Cell Division: Mechanisms in Prokaryotes and Eukaryotes

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Cell Division

Introduction to Cell Division

Cell division is the biological process by which a parent cell divides into two or more daughter cells. This process is fundamental for growth, development, tissue repair, and reproduction in all living organisms. Depending on the mechanism, daughter cells may be genetically identical (asexual reproduction) or genetically distinct (sexual reproduction).

Asexual reproduction: Produces genetic clones of the parent cell.
Sexual reproduction: Produces genetically and phenotypically distinct offspring.

When and Why Do Cells Divide?

Cell division serves different purposes in unicellular and multicellular organisms:

Unicellular organisms (prokaryotes): Cell division is used solely for asexual reproduction.
Multicellular organisms: Cell division is essential for tissue growth, tissue homeostasis, repair/wound healing, and sexual reproduction.

Mechanisms of Cell Division

Prokaryotes

Prokaryotes (bacteria and archaea) primarily reproduce by binary fission, a simple and efficient process.

Simple: Prokaryotic cells lack a nucleus and complex organelles, possessing a single circular chromosome.
Efficient: Cell growth, DNA replication, and division occur simultaneously.
Fast: Rate of division is influenced by environmental factors (temperature, nutrients, pH).
Asexual reproduction: Results in genetically identical clones.

Binary Fission Process

DNA replication
Chromosome segregation
Cytokinesis (cell splits)

Role of FtsZ Proteins

Binary fission is orchestrated by the divisome, a protein complex centered on the tubulin homolog FtsZ.

FtsZ polymerizes into a dynamic ring (the Z-ring) that defines the division site.
Recruits downstream proteins and directs peptidoglycan synthesis for cell wall constriction and division.

Evidence for FtsZ Function

Conditional mutants (e.g., temperature-sensitive G105S mutation in Staphylococcus aureus FtsZ) demonstrate that non-functional FtsZ impairs cell division.

Regulation of Z-ring Localization

Min proteins: Prevent Z-ring assembly at cell poles, ensuring correct division site.
Min mutants result in mislocalized Z-rings and abnormal cell division.

Alternative Prokaryotic Cell Division Mechanisms

Budding: New cell forms as a bud from the parent (e.g., Hyphomicrobium polymorphum).
Fragmentation: Filamentous bacteria break into fragments, each becoming a new cell (e.g., cyanobacteria).
Sporulation: Formation of spores for survival under adverse conditions (e.g., Bacillus species).

Genetic Variation in Prokaryotes

Random genetic mutations
Horizontal gene transfer:
- Conjugation
- Transformation
- Transduction
Natural selection acts on genetic variation.

Eukaryotes

Cell division in eukaryotes is more complex, involving distinct phases and specialized structures.

Comparison: Prokaryotic vs. Eukaryotic Cell Division

Feature	Prokaryotes (Binary Fission)	Eukaryotes (Mitosis/Meiosis)
DNA Replication	Single circular chromosome, simultaneous with division	Multiple linear chromosomes, distinct S phase
Cell Growth	Continuous	G1, S, G2 phases
Division Mechanism	Binary fission	Mitosis (somatic cells), Meiosis (gametes)
Genetic Variation	Mutation, horizontal gene transfer	Sexual reproduction, recombination

Eukaryotic Cell Cycle Phases

Interphase:
- G1 phase: Cell growth, organelle duplication
- S phase: DNA replication
- G2 phase: Preparation for mitosis, centrosome/MTOC duplication
M phase: Nuclear division (mitosis or meiosis) and cytokinesis

Key Events During Interphase

Growth: Cells increase in size and mass to maintain stable size distribution.
Mitochondrial/Chloroplast Duplication: Occurs by binary fission during G1 phase.
DNA Replication: Occurs during S phase; sister chromatids remain attached until M phase.
Centrosome/MTOC Duplication: Occurs during S and G2 phases; essential for spindle formation.
Kinetochore Assembly: Mature kinetochores assemble on centromeric DNA at G2/M transition.

Microtubule Organising Centres (MTOCs) and Centrosomes

Microtubule fibers are organized by the MTOC; in animal cells, this is the centrosome.
Centrosome cycle coordinates with cell cycle phases.

M Phase: Mitosis

Mitosis is the process of nuclear division, followed by cytokinesis. It ensures accurate transmission of genetic material to daughter cells.

Stages of Mitosis

Prophase:
- Centrosomes migrate to opposite poles
- Nucleolus disappears
- Nuclear envelope breakdown begins
- Chromatin condenses into chromosomes
- Mitotic spindle begins to form
Prometaphase:
- Nuclear envelope breakdown complete
- Spindle fibers attach to kinetochores on chromosomes
- Chromosomes align at metaphase plate
Metaphase:
- Chromosomes fully aligned at metaphase plate
- Critical checkpoint ensures proper attachment before anaphase
Anaphase:
- Sister chromatids separate and move to opposite poles
- Errors can lead to aneuploidy
Telophase:
- Chromosomes arrive at poles
- New nuclear envelopes form
- Chromosomes decondense
Cytokinesis:
- Division of cytoplasm to form two daughter cells
- Animal cells: cleavage furrow formation
- Plant cells: cell plate formation

Chromosome Condensation

Condensation of DNA into compact chromosomes is essential for accurate segregation during M phase.

Microtubule Dynamics

Microtubules grow and shrink by addition/removal of tubulin dimers ( drives dissociation).

Correct Spindle Attachment

Stable attachment of spindle microtubules to kinetochores is required for progression through M phase.

Cytokinesis in Plant vs. Animal Cells

Feature	Animal Cells	Plant Cells
Mechanism	Cleavage furrow, contractile ring	Cell plate formation, vesicle fusion
Outcome	Two daughter cells	Two daughter cells with new cell wall