Cell Cycle and Cell Division Lecture Notes 09

Lecture Overview

  • Topic: Cell Cycle and Cell Division

  • Instructor: Prof. Yan Wang

  • Institution: Biological Sciences, UTSC

  • Relevant Chapters:

    • Chapter 1.3: Introduction & road map of the course

    • Chapter 3: Cell structure and components

    • Chapter 6.1, 6.2, 6.3: Metabolism and energy

    • Chapter 6.4 & 6.5: Chemical reactions and enzymes

    • Chapter 7.1-7.5: Cellular respiration I

    • Chapter 7.6 & 7.7: Cellular respiration II

    • Chapter 8.1, 8.2, 8.3: Photosynthesis I

    • Chapter 8.4 & 8.5: Photosynthesis II

    • Chapter 11.1, 11.4 & 14.1: Cell cycle and cell division

    • Chapter 11.5 & 14.2: Regulation of the cell cycle and cancer

    • Chapter 32.1 & 32.2: Tree of Life: fungal cells

    • Chapter 32.3: Tree of Life: fungal diversity

Core Concepts

  1. The process of cell division increases cell number and occurs for:

    • Growth

    • Cell replacement

    • Healing

    • Reproduction

  2. Types of Cell Division:

    • Mitotic Cell Division:

      • Basis for asexual reproduction in unicellular eukaryotes.

      • In multicellular eukaryotes, responsible for cellular division processes.

    • Meiotic Cell Division:

      • Essential for sexual reproduction, results in offspring combining genetic material from two parents.

  3. The two daughter cells must each receive the full complement of genetic material from the parent cell.

Binary Fission Steps

  • The process occurs in prokaryotes:

    1. The circular bacterial DNA is attached by proteins to the inner membrane.

    2. DNA replication starts at a specific location and proceeds bidirectionally around the circle.

    3. Newly synthesized DNA is attached to the inner membrane near the original attachment site.

    4. The cell elongates symmetrically around the midpoint, separating the DNA attachment sites.

    5. Cell division begins with the synthesis of new membrane and wall material at the midpoint.

    6. Continued synthesis completes separation, leading to two daughter cells.

Comparison of Eukaryotes and Prokaryotes

  • Eukaryotes:

    • Genome: large and linear

    • DNA is located in the nucleus

  • Prokaryotes:

    • Genome: small and circular

    • DNA is located in the cytoplasm

Eukaryotic Cell Cycle

  • M Phase:

    • Time during which the parent cell divides into two daughter cells.

  • Interphase:

    • Time between two successive M phases, cells prepare for division through DNA replication and size increase.

Key Regulators of the Cell Cycle

  • Using model organisms such as Schizosaccharomyces pombe and Saccharomyces cerevisiae to study cell cycle regulation.

  • Defects in cell cycle control may lead to chromosome alterations seen in cancer cells.

DNA Organization in Eukaryotes

  • DNA is organized with histones and other proteins into chromatin.

  • Chromatin can be looped and packaged to form chromosomes.

  • Karyotype: visual representation of an organism's chromosomes.

    • Homologous chromosomes: pairs of chromosomes, one from each parent.

    • Sex chromosomes: XX for females and XY for males.

Mitosis Stages

  • Prophase: Chromosomes condense, centrosomes migrate to opposite poles, mitotic spindle forms.

  • Prometaphase: Microtubules attach to chromosomes, nuclear envelope breaks down.

  • Metaphase: Chromosomes align in the center of the cell.

  • Anaphase: Sister chromatids separate as centromeres split and move toward opposite poles.

  • Telophase: Nuclear envelope re-forms, chromosomes decondense, marks the end of mitosis.

  • Cytokinesis in Animal Cells: A contractile ring forms, involving actin filaments, leading to cell separation.

  • Cytokinesis in Plant Cells: Formation of a cell plate that will develop into the cell wall separating daughter cells.

Meiotic Cell Division

  • Results in four daughter cells, each with half the number of chromosomes compared to the parent cell.

  • Each daughter cell is genetically unique.

  • Meiosis I: Homologous chromosomes separate.

  • Meiosis II: Sister chromatids separate, resembling mitosis.

Steps of Meiosis I

  • Prophase I:

    1. Chromosomes become visible as threads.

    2. Homologous chromosomes condense and undergo synapsis, forming bivalents.

    3. Crossing over occurs, allowing exchange of DNA segments.

  • Prometaphase I: Spindles attach to kinetochores on chromosomes.

  • Metaphase I: Homologous pairs align at the cell's center in bivalent formation.

  • Anaphase I: Reductional division occurs, centromeres do not split.

  • Telophase I: Completion of the first meiotic division.

  • Meiosis II: Resembles mitosis, with no DNA synthesis between meiotic divisions, equational division occurs.

  • Variations occur in cytoplasmic division during female and male meiosis, leading to different outcomes.

    • In females: Unequal division resulting in polar bodies.

    • In males: Equal division resulting in functional sperm.

Summary Diagram

  • Mitosis vs. Meiosis:

    • Mitosis: One division, produces two genetically identical daughter cells.

    • Meiosis: Two divisions, produces four genetically diverse daughter cells.

Ethics and Implications

  • Understanding cell cycle and division is crucial for biological research, particularly in cancer studies and developmental biology ensuring proper reproductive health.

  • Provides insights into evolutionary relationships and genetic diversity among organisms.