Campbell Biology Twelfth Edition: Chapter 12 The Cell Cycle

The Lifecycle of a Cell

• The Cell Cycle: The lifecycle of a cell is fundamentally described by the cell cycle, which encompasses both cell growth and cell division.

• Genetic Fidelity: Genetic information ($DNA$) must be faithfully copied and subsequently transmitted to the next generation of cells, known as daughter cells.

• Basis of Cancer: Errors that occur during $DNA$ replication or within the regulation of the cell cycle are the foundational cause of cancer.

Alternation Between Growth and Division

• Interphase: This phase accounts for approximately $90\%$ of the cell cycle and is dedicated to cell growth. It is subdivided into three distinct phases:     * $G_1$ Phase (\"first gap\"): Characterized by metabolic activity and growth.     * $S$ Phase (\"synthesis\"): Characterized by metabolic activity, growth, and the Synthesis (replication) of $DNA$.     * $G_2$ Phase (\"second gap\"): Characterized by metabolic activity, growth, and specific preparations for cell division.

• Mitotic (M) Phase: This is when cell division occurs. It consists of two main processes:     * Mitosis: The process of nuclear division where chromosomes are distributed into two daughter nuclei.     * Cytokinesis: The division of the cytoplasm, which results in two distinct daughter cells. Each daughter cell is then capable of starting its own new cell cycle.

• Cell Cycle Exit ($G_0$ Phase): Many types of cells will stop dividing once they reach maturity. These cells exit the active cell cycle and enter a non-dividing state called the $G_0$ phase.

Conceptual Roles of Cell Division

• Single-Celled Organisms: Cell division is the mechanism by which single-celled organisms give rise to entirely new organisms (reproduction).

• Multicellular Eukaryotes: These organisms develop from a single cell (zygote) into complex organisms through a combination of cell division and differentiation.

• Renewal and Repair: In fully grown multicellular eukaryotes, cell division continues to function for the renewal and repair of tissues.

• Accuracy: Cell division is typically highly accurate in the transmission of $DNA$ from one generation of cells to the next.

Organization of Genetic Material

• Chromosomes: $DNA$ molecules are packaged into structures called chromosomes. A single chromosome can carry several hundred to a few thousand genes.

• Chromatin: In eukaryotic cells, chromosomes consist of chromatin, which is a complex consisting of $DNA$ and proteins.

• Species Specificity: Every eukaryotic species possesses a characteristic number of chromosomes within each cell nucleus.

• Cell Types:     * Somatic Cells: These are non-reproductive cells. They contain two sets of chromosomes, making them diploid. They divide via the process of mitosis.     * Gametes: These are reproductive cells, such as sperm and eggs. They contain half the number of chromosomes found in somatic cells, making them haploid. They are formed through the process of meiosis.

Distribution of Chromosomes During Division

• Preparation: In preparation for division, $DNA$ is replicated and the chromosomes condense.

• Sister Chromatids: Each duplicated chromosome consists of two sister chromatids. These remain attached to one another until the process of mitosis.

• Centromere: This is the narrow \"waist\" of the duplicated chromosome where the two sister chromatids are most closely attached.

• Separation: During mitosis, the two sister chromatids of every duplicated chromosome are separated. Once separated, these chromatids are considered individual chromosomes in the new daughter cells.

The Five Stages of Mitosis

• Classification: Mitosis is conventionally divided into five specific stages:     1. Prophase: Chromosomes condense and nucleoli disappear. In animal cells, the centrosome begins to organize microtubules and migrates to opposite sides of the cell. The mitotic spindle and shorter radial fibers called asters begin to form.     2. Prometaphase: The nuclear envelope fragments as chromosomes become more condensed. A kinetochore forms at the centromere of each sister chromatid. Microtubules attach to these kinetochores and begin tugging the chromosomes. Non-kinetochore microtubules push against those from the opposite pole to lengthen the cell.     3. Metaphase: Centrosomes reach opposite poles. Chromosomes align at the metaphase plate, an imaginary plane equidistant between the two spindle poles. Kinetochores of sister chromatids are attached to microtubules coming from opposite poles.     4. Anaphase: Sister chromatids separate and are now considered individual chromosomes. They migrate toward opposite poles while the cell continues to lengthen.     5. Telophase: Two new nuclear envelopes form around the separated sets of chromosomes. Nucleoli reappear, chromosomes begin to decondense, and the spindle microtubules depolymerize.

• Cytokinesis Overlap: The division of the cytoplasm (cytokinesis) overlaps with the final stage of mitosis, telophase.

Cytokinesis in Animal and Plant Cells

• Animal Cells (Cleavage): Cytokinesis occurs via a process known as cleavage.     * The first sign is the appearance of a cleavage furrow, which is a shallow groove near the old metaphase plate.     * Contraction of the cleavage furrow is driven by actin-myosin interaction.     * A contractile ring of microfilaments pinches the cell into two daughter cells.

• Plant Cells (Cell Plate): Because plants have rigid cell walls, they do not form a cleavage furrow.     * Vesicles form a cell plate during cytokinesis.     * The cell plate grows outward until it fuses with the plasma membrane, forming a new cell wall that separates the daughter cells.

Binary Fission in Bacteria

• Definition: Prokaryotes (bacteria and archaea) reproduce by a type of cell division called binary fission.

• Process:     1. Chromosome replication begins at a specific site called the origin of replication.     2. As the chromosome replicates, the two copies of the origin move toward opposite ends of the cell.     3. The chromosome is attached to the cell membrane, and the copies are separated as the cell elongates.     4. Replication finishes, the plasma membrane pinches inward, and two daughter cells result.

Molecular Regulation of the Cell Cycle

• Control System: The speed and frequency of the cell cycle are regulated at the molecular level through a cycle of protein accumulation.

• Checkpoints: Several \"checkpoints\" exist to ensure that various biological processes are completed before the cell proceeds to the next phase.

• Regulatory Proteins:     * Cyclins: Proteins that accumulate and degrade in a cyclic fashion.     * Cyclin-Dependent Kinases (Cdks): Enzymes that remain at a relatively constant concentration but are only active when bound to a cyclin. Cdks modify other proteins to activate or inactivate them.

• Maturation-Promoting Factor (MPF): A specific cyclin-Cdk complex that triggers the cell to pass the $G_2$ checkpoint and enter the $M$ phase. Peaks of MPF activity correspond to peaks of cyclin concentration.

Internal and External Cell Cycle Signals

• G1 Checkpoint (The \"Restriction Point\"): For many cells, this is the most important checkpoint.     * Go-ahead Signal: If received, the cell usually completes the $S$, $G_2$, and $M$ phases and divides.     * No Go-ahead: The cell exits the cycle and enters the non-dividing $G_0$ phase.

• M Checkpoint: This internal signal ensures that all sister chromatids are properly aligned and attached to the spindle at the metaphase plate before anaphase begins, ensuring equal chromosome division.

• External Factors:     * Growth Factors: Proteins released by certain cells that stimulate other cells to divide.     * Density-Dependent Inhibition: A physical factor where crowded cells stop dividing.     * Anchorage Dependence: Most animal cells must be attached to a substratum (like a tissue surface) to divide.

• Cancer and Regulation: Cancer cells are characterized by their escape from these usual internal and external controls, leading to uncontrolled growth.