Chapter 6: Cell Division

Learning Objectives

By the end of this chapter, you should be able to:

• Describe the cell cycle and its phases
• Explain the process and significance of mitosis
• Understand meiosis and its role in sexual reproduction
• Differentiate between mitosis and meiosis
• Explain the relationship between cell division and cancer

Overview

Cell division is essential for growth, repair, and reproduction in multicellular organisms. The cell cycle coordinates DNA replication and cell division, ensuring genetic continuity. Mitosis produces genetically identical cells for growth and repair, while meiosis produces genetically diverse gametes for sexual reproduction. Understanding these processes is crucial for comprehending development, genetics, and disease.

The Cell Cycle

Definition and Phases

The cell cycle is the sequence of events that takes place in a cell leading to its division and duplication. It consists of:

Interphase

The longest phase where the cell prepares for division, consisting of three sub-phases:

G1 Phase (Gap 1)

• Duration: Variable (6-24 hours in mammalian cells)
• Activities: Cell growth, organelle duplication, protein synthesis
• Checkpoint: G1 checkpoint checks for DNA damage and adequate resources

S Phase (Synthesis)

• Duration: 8-12 hours in mammalian cells
• Key Event: DNA replication - chromosomes duplicate
• Result: Each chromosome consists of two sister chromatids
• DNA Content: $2n \rightarrow 4n$

G2 Phase (Gap 2)

• Duration: 4-6 hours in mammalian cells
• Activities: Final preparations for division
• Checkpoint: G2 checkpoint checks for DNA replication completion and damage

Mitotic Phase (M Phase)

The phase where actual cell division occurs, including mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Did You Know? In a typical human cell, the cell cycle takes about 24 hours to complete, with mitosis taking only 1-2 hours. The rest of the time is spent in interphase, preparing the cell for division!

Mitosis: Cell Division for Growth and Repair

Definition and Significance

Mitosis is the process of nuclear division that results in two daughter cells with genetically identical chromosomes to the parent cell.

Significance:

• Growth: Increases cell number in multicellular organisms
• Repair: Replaces damaged or dead cells
• Asexual Reproduction: Produces genetically identical offspring
• Development: Essential for embryonic development

Stages of Mitosis

Mitosis consists of four distinct phases:

Prophase

Key Events:

• Chromatin condenses into visible chromosomes
• Chromosomes appear as X-shaped structures with two sister chromatids
• Nuclear envelope breaks down
• Spindle fibers begin to form from centrioles

Visual Changes: Nucleus disappears, chromosomes become visible

Metaphase

Key Events:

• Chromosomes align at the equator (metaphase plate) of the cell
• Spindle fibers attach to centromeres of chromosomes
• Chromosomes are maximally condensed and easiest to count

Visual Changes: Chromosomes arranged in a single line across the cell center

Anaphase

Key Events:

• Sister chromatids separate and move to opposite poles
• Each chromatid is now considered an independent chromosome
• Spindle fibers shorten, pulling chromosomes apart

Visual Changes: Chromosomes moving away from center toward poles

Telophase

Key Events:

• Chromosomes arrive at opposite poles
• Chromosomes begin to decondense back to chromatin
• Nuclear envelopes reform around each set of chromosomes
• Nucleoli reappear

Visual Changes: Two distinct nuclei forming, chromosomes becoming less distinct

Cytokinesis: Cell Division

Definition: The division of cytoplasm to form two separate daughter cells

In Animal Cells:

• Cleavage furrow forms in the middle of the cell
• Actin filaments contract, pinching the cell in two
• Results in two approximately equal daughter cells

In Plant Cells:

• Cell plate forms in the middle of the cell
• Vesicles from Golgi apparatus fuse to form the plate
• Cell plate develops into new cell wall
• Results in two daughter cells each with its own cell wall

Mitosis Summary:

Meiosis: Sexual Reproduction

Definition and Purpose

Meiosis is the specialized cell division that produces gametes (sperm and egg cells) with half the chromosome number of the parent cell.

Purpose:

• Reduce chromosome number from diploid (2n) to haploid (n)
• Generate genetic diversity through crossing over and independent assortment
• Ensure chromosome number remains constant across generations

Meiosis Equation:

$$2n \xrightarrow{\text{Meiosis I}} n \xrightarrow{\text{Meiosis II}} n$$

Meiosis vs. Mitosis Comparison

Stages of Meiosis

Meiosis I: Reduction Division

Prophase I

• Longest and most complex phase
• Synapsis: Homologous chromosomes pair up
• Crossing over: Exchange of genetic material between homologous chromosomes
• Tetrads (pairs of homologous chromosomes) form
• Chiasmata (sites of crossing over) visible

Metaphase I

• Homologous chromosome pairs align at the equator
• Independent assortment occurs
• Each pair consists of two chromosomes (each with two chromatids)

Anaphase I

• Homologous chromosomes separate
• Sister chromatids remain together
• Each pole receives one chromosome from each homologous pair

Telophase I and Cytokinesis

• Chromosomes arrive at poles
• Nuclear envelopes may reform temporarily
• Cytokinesis produces two haploid daughter cells

Meiosis II: Equational Division

Similar to mitosis but with haploid cells:

Prophase II

• Chromosomes condense if they decondensed
• No DNA replication occurs

Metaphase II

• Chromosomes align at equator (individually)

Anaphase II

• Sister chromatids separate and move to opposite poles

Telophase II and Cytokinesis

• Four haploid daughter cells formed
• Each with half the chromosome number of original cell

Genetic Significance of Meiosis:

1. Crossing Over: Creates new combinations of alleles on chromosomes
2. Independent Assortment: Random alignment of homologous pairs produces diverse gametes
3. Random Fertilization: Any sperm can fertilize any egg, increasing diversity

SPM Exam Tip: When comparing mitosis and meiosis, remember that meiosis produces 4 genetically different haploid cells, while mitosis produces 2 identical diploid cells. Crossing over and independent assortment are key sources of genetic diversity in meiosis!

