Cell Cycle

Midterm Information

  • Date: Tuesday, Feb 24th, 2026

  • Time: 4:00 PM – 6:00 PM (after lecture)

  • Permitted Materials: One-sided, handwritten cheat sheet

  • Coverage: Content from Weeks 1 through 6

  • Format:

    • 35 Multiple Choice Questions

    • 5 Short Answer Questions

Course Announcements

Contributors

  • Third Canadian Edition Authors:

    • Freeman

    • Quillin

    • Allison

    • Black Podgorski

    • Taylor Harrington Sharp

Chapter Overview: The Cell Cycle

Context:

  • Course Code: CBLG143: Biology I

  • Faculty: Faculty of Science, Department of Chemistry & Biology, Toronto Metropolitan University

Introduction to the Cell Cycle

  • Cells arise through cell division of pre-existing cells.

  • Observations show:

    • Plants and animals start as single-celled embryos.

    • They grow through a series of cell divisions.

  • Types of Cell Division:

    • Meiosis: Produces reproductive cells (gametes).

    • Mitosis: Produces all other cell types (somatic cells).

Mitosis and Cytokinesis

  • Both processes typically accompanied by cytokinesis, the division of the cytoplasm into two daughter cells.

  • These processes are crucial for one of life's fundamental attributes: reproduction.

Mitosis vs. Meiosis

  • Mitosis:

    • Genetic material is copied and divided equally between two cells (cellular replication).

    • Daughter cells are genetically identical to the parent cell and one another.

  • Meiosis:

    • Produces daughter cells that are genetically different from each other.

    • Each has half of the hereditary material compared to the parent cell.

Steps in Cellular Replication

  1. Copying the DNA

  2. Separating the copies

  3. Dividing the cytoplasm to create two complete cells

What Is a Chromosome?

  • Definition: A chromosome is a single, long double helix of DNA wrapped around proteins called histones.

  • DNA encodes genetic information.

  • Gene: A section of DNA that codes for a specific RNA; thus, it codes for a specific protein.

  • Chromosomes are visible during specific stages, existing as “X” shapes for a short time.

Chromosome Structure

  • Sister Chromatids: Two chromatids attached at the centromere are considered one chromosome.

  • Morphology Changes:

    • Unreplicated Chromosome: A single long DNA double helix.

    • Replicated Chromosome: Composed of two copies of the same DNA double helix.

    • Condensed Replicated Chromosome: DNA compacted around proteins, resulting in a structure 10,000 times shorter than the original length.

Cell Cycle Phases

  • M Phase:

    • Defined as the dividing phase where chromosomes condense into compact structures.

  • Interphase:

    • A nondividing phase where chromosomes are uncoiled and cells are either growing and preparing for division or fulfilling specialized functions.

  • Most cells spend the majority of their time in interphase.

Discovery of the S Phase

  • Researchers Alma Howard and Stephen Pelc discovered the S phase using radioactive labeling techniques:

    • Labeled DNA during replication.

    • Observed that DNA replication occurs in a specific phase during interphase, termed the S (synthesis) phase.

Phases of Interphase

  • Gap Phases:

    • G1 Phase: First gap occurring between M phase and S phase.

    • G2 Phase: Second gap occurring between S phase and mitosis, allowing cells to grow and replicate organelles.

Understanding M Phase

  • M Phase Components:

    1. Mitosis: Division of the replicated chromosomes.

    2. Cytokinesis: Division of the cytoplasm.

  • Eukaryotic species possess a characteristic number of chromosomes; for instance, human somatic cells have 46 chromosomes (2 × 23).

Chromatin and Mitosis

  • During interphase, chromatin—DNA wrapped around histone proteins—is in a relaxed state.

  • Chromosomes replicate during the S phase; each consists of two sister chromatids during mitosis.

Proteins Involved in Mitosis

  • Cohesins: Hold sister chromatids together.

  • Microtubules: Move chromosomes during mitosis.

  • Kinetochore Proteins: Serve as attachment sites for microtubules.

  • Nuclear Lamins: Intermediate filaments that maintain the nuclear envelope structure.

  • Condensins: Proteins that condense DNA for mitosis.

  • Centrosomes: Microtubule-organizing centers in animal cells during mitosis.

Mitosis Phases

  • Prophase:

    • Chromosomes condense and become visible.

    • The spindle apparatus forms, facilitating chromosome movement.

  • Prometaphase:

    • Breakdown of the nuclear envelope.

    • Microtubules attach to chromosomes at kinetochores.

  • Metaphase:

    • Chromosomes align on the metaphase plate, held in place by microtubules.

  • Anaphase:

    • Cohesions break, pulling sister chromatids toward opposite poles.

  • Telophase:

    • Nuclear envelope reforms around each set of chromosomes, and chromosomes decondense.

Cytokinesis

  • In Animals: A contractile ring of actin and myosin forms to pinch the cell membrane inward, leading to the cleavage furrow.

  • In Plants: Vesicles from the Golgi apparatus form a cell plate to divide the cell.

Bacterial Cell Replication

  • Bacteria divide through binary fission—similar to eukaryotic M phase.

  • The process involves replication of the bacterial chromosomes and the separation of cytoplasmic contents.

Cell Cycle Control

  • Cell-cycle length varies significantly among cell types, primarily due to different G1 phase lengths.

  • Rapidly dividing cells may eliminate G1, while non-dividing cells may enter a G0 state (e.g., nerve and muscle cells).

  • Example Rates:

    • Intestinal cells divide twice daily; liver cells can divide once a year or faster if damaged.

Experimental Evidence for Cell-Cycle Control

  • Hypothesis: Regulatory molecules in the cytoplasm control entry into M phase.

    • Experiment setup involved microinjection of cytoplasm from M-phase and interphase cells into oocytes to observe transitions.

M Phase-Promoting Factor (MPF)

  • Composition:

    • A protein kinase (enzyme) and a cyclin (protein whose concentration varies throughout the cycle).

    • MPF is responsible for the initiation of mitosis.

The Discovery of MPF

  • Experiment by Yoshio Masui determined that MPF is a protein, as treatments to break down proteins eliminated MPF activity, while RNA treatments did not.

Regulation of MPF Activity

  • Activation:

    • Cyclin binds to MPF's Cdk subunit, which is phosphorylated at two sites, enabling the kinase to activate proteins for mitosis.

  • Deactivation:

    • Negative feedback via protein destruction marks cyclin for degradation, leading to MPF inactivation.

Cell Cycle Checkpoints

  • Cell-cycle checkpoints monitor the cell's progression through critical points:

    1. G1 Checkpoint: Checks cell size, nutrients, growth signals, and DNA integrity.

    2. G2 Checkpoint: Verifies DNA replication and damage before M phase.

    3. M-Phase Checkpoint: Ensures proper chromosome attachment and separation.

Cancer and Cell Cycle Defects

  • Cancer arises from cell-cycle checkpoint failures, resulting in uncontrolled cell division.

  • Types of defects:

    1. Activating proteins for cell growth inappropriately.

    2. Preventing tumor suppressor genes from shutting down the cell cycle (e.g., loss of p53).

  • Two tumor types:

    • Malignant tumours: Cancerous, invasive, spreading via blood/lymph (metastasis).

    • Benign tumours: Non-cancerous, non-invasive.

Loss of Social Control and p53 Proteins

  • Cell division is regulated by external signals (growth factors).

  • Cancer cells often ignore these signals and divide uncontrollably.

  • p53 Mutation: Commonly associated with lung cancer due to smoking; over half of cancers involve p53 impairment.