Mitosis and the Cell Cycle: Study Guide for BIOL150

MITOSIS: THE CELL CYCLE

Overview of Cell Division

Cell division is a fundamental process in biology, essential for growth, repair, and replacement of cells in multicellular organisms. The cell cycle describes the ordered sequence of events that a cell undergoes to divide and produce two genetically identical daughter cells.

• Purposes of Cell Division: Growth, repair, and replacement of cells.

• Cell Cycle: Consists of interphase (cell growth and DNA replication), mitosis (separation of chromosomes), and cytokinesis (division of the cytoplasm).

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

• Mitosis: Nuclear division resulting in two genetically identical daughter cells. Involves five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

• Meiosis: Specialized nuclear division that reduces chromosome number by half, producing gametes.

• Genome: The complete set of DNA in a cell.

Chromosome Structure

• Chromatin: DNA-protein complex that makes up chromosomes.

• Sister Chromatids: Identical copies of a chromosome, joined at the centromere by cohesins.

• Centromere: The constricted region where sister chromatids are most closely attached.

Somatic Cells vs. Gametes

• Somatic Cells: All body cells except reproductive cells; diploid (contain two sets of chromosomes).

• Gametes: Reproductive cells (sperm and egg); haploid (contain one set of chromosomes).

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

The cell cycle is divided into interphase and the mitotic (M) phase. Interphase is the period of cell growth and DNA replication, while the M phase includes mitosis and cytokinesis.

Interphase

• G1 Phase: Cell growth and normal functions.

• S Phase: DNA replication occurs.

• G2 Phase: Preparation for mitosis; nuclear envelope is intact, and centrosomes duplicate.

Mitotic Phase (M Phase)

• Prophase: Chromatin condenses into chromosomes; mitotic spindle begins to form.

• Prometaphase: Nuclear envelope breaks down; spindle microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate; kinetochores attach to spindle fibers from opposite poles.

• Anaphase: Cohesins are cleaved; sister chromatids separate and move to opposite poles; cell elongates.

• Telophase: Nuclear envelopes reform; chromosomes decondense; nucleoli reappear; spindle disassembles.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Mitotic Spindle Components

• Centrosomes: Microtubule-organizing centers in animal cells.

• Kinetochores: Protein complexes at the centromere where spindle fibers attach.

• Metaphase Plate: Imaginary plane equidistant between the spindle's two poles where chromosomes align during metaphase.

Cytokinesis in Animal vs. Plant Cells

Feature	Animal Cells	Plant Cells
Mechanism	Cleavage furrow forms	Cell plate forms
Description	Shallow groove near old metaphase plate	New cell wall forms between daughter cells

Binary Fission in Bacteria

• Origin of Replication: Specific site where DNA replication begins in prokaryotes.

• Bacteria divide by binary fission, a simpler process than mitosis.

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

The cell cycle is tightly regulated by checkpoints and molecular signals to ensure proper division and prevent errors.

Cell Cycle Control System

• Checkpoints: Critical control points where stop and go-ahead signals regulate the cycle.

• G1 Checkpoint: Most important; if passed, the cell usually completes the cycle. Checks for sufficient resources before DNA synthesis.

• G0 Phase: Non-dividing state entered if the cell does not receive the go-ahead signal at G1.

• G2 Checkpoint: Ensures DNA replication is complete and undamaged before mitosis. Involves cyclins, cyclin-dependent kinases (Cdks), and maturation-promoting factor (MPF).

• M Checkpoint: Ensures all chromosomes are properly attached to the spindle before anaphase.

Regulatory Molecules

• Cyclins: Proteins whose levels fluctuate during the cell cycle.

• Cyclin-dependent Kinases (Cdks): Enzymes activated by cyclins; regulate cell cycle progression.

• MPF (Maturation-Promoting Factor): Cyclin-Cdk complex that triggers passage from G2 to M phase.

External Signals Influencing Cell Division

• Growth Factors: Proteins released by certain cells that stimulate others to divide.

• Density-Dependent Inhibition: Cells stop dividing when crowded.

• Anchorage Dependence: Cells must be attached to a substrate to divide.

Cancer Cells and Loss of Cell Cycle Control

• Transformation: Process by which cells acquire the ability to divide indefinitely.

• Benign Tumor: Abnormal cells remain at the original site.

• Malignant Tumor: Invades surrounding tissues and can spread (metastasis) to other parts of the body, forming new tumors.

Summary Table: Key Differences in Cell Division

Process	Purpose	Result
Mitosis	Growth, repair, replacement	2 genetically identical diploid cells
Meiosis	Sexual reproduction	4 genetically unique haploid cells
Binary Fission	Prokaryotic cell division	2 genetically identical cells

Key Equations and Concepts

• Chromosome Number in Mitosis: If a cell starts with n chromosomes, each daughter cell will have n chromosomes after mitosis.

• Cell Cycle Regulation: Progression through the cell cycle is regulated by the concentration of cyclins and the activity of Cdks.

Example: In human somatic cells (2n = 46), mitosis produces two daughter cells each with 46 chromosomes, while meiosis produces gametes with 23 chromosomes each.

Additional info: The cell cycle is a highly conserved process across eukaryotes, and errors in regulation can lead to diseases such as cancer. Understanding the molecular mechanisms of cell cycle control is crucial for advances in cancer therapy and regenerative medicine.