bio notes

Why Do Cells Divide?

  • Reproduction

    • Asexual reproduction

    • One-celled organisms

  • Growth

    • From fertilized egg to multi-celled organism

  • Repair & Renewal

    • Replace cells that die from normal wear and tear or injury

  • Nucleus

    • Contains chromosomes

    • Contains DNA

  • Cytoskeleton

    • Contains centrioles

Structure of the Nucleus

  • Function

    • Protects DNA

  • Structure

    • Nuclear envelope: double membrane

    • Membrane fused in spots to create pores

      • Allows large macromolecules to pass through

Cytoskeleton

  • Function

    • Provides structural support

    • Maintains shape of cell

    • Provides anchorage for organelles

  • Components

    • Protein fibers: microfilaments, intermediate filaments, microtubules

  • Motility

    • Cell locomotion, through cilia and flagella

  • Regulation

    • Organizes structures and activities of the cell

Centrioles

  • Role in Cell Division

    • In animal cells, pairs of centrioles organize microtubules into spindle fibers

    • Guide chromosomes during mitosis

Mitosis Overview

  • Mitosis and Cytokinesis

    • Exact copy of genetic material (DNA) passed on to daughter cells

    • Includes organelles, cytoplasm, cell membrane, enzymes

  • Interphase

    • Accounts for 90% of the cell life cycle

    • Cell performs its "everyday job"

      • Produces RNA, synthesizes proteins/enzymes

      • Prepares for duplication if triggered

    • Nucleus during Interphase

      • Well-defined with DNA loosely packed in long chromatin fibers

      • Prepares for mitosis by replicating chromosomes (DNA and proteins)

S Phase (Synthesis Phase)

  • Role in DNA Replication

    • Cell replicates DNA

      • Human cells duplicate approximately 3 meters of DNA

      • Each daughter cell receives complete identical copy

      • Error rate of replication is about 1 per 100 million bases

      • With 3 billion base pairs in mammalian genome, ~30 errors per cell cycle

      • Can lead to mutations in somatic (body) cells

  • Organizing DNA

    • DNA organized into chromosomes

      • Double helix DNA molecule wound around histone proteins

      • DNA-protein complex forms chromatin

      • Condensed further during mitosis

Copying and Packaging DNA

  • After DNA Duplication

    • Chromatin condenses through coiling and folding to make smaller packages

  • Mitotic Chromosome

    • Duplicated chromosome: consists of 2 sister chromatids

    • Narrow at centromeres, containing identical copies of original DNA

Mitosis Phases

  • Overview of Mitosis

    • Process that divides the cell's DNA between two daughter nuclei

    • Involves the "dance of the chromosomes"

  • Four Phases of Mitosis

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

  • Prophase

    • Chromatin condenses into visible chromosomes

    • Centrioles move to opposite poles of the cell

    • Protein fibers cross the cell to form the mitotic spindle (microtubules)

    • Nucleolus disappears and nuclear membrane breaks down

  • Transition to Metaphase (Prometaphase)

    • Spindle fibers attach to centromeres

    • Kinetochores form at centromeres

    • Chromosomes begin moving

  • Metaphase

    • Chromosomes align along the middle of the cell (metaphase plate)

    • Spindle fibers coordinate movement to ensure correct separation

Anaphase

  • Process

    • Sister chromatids separate at kinetochores

    • Chromatids are pulled to opposite poles by motor proteins

    • Poles move apart as polar microtubules lengthen

  • Separation of Chromatids

    • Proteins holding sister chromatids are inactivated, turning them into individual chromosomes

Telophase

  • Completion of Mitosis

    • Chromosomes arrive at opposite poles

    • Daughter nuclei form, nucleoli reappear

    • Chromosomes disperse; become non-visible under light microscope

    • Spindle fibers disperse

    • Cytokinesis begins, signaling cell division

  • Cytokinesis in Animals

    • Constriction belt of actin microfilaments around equator of cell creates cleavage furrow, splitting the cell in two

  • Cytokinesis in Plants

    • Cell plate forms, vesicles derive from the Golgi and fuse to create new membranes

