Cell cycle and cell division

Introduction to Cell Division

  • All organisms start their life from a single cell.

  • Growth and reproduction are key characteristics of cells.

  • Cells divide to form daughter cells which can further grow and divide, allowing for exponential growth in cell populations.

Cell Cycle Overview

  • Cell Cycle: The sequence of events through which a cell duplicates its genome, synthesizes other cell components, and divides.

    • Cell division requires DNA replication and cell growth to occur in a coordinated manner.

    • The cell cycle consists of various stages, though DNA synthesis occurs during one specific stage.

Phases of the Cell Cycle

  1. Interphase: Predominantly a preparatory phase, lasting over 95% of the cycle.

    • Divided into:

      • G1 Phase (Gap 1): Cell grows and metabolically active.

      • S Phase (Synthesis): DNA replication occurs, doubling the DNA content.

      • G2 Phase (Gap 2): Further growth and preparation for mitosis.

  2. M Phase: The actual division of the cell.

    • Consists of karyokinesis (nuclear division) and cytokinesis (cytoplasm division).

M Phase: Key Features

  • M Phase is marked by major cellular reorganizations and comprises four stages of mitosis:

    1. Prophase: Chromatin condenses into visible chromosomes; centrosomes move to opposite poles.

    2. Metaphase: Chromosomes align at the cell equator; spindle fibers attach to kinetochores.

    3. Anaphase: Sister chromatids separate and move towards opposite poles.

    4. Telophase: Chromosomes de-condense; nuclear envelopes form around each set; nucleolus reappears.

  • Cytokinesis: Separates the cytoplasm of the parent cell into two daughter cells, differing in mechanism between plant and animal cells.

Significance of Mitosis

  • Mitosis is crucial for growth, healing, and maintaining the proper nucleus-to-cytoplasm ratio in cells.

  • Mostly occurs in diploid somatic cells but can also happen in haploid cells in certain lower organisms.

  • Enables continuous growth in plants via mitotic divisions in meristematic tissues.

Meiosis: Overview and Importance

  • Meiosis: Specialized type of cell division that reduces the chromosome number by half, forming haploid gametes necessary for sexual reproduction.

  • Involves two rounds of division (Meiosis I and Meiosis II) but only one DNA replication cycle.

  • Key features include:

    • Pairing of homologous chromosomes and genetic recombination through crossing over, enhancing genetic variability.

Phases of Meiosis

  • Meiosis I:

    • Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis) forming bivalents.

    • Metaphase I: Bivalents align on the equatorial plane.

    • Anaphase I: Homologous chromosomes separate; sister chromatids remain together.

    • Telophase I: Two daughter cells form, each with half the chromosome number, followed by cytokinesis.

  • Meiosis II: Similar to mitosis; results in four haploid cells at the end.

    • Prophase II, Metaphase II, Anaphase II, Telophase II are similar to their counterparts in mitosis.

Cell Cycle Summary

  • The cell cycle is the life of a cell, ranging from growth through division.

  • Essential for development and reproduction in organisms.

  • Mitosis conserves the chromosome number; meiosis reduces it, ensuring genetic diversity in future generations.

Exercises

  1. Identify the average cell cycle span for a mammalian cell.

  2. Explain the differences between cytokinesis and karyokinesis.

  3. Describe interphase events in detail.

  4. Define the G0 (quiescent phase).

  5. Why is mitosis referred to as an equational division?

  6. Locate stages of the cell cycle for specific events like chromosome alignment and centromere splitting.