Cell Cycle Study Notes

Chapter 12: The Cell Cycle

Learning Objectives

  • Key Roles of Cell Division

    • Understand that cell division functions in reproduction, growth, renewal, and repair.

    • Explain how chromatin, chromosomes, and genomes relate to one another.

    • Describe the difference between a somatic cell and a gamete.

  • The Mitotic Cell Cycle

    • Describe the process of binary fission in bacteria.

    • List the phases of the cell cycle and describe the sequence of events occurring during each phase.

    • List the phases of mitosis and describe the major events characteristic of each phase.

    • Recognize the phases of mitosis from diagrams and micrographs.

    • Be able to draw the phases of mitosis given a specific number of chromosomes.

    • Draw or describe the mitotic spindle, including centrosomes, kinetochore microtubules, nonkinetochore microtubules, asters, and centrioles (in animal cells).

  • Regulation of the Cell Cycle

    • Describe the roles of checkpoints, cyclins, Cdks, and MPF in cell cycle control.

    • Describe the internal and external factors that influence the cell cycle control system.

    • Be able to give examples of each.

The Key Roles of Cell Division

  • Distinction of Living Organisms

    • The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter.

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

  • Cell Division in Unicellular and Multicellular Organisms

    • In unicellular organisms, division of one cell reproduces the entire organism.

    • Multicellular eukaryotes depend on cell division for:

    • Development from a fertilized cell.

    • Growth.

    • Repair.

  • Integral Part of Cell Cycle

    • Cell division is an integral part of the cell cycle, which is the life of a cell from formation to its own division.

Distribution of Chromosomes During Eukaryotic Cell Division

  • Genetic Identity in Cell Division

    • Most cell division results in genetically identical daughter cells with identical genetic information (DNA).

    • Exception: Meiosis, a special type of division that produces sperm and egg cells (gametes).

  • Cellular Organization of Genetic Material

    • All DNA in a cell constitutes that cell's genome.

    • A genome can consist of:

    • A single DNA molecule (common in prokaryotic cells).

    • A number of DNA molecules (common in eukaryotic cells).

    • DNA molecules are packaged into chromosomes.

  • Eukaryotic Chromosomes

    • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.

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

    • Somatic cells (nonreproductive cells) have two sets of chromosomes.

    • Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells.

The Mitotic Phase

  • Phases of the Cell Cycle

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

    • Interphase: Cell growth and copying of chromosomes in preparation for cell division, occupying about 90% of the cell cycle.

    • Interphase can be divided into subphases:

    • G1 Phase: “First gap.”

    • S Phase: “Synthesis,” when chromosomes are duplicated.

    • G2 Phase: “Second gap.”

  • Key Phases of Mitosis

    1. Prophase

    2. Prometaphase

    3. Metaphase

    4. Anaphase

    5. Telophase

Mitosis: A Closer Look

  • Mitotic Spindle

    • The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis.

    • In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center, which replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase.

  • Kinetochore and Chromosome Movement

    • During prometaphase, some spindle microtubules attach to kinetochores of chromosomes and start moving them.

    • Kinetochores are protein complexes associated with centromeres, while at metaphase, chromosomes align along the metaphase plate.

  • Anaphase Mechanism

    • In anaphase, cohesins are cleaved by an enzyme called separase, allowing sister chromatids to separate and move toward opposite cell ends along kinetochore microtubules.

    • Nonkinetochore microtubules from opposite poles overlap, pushing against each other to elongate the cell.

Cytokinesis

  • Cytokinesis Mechanisms

    • In animal cells, cytokinesis occurs by cleavage, forming a cleavage furrow.

    • In plant cells, a cell plate forms during cytokinesis.

Binary Fission in Bacteria

  • Process Details

    • Prokaryotes (bacteria and archaea) reproduce via binary fission, beginning at the origin of replication where the chromosome duplicates and the two daughter chromosomes move apart.

    • The plasma membrane pinches inward, ultimately dividing the cell into two.

  • Evolution of Mitosis

    • Mitosis likely evolved from binary fission, as certain protists display cell division types intermediate between binary fission and mitosis.

Regulation of the Cell Cycle

  • Cell Cycle Control System

    • The cell cycle is directed by a specific control system similar to a clock, regulated by both internal and external controls, including checkpoints that ensure proper timing and progression through the cell cycle phases.

  • Cyclins and Cyclin-Dependent Kinases (Cdks)

    • Two types of regulatory proteins involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks).

    • MPF (Maturation-Promoting Factor) is a cyclin-Cdk complex that triggers the passage of a cell from G2 into the M phase.

  • Checkpoint Signals

    • G1 is the primary checkpoint (

    • If a cell receives a go-ahead signal at the G1 checkpoint, it will typically complete S, G2, and M phases and divide.

    • If no signal is received, the cell enters a nondividing state called G0 phase.)

    • Internal signals ensure that chromatids are properly attached to the spindle before anaphase begins.

    • External factors include specific growth factors, like platelet-derived growth factor (PDGF), which stimulate cell division.

Implications of Cell Cycle Control

  • Cancer Cell Characteristics

    • Cancer cells do not respond normally to body controls and may grow and divide without the usual external growth signals.

    • Cancer cells can divert growth signals and exhibit an abnormal cell cycle control system.

    • Transformation is the process by which normal cells are converted into cancerous cells; tumors can be benign (localized) or malignant (metastatic).

  • Advances in Cancer Treatment

    • New understandings of cell cycle dynamics contribute to more personalized cancer treatments.

    • High-energy radiation affects localized tumors, while chemotherapies target specific cell cycle phases in metastatic cancers.