    • A new cell wall is laid between the membranes, fusing with the existing wall

Evolution of Mitosis

  • Origin in Eukaryotes

    • Likely evolved from binary fission in bacteria

    • Indicative of a progression between binary fission and modern mitosis

  • Intermediate Structures

    • Found in modern organisms like dinoflagellates and diatoms

Kinetochore

  • Each chromatid has its own kinetochore proteins

  • Microtubules attach to these kinetochore proteins during cell division

Cell Division Cycle

  • Phases of Dividing Cell's Life

    • Interphase

    • Cell grows, replicates chromosomes, produces new organelles, enzymes, membranes (G1, S, G2)

    • Mitotic phase

    • Cell separates and divides chromosomes (mitosis)

    • Divides cytoplasm and organelles (cytokinesis)

  • Reproduction

    • Mitosis produces identical daughter cells (clones)

    • Each has an equal amount of DNA, same number of chromosomes, and genetic information

  • Asexual Reproduction

    • Simple eukaryotes (yeast, Paramecium, Amoeba) and multicellular eukaryotes (Hydra) reproduce asexually

Sexual Reproduction and Meiosis

  • Alternating Processes

    • Meiosis produces gametes; reduces chromosomal numbers from diploid (2n) to haploid (n)

    • Restoration through fertilization

  • Mitosis vs. Meiosis

    • Meiosis involves special cell division to generate gametes, reducing chromosome number

Coordination of Cell Division

  • Multicellular organisms need coordinated timing across different parts

    • Critical for growth, development, and maintenance

  • Frequency of Cell Division

    • Varies between cell types:

      • Skin cells divide frequently

      • Liver cells have retained division ability but reserve it

      • Mature nerve and muscle cells do not divide after maturity

Cell Cycle Control

  • Irreversible Points in Cell Cycle

    • Genetic material replication

    • Separation of sister chromatids

  • Checkpoints

    • Control system regulates the cycle at critical points based on cellular process completion

  • Major Checkpoints

    • G1

    • G2

    • M phases (spindle checkpoint)

  • G1 Checkpoint

    • Most critical decision point

    • Receives "go" signal to divide or exits to G0 phase (non-dividing state)

Activation of Cell Division

  • Cell Communication

    • Signals (chemical cues) prompt cell division

    • Usually involve proteins, including activators and inhibitors

  • "Go-ahead" Signals

    • Promote growth and division

    • Include cyclins and cyclin-dependent kinases (Cdks)

  • Regulatory Proteins and Stages

    • Cyclins govern progression through the cell cycle

    • MPF (maturation-promoting factor) and APC (anaphase-promoting complex) play crucial roles

  • Conservation of Genes

    • Genes for regulatory proteins are highly conserved across species

External Signals

  • Growth Factors

    • Protein signals that stimulate cells to divide

    • Density-Dependent Inhibition

      • Crowded cells stop dividing

      • Mass of cells uses up growth factors

    • Anchorage Dependence

      • Cells must be attached to divide

  • Example Growth Factor

    • Platelet-Derived Growth Factor (PDGF): stimulates fibroblast cell division aiding in wound repair

Growth Factors and Cancer

  • Influence of Growth Factors

    • Proto-oncogenes can become oncogenes (cancer-causing) when mutated

    • Normal genes can lead to rapid cell growth if activated

    • Tumor-suppressor genes inhibit cell division; mutations can result in cancer

  • Role of p53 Gene

    • Plays a crucial role in DNA checkpoint regulation

    • Halts cell division in response to damaged DNA

    • Stimulates repair enzymes or induces apoptosis in severely damaged cells

Development of Cancer

  • Cumulative Mutations

    • Cancer stems from approximately 6 key mutations:

      • Unlimited growth: activation of growth promoters

      • Ignoring checkpoints: malfunction in tumor suppressor genes

      • Escaping apoptosis: turning off suicide genes

      • Immortality: enabling unlimited divisions

      • Promoting blood vessel growth: activating related genes

      • Overcoming Anchorage & Density Dependence: turning off inhibitory genes

Causes of "Hits"

  • Can result from exposure to:

    • UV radiation

    • Chemical exposure

    • Radiation exposure

    • Heat

Tumors

  • Benign Tumor

    • Abnormal cells remain at the original site as a lump

    • Often manageable; may be removed surgically

  • Malignant Tumor

    • Cells break away from the original site

    • Spread to other tissues via blood or lymph (metastasis)

    • Can impair organ function

Traditional Cancer Treatments

  • Target rapidly dividing cells

    • High-energy radiation and chemotherapy with toxic drugs aim to kill rapidly dividing cells