Cell Division and the Cell Cycle: Binary Fission, Mitosis, and Cytokinesis

Continuity of Life: Cell Division

Principle of Cellular Continuity

All cells arise from pre-existing cells, ensuring genetic continuity and heredity. Cell division is the central phenomenon in organic reproduction, allowing organisms to grow, develop, and maintain tissues.

• Organic reproduction involves the transmission of genetic material from parent to offspring.

• Cell division is essential for genetic continuity and heredity.

• Reference: E.B. Wilson, The Cell in Development and Heredity, 1925.

Types of Cell Division

Overview: Prokaryotic vs. Eukaryotic Cell Division

Eukaryotic cells utilize two main types of cell division, while prokaryotes use a simpler process.

• Prokaryotic Binary Fission: Asexual, produces genetically identical daughter cells.

• Eukaryotic Mitosis: Asexual, produces genetically identical daughter cells (clones).

• Eukaryotic Meiosis: Sexual, produces gametes/spores with half the chromosome number.

Type	Organism	Purpose	Genetic Outcome
Binary Fission	Prokaryotes	Growth, reproduction	Clones
Mitosis	Eukaryotes	Growth, repair, asexual reproduction	Clones
Meiosis	Eukaryotes	Sexual reproduction	Genetic diversity

Binary Fission in Bacteria

Mechanism and Consequences

Bacteria reproduce by binary fission, a rapid and efficient process.

• Asexual reproduction: No genetic recombination; offspring are clones.

• Chromosomes are replicated and separated using cytoskeletal proteins.

• Cell divides, resulting in two identical daughter cells.

• Population growth is exponential: $\text{Number of cells} = 2^n$, where $n$ is the number of divisions.

The Eukaryotic Cell Cycle

Phases of the Cell Cycle

The cell cycle is divided into periods of cell division (mitosis) and non-division (interphase).

• Mitosis (M phase): Division of the nucleus and cytoplasm.

• Interphase: Cell growth, DNA replication, and preparation for division.

Phase	Main Events
G1 (First Gap)	Cell grows, produces proteins, RNA, lipids; prepares for DNA synthesis
S (Synthesis)	DNA replication, centriole replication, chromosomal protein synthesis
G2 (Second Gap)	Cell grows, produces RNA, proteins, lipids; prepares for mitosis

Chromosome Structure and Replication

Eukaryotic Chromosome Features

Eukaryotic chromosomes are linear double helices of DNA, with specific structural regions.

• Centromere: Central region where kinetochores form and sister chromatids attach.

• Telomeres: Protective ends of chromosomes.

DNA Replication and Sister Chromatids

During S phase, DNA replication produces sister chromatids.

• Each chromosome consists of two identical DNA molecules (sister chromatids) after replication.

• Cohesin proteins hold sister chromatids together until anaphase.

• Chromosome number remains constant; only DNA amount doubles.

Stage	Chromosome Number	DNA Amount
Before S phase	46	X
After S phase	46	2X (with sister chromatids)

Terminology Clarification

Key Terms in Chromosome Biology

• Chromosome: A single, long DNA molecule with associated proteins.

• Chromatin: The complex of DNA and proteins that forms chromosomes.

• Sister Chromatid: One of two identical DNA molecules produced by replication, joined at the centromere.

• Centromere: Region of chromosome where sister chromatids are joined and kinetochores form.

• Centrosome: Organelle that organizes microtubules and forms spindle poles.

• Kinetochore: Protein structure at the centromere that attaches chromosomes to spindle microtubules.

Mitosis: Stages and Mechanisms

Overview of Mitosis

Mitosis is the process by which a eukaryotic cell divides its nucleus, ensuring each daughter cell receives identical genetic material.

• Division of one nucleus into two genetically identical nuclei.

• Essential for growth, development, and tissue repair.

Stages of Mitosis

• Interphase: DNA replication and preparation for mitosis.

• Prophase: Chromosomes condense; mitotic spindle begins to form; centrosomes separate; kinetochores start to form; condensin proteins promote condensation.

• Prometaphase: Nuclear membrane breaks down; spindle microtubules attach to kinetochores; chromosomes begin to move.

• Metaphase: Chromosomes align at the metaphase plate; spindle is mature; kinetochores of sister chromatids attach to microtubules from opposite poles.

• Anaphase: Cohesin proteins are cleaved; sister chromatids separate and move to opposite poles; spindle fibers shorten.

• Telophase: Chromosomes reach poles; nuclear membrane reforms; chromosomes de-condense; spindle reverts to interphase state; cytokinesis begins.

Mitotic Spindle and Microtubules

The mitotic spindle is essential for chromosome movement during mitosis.

• Kinetochore microtubules: Attach to chromosomes at kinetochores.

• Nonkinetochore (polar) microtubules: Overlap and interact with microtubules from the opposite pole.

• Astral microtubules: Radiate toward the plasma membrane.

Cytokinesis: Division of the Cytoplasm

Animal Cells

Cytokinesis in animal cells occurs via a contractile ring composed of actin and myosin.

• Contractile ring forms at the cell equator.

• Actin-myosin contraction pinches the cell into two daughter cells.

Plant Cells

Plant cells undergo cytokinesis by forming a cell plate.

• No contractile ring or cleavage furrow.

• Cell plate forms at the equator, separating the cytoplasm.

• Cell plate develops into a new cell wall for each daughter cell.

Summary Table: Mitosis Stages and Key Events

Stage	Main Events
Interphase	DNA replication, cell growth, preparation for mitosis
Prophase	Chromosome condensation, spindle formation, centrosome separation
Prometaphase	Nuclear envelope breakdown, spindle attachment to kinetochores
Metaphase	Chromosomes align at metaphase plate, spindle fully formed
Anaphase	Sister chromatids separate, move to opposite poles
Telophase	Chromosomes reach poles, nuclear envelope reforms, cytokinesis begins
Cytokinesis	Division of cytoplasm (contractile ring in animals, cell plate in plants)

Additional info:

• Cell division is fundamental for multicellular life, enabling growth, development, and maintenance of tissues.

• Errors in cell division can lead to genetic disorders or cancer.