Issues in Cell Division: Cancer

Definition of Cancer

Cancer is characterized by uncontrolled cell division due to mutations in genes that regulate the cell cycle.

Cell Cycle Control

Key Regulatory Proteins:

• Cyclins: Proteins that activate cyclin-dependent kinases (CDKs)
• CDKs: Enzymes that phosphorylate target proteins to control cell cycle progression
• Tumor Suppressor Genes: Inhibit cell division (e.g., p53, Rb)
• Proto-oncogenes: Promote normal cell division (when mutated, become oncogenes)

Cell Cycle Control Equation:

$$\text{Cell Division} \xrightarrow{\text{Cyclins + CDKs}} \text{Progression} \xrightarrow{\text{Checkpoint}} \text{Division or Arrest}$$

Cancer Development

Multi-step Process:

1. Initiation: DNA damage/mutation occurs
2. Promotion: Damaged cells divide and accumulate mutations
3. Progression: Cancer cells acquire invasive properties

Cancer Development Equation:

$$\text{DNA Mutation} \xrightarrow{\text{Checkpoint Failure}} \text{Uncontrolled Division} \xrightarrow{\text{Accumulation}} \text{Tumor Formation}$$

Hallmarks of Cancer Cells:

• Uncontrolled proliferation: Ignored growth signals
• Evading growth suppressors: Resistant to inhibitory signals
• Resisting cell death: Avoid apoptosis
• Replicative immortality: Unlimited division capacity
• Angiogenesis: Stimulating blood vessel formation
• Metastasis: Ability to spread to other locations

Common Cancer Types and Their Characteristics

Cancer Detection and Prevention

Early Detection Methods:

• Biopsy: Examination of tissue samples
• Imaging: X-rays, CT scans, MRI
• Blood tests: Tumor markers, genetic testing
• Screening programs: Mammograms, Pap smears, colonoscopies

Prevention Strategies:

• Lifestyle changes: Healthy diet, regular exercise, avoiding tobacco
• Environmental protection: Reducing exposure to carcinogens
• Regular check-ups: Early detection and treatment
• Genetic counseling: For hereditary cancer syndromes

Laboratory Techniques in Cell Division Studies

Microscopy Techniques

Light Microscopy:

• Staining techniques (Feulgen stain for DNA)
• Phase-contrast microscopy for living cells
• Time-lapse photography of cell division

Electron Microscopy:

• Transmission EM for detailed chromosome structure
• Scanning EM for cell surface changes during division

Molecular Biology Techniques

Flow Cytometry:

• Analyzes DNA content and cell cycle phases
• Can detect abnormal DNA content (aneuploidy)

Fluorescence Microscopy:

• Fluorescent labels for specific proteins and DNA
• Live-cell imaging of cell division processes

Chromosome Analysis:

• Karyotyping: Visual examination of chromosome number and structure
• FISH (Fluorescence In Situ Hybridization): Specific chromosome labeling

Practice Tips for SPM Students

Memorization Strategies

1. Create flowcharts showing mitosis and meiosis stages
2. Use mnemonics for phase names (e.g., "Please My Aunt Teases" for Prophase, Metaphase, Anaphase, Telophase)
3. Draw comparison tables for mitosis vs. meiosis
4. Practice labeling diagrams of cell division stages

Conceptual Understanding

1. Understand chromosome behavior in each phase
2. Relate structure to function (e.g., spindle fibers for chromosome movement)
3. Apply knowledge to genetic inheritance problems
4. Connect to real-world applications (cancer, genetics)

Experimental Skills

1. Identify phases from microscope images
2. Calculate chromosome numbers at different stages
3. Interpret karyotypes for genetic abnormalities
4. Design experiments to study cell cycle regulation

Environmental and Health Connections

Environmental Factors Affecting Cell Division

• Radiation: UV radiation, X-rays can damage DNA and cause mutations
• Chemical carcinogens: Tobacco smoke, industrial chemicals
• Viruses: Some viruses integrate into host DNA and disrupt cell cycle
• Diet and lifestyle: Obesity, poor nutrition can increase cancer risk

Medical Applications

• Cancer treatments: Chemotherapy, radiation therapy target rapidly dividing cells
• Stem cell therapies: Understanding cell division for regenerative medicine
• Genetic counseling: Understanding inheritance patterns and genetic disorders

Agricultural Applications

• Plant breeding: Understanding meiosis for crop improvement
• Tissue culture: Using cell division for plant propagation
• Genetically modified organisms: Manipulating cell division processes

Summary

• The cell cycle coordinates DNA replication and cell division
• Mitosis produces genetically identical cells for growth and repair
• Meiosis produces genetically diverse gametes for sexual reproduction
• Cell cycle regulation is essential for preventing cancer
• Understanding cell division is crucial for genetics, medicine, and biotechnology
• Cancer results from uncontrolled cell division due to genetic mutations

Related Topics

• Chapter 4: Chemical Composition in Cells
• Chapter 5: Metabolism and Enzymes
• Chapter 7: Respiratory Systems in Humans and Animals