THE CELL CYCLE AND CELL DIVISION

THE CELL CYCLE AND CELL DIVISION

OVERVIEW & INTRODUCTION

  • Importance of Cell Division:

    • Organisms need to make more cells for various reasons:

    • Unicellular organisms use cell division for reproduction.

    • Multicellular organisms utilize it for growth, development, tissue repair, and production of reproductive cells.

MODES OF CELL REPRODUCTION

CONCEPT 7.1: DIFFERENT LIFE CYCLES USE DIFFERENT MODES OF CELL REPRODUCTION

  • Key Functions of Cell Division:

    • Reproduction

    • Growth & Development

    • Tissue Repair

    • Production of Reproductive Cells

PROKARYOTIC REPRODUCTION

  • Prokaryotes reproduce asexually.

  • Asexual Reproduction: Offspring are clones, genetically identical to the parent.

  • Genetic variations arise through mutations post-division.

  • Binary Fission Process:

    • A unicellular prokaryote reproduces via binary fission:

    1. Young cell (non-dividing) growth.

    2. Preparation for division: enlarging cell wall, membrane, volume; chromosome duplicates.

    3. Formation of early transverse septum.

    4. Septum growth occurs; chromosomes move to opposite ends.

    5. Completion of septum results in two cell chambers.

    6. Daughter cells may separate completely or remain attached (chains or doublets).

EUKARYOTIC REPRODUCTIVE CYCLES

STRUCTURE OF DNA IN EUKARYOTIC CELLS

  • DNA is organized into linear chromosomes, with each consisting of a single DNA molecule and proteins.

  • Types of Cells:

    • Somatic Cells: Non-reproductive cells; each human somatic cell contains two sets of chromosomes (homologs).

    • Gametes: Reproductive cells, e.g., sperm and egg (ovum).

HAPLOID vs DIPLOID

  • Haploid (n): One copy of each chromosome.

  • Diploid (2n): Two copies of each chromosome.

  • The difference is illustrated by the number of homologous pairs in a given organism's chromosomes.

MEIOSIS & MITOSIS

  • Eukaryotic Cell Division:

    • Mitosis: Divides cell nucleus into two genetically identical daughter cells, followed by cytokinesis (division of cytoplasm).

    • Meiosis: Reduces chromosome number by half, produces four genetically different daughter cells. Meiosis consists of two stages, each followed by cytokinesis.

    • Uses in Animals: Mitosis for growth or tissue repair, meiosis for gamete production.

MODES OF REPRODUCTION IN EUKARYOTES

  • Unicellular eukaryotes generally reproduce asexually.

  • Multicellular eukaryotes can reproduce both asexually or sexually:

    • Asexual Reproduction: Includes budding and fragmentation.

    • Sexual Reproduction: Involves fusion of gametes.

GAMETES AND FERTILIZATION

  • Gametes are the product of meiosis and have complete genetic information.

  • Each gamete in humans has one set of chromosomes (haploid, n).

  • Fertilization involves fusion of two haploid gametes, forming a diploid zygote (2n).

SEXUAL REPRODUCTION LIFE CYCLES IN MULTICELLULAR EUKARYOTES

TYPES OF LIFE CYCLES

  • Haplontic Life Cycle:

    • Found in protists, fungi, and some algae; zygote is the only diploid stage, undergoes meiosis to form haploid spores.

  • Alternation of Generations:

    • Common in most plants and some protists; meiosis produces haploid spores which divide into a gametophyte that forms gametes.

  • Diplontic Life Cycle:

    • Present in animals and some plants where mature organisms are diploid and gametes are the only haploid stage.

CELL DIVISION PROCESSES

FOUR KEY EVENTS FOR CELL DIVISION

  1. Reproductive Signal: Initiates cell division.

  2. Replication of DNA.

  3. Segregation: Distribution of DNA into the new cells.

  4. Cytokinesis: Division of cytoplasm.

CELL CYCLE OVERVIEW

  • Defined as an ordered series of events involving cell growth and division, producing two new daughter cells.

  • In eukaryotes, divided into M phase (mitosis and cytokinesis) and interphase (G1, S, G2).

INTERPHASE SUBPHASE DETAILS

  • G1 Phase (Gap 1): Cell performs normal functions; variable duration.

  • S Phase (Synthesis): DNA replication occurs.

  • G2 Phase (Gap 2): Prepares for mitosis; synthesizes microtubules.

M PHASE DESCRIPTION

  • Divided into:

    • Mitosis: The nucleus divides; processes include prophase, prometaphase, metaphase, anaphase, and telophase.

    • Cytokinesis: Divides cytoplasm, differs between plant and animal cells.

MITOSIS PHASES EXPLAINED
PROPHASE

  • Chromosomes condense and centrosomes migrate to opposite sides; spindle formation initiates.

PROMETAPHASE

  • Nuclear envelope breaks down, chromosomes attach to kinetochore microtubules.

METAPHASE

  • Chromosomes align at the equatorial plane, preparing for separation.

ANAPHASE

  • Sister chromatids separate and move towards spindle poles due to forces exerted on them.

TELOPHASE

  • Chromosomes decondense, nuclear envelope reforms, resulting in two nuclei.

CYTOKINESIS

  • In animal cells, cytoplasm divides via a contractile ring; in plant cells, vesicles form a cell plate to create a new cell wall.

REGULATION OF CELL DIVISION

CONTROL MECHANISMS

  • External signals (e.g., growth factors) stimulate cell division; examples include Platelet Derived Growth Factor (PDGF).

  • Anchorage dependence: Cells must be attached to a substratum to divide.

INTERNAL CELL CYCLE REGULATION

  • Cyclins and Cyclin-Dependent Kinases (CDKs): Proteins that regulate the cell cycle; involve in phosphorylating target proteins to signal transitions between phases.

  • Each phase of the cycle is controlled through specific checkpoints, ensuring proper progression.

CELL DEATH MECHANISMS

NECROSIS

  • Occurs when cells are damaged, leading to swelling and bursting, releasing contents into the extracellular environment.

APOPTOSIS

  • Programmed cell death, characterized by:

    • Detaching from neighbors.

    • Fragmentation of chromatin.

    • Formation of “blebs” and subsequent engulfment by neighboring cells.

MEIOSIS INTENDED FUNCTION

  • Consists of two nuclear divisions, reducing chromosome number by half, generating genetic diversity.

MEIOSIS I PROCESS

  • Prophase I: Homologous chromosomes align and crossover.

  • Metaphase I: Homologs align at the equatorial plane; independent assortment occurs.

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

  • Telophase I: Two nuclei form, each with a reduced number of chromosomes.

MEIOSIS II PROCESS

  • Sister chromatids separate during Meiosis II to produce haploid daughter cells that are genetically diverse.