Cell Cycle

Chapter 12: The Cell Cycle

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Key Questions and Concepts

  • What is the product of mitosis?

  • What happens at each step in interphase?

  • How many copies of DNA are present in each step in interphase?

  • What happens at each individual step of mitosis?

  • How do bacteria replicate?

  • How is mitosis regulated in eukaryotic cells?

  • What affects mitosis to cause cancer?

Concept 12.1: Most Cell Division Results in Genetically Identical Daughter Cells

  • Definition of Cell Division: The ability of organisms to produce more of their own kind (cell division) is a characteristic distinguishing living things from nonliving matter.

  • Functions of Cell Division:

    • Single-celled organisms give rise to new organisms.

    • Multicellular eukaryotes undergo embryonic development.

    • Cell division is vital for renewal and repair in fully grown multicellular organisms.

    • Distribution of identical genetic material to daughter cells.

Cellular Organization of the Genetic Material

  • All DNA in a cell constitutes the cell's genome, packaged into chromosomes.

    • A genome can consist of a single DNA molecule (common in prokaryotic cells) or multiple DNA molecules (common in eukaryotic cells).

    • Eukaryotic chromosomes consist of chromatin.

    • Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

    • Somatic cells have two sets of chromosomes.

    • Gametes have half as many chromosomes as somatic cells.

Distribution of Chromosomes During Eukaryotic Cell Division

  • Preparation for Cell Division:

    • DNA is replicated and chromosomes condense.

    • Each duplicated chromosome has two sister chromatids attached by cohesins at the centromere.

    • Eukaryotic cell division of somatic cells consists of mitosis followed by cytokinesis.

Concept 12.2: The Mitotic Phase Alternates with Interphase in the Cell Cycle

  • The Cell Cycle:

    • The cell cycle consists of two main phases:

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

      • Interphase: involves cell growth and copying of chromosomes in preparation for cell division.

Interphase

  • Three Phases of Interphase:

    • G1 phase (“first gap”).

    • S phase (“synthesis”) where chromosomes are duplicated.

    • G2 phase (“second gap”).

    • Growth: the cell grows during all three phases, but chromosomes are duplicated only during the S phase.

Mitosis

  • Stages of Mitosis:

    • Prophase: Chromatin condenses, mitotic spindle forms, nuclear envelope disappears.

    • Prometaphase: Nuclear membrane breaks down, allowing spindle microtubules to attach to kinetochores of chromosomes.

    • Metaphase: Chromosomes align at the metaphase plate.

    • Anaphase: Chromosomes are separated; cohesins are cleaved by separase, moving them via microtubules.

    • Telophase: Chromosomes are at opposite sides; nuclear envelope reforms, cytokinesis occurs.

    • Cytokinesis may begin during anaphase but is completed during telophase.

The Mitotic Spindle: A Closer Look

  • Definition: A structure made of microtubules that controls chromosome movement during mitosis.

    • The spindle includes centrosomes, spindle microtubules, and asters.

    • In animal cells, assembly begins in the centrosome, which replicates during interphase.

    • Two centrosomes migrate to opposite ends of the cell during prophase and prometaphase, extending asters.

    • Each sister chromatid has a kinetochore.

Cytokinesis: A Closer Look

  • In Animal Cells:

    • Cytokinesis occurs through a process called cleavage.

    • Cleavage furrow: A shallow groove appears in the cell surface near the metaphase plate.

  • In Plant Cells:

    • A cell plate forms during cytokinesis.

Binary Fission in Bacteria

  • Process of Binary Fission:

    • Prokaryotes (bacteria and archaea) reproduce through binary fission.

    • The chromosome replicates, beginning at the origin of replication.

    • Daughter chromosomes actively move apart as the plasma membrane pinches inward.

    • Research is ongoing regarding how bacterial chromosomes move and their location establishment.

Concept 12.3: The Eukaryotic Cell Cycle is Regulated by a Molecular Control System

  • Cell Cycle Regulation:

    • The frequency of cell division varies with cell type due to regulation at the molecular level.

    • Cancer cells can escape these controls.

    • The cell cycle is driven by specific signaling molecules present in the cytoplasm.

Cell Control System

  • Definition: The sequential events of the cell cycle are directed by a distinct control system.

  • Controls: The system is regulated by both internal and external signals with specific checkpoints that halt the cycle until a go-ahead signal is received.

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

  • Regulatory Proteins:

    • Two key types are cyclins and cyclin-dependent kinases (Cdks).

    • Cyclins: Named for their cyclically fluctuating concentrations in the cell.

    • Cdks: Must attach to a cyclin to become active.

    • MPF: A cyclin-Cdk complex that triggers passage past the G2 checkpoint into M phase.

    • Peaks of MPF activity correspond with peaks of cyclin concentration.

Internal and External Signals at the Checkpoints

  • Sources of Signals:

    • Signals come from cellular surveillance mechanisms and external factors.

    • Three critical checkpoints: G1, G2, and M phases.

    • G1 checkpoint is often deemed the most important; receiving a go-ahead signal allows progression through the rest of the cell cycle.

    • If a signal isn’t received, the cell may exit into a nondividing state known as the G0 phase.

External Factors Influencing Cell Division

  • Growth Factors:

    • Released by certain cells, stimulating others to divide (e.g., platelet-derived growth factor (PDGF)).

  • Density-Dependent Inhibition:

    • Crowded cells will stop dividing; most animal cells exhibit anchorage dependence, requiring attachment to a substratum to divide.

Loss of Cell Cycle Controls in Cancer Cells

  • Cancer Cells Behavior:

    • Do not heed normal signals regulating the cell cycle.

    • Continue to divide when growth factors are depleted or may produce their own growth factors.

    • Have abnormal cell cycle control systems.

Tumors and Transformation

  • Cells that can divide indefinitely have undergone transformation.

  • Tumors: Abnormal masses of cells; if they remain at the original site, they're referred to as benign tumors, which typically do not cause major problems.

  • Malignant Tumors: Invade surrounding tissues and can undergo metastasis, spreading cancer cells to form additional tumors elsewhere in the body.