
• Anaphase I: Homologous chromosomes separate and move to opposite poles. (Sister chromatids remain attached!).
• Telophase I & Cytokinesis: Two haploid cells are formed.

Meiosis II (Equational Division)

This division is identical to mitosis. Sister chromatids are separated.

• Prophase II: Spindle forms.
• Metaphase II: Chromosomes line up at the metaphase plate.
• Anaphase II: Sister chromatids separate and move to opposite poles.
• Telophase II & Cytokinesis: The final result is four genetically unique haploid daughter cells.

Role of Centromere, Kinetochore and Spindle Apparatus

These three components are the "machinery" of mitosis and meiosis.

• Centromere: The constricted region on a chromosome that holds the two sister chromatids together.
• Kinetochore: A complex of proteins that assembles on the centromere. This is the actual attachment point for the spindle fibers.
• Spindle Apparatus (Mitotic Spindle): The structure made of microtubules that originates from the centrosomes (poles). The microtubules (spindle fibers) attach to the kinetochores and are responsible for pulling the chromosomes apart.