Cell Division: Mitosis

Overview of Cell Division

Cell division is a fundamental process in eukaryotic organisms, essential for growth, development, repair, and maintenance. Mitosis is the process by which somatic (body) cells divide to produce two genetically identical daughter cells.

• Mitosis is used for growth, repair, and replacement of cells.

• It is a form of asexual reproduction in multicellular organisms.

• Daughter cells are genetically identical to the parent cell.

Comparison: Binary Fission vs. Mitosis

Binary fission and mitosis are both mechanisms of cell division, but they occur in different types of organisms and have distinct processes.

• Binary Fission: Occurs in prokaryotes (bacteria and archaea). The cell duplicates its DNA and divides into two identical cells.

• Mitosis: Occurs in eukaryotes. Involves complex steps including nuclear division and cytokinesis.

• Key Difference: Mitosis involves the formation of a mitotic spindle and organized chromosome segregation, while binary fission does not.

Structure and Packaging of Eukaryotic Chromosomes

Eukaryotic chromosomes are highly organized structures that package DNA efficiently within the nucleus.

• Karyotype: The particular array of chromosomes in an individual organism, arranged by size, staining properties, or centromere location.

• Haploid (1n): One complete set of chromosomes (23 in humans).

• Diploid (2n): Two complete sets of chromosomes (46 in humans).

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles; each is a homologue.

Chromosome Replication

Before a cell divides, its chromosomes must be replicated so that each daughter cell receives a complete set.

• Centromere: The visible constriction on a chromosome, serving as the attachment point for spindle fibers.

• Cohesion Proteins: Hold sister chromatids together after DNA replication.

• Kinetochore: Protein structure on chromatids where spindle fibers attach during cell division.

Eukaryotic Cell Cycle

The eukaryotic cell cycle consists of a series of phases that prepare the cell for division and ensure accurate DNA replication and segregation.

• Interphase: Includes G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis).

• M Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Stages of M Phase

Mitosis is divided into several stages, each characterized by specific events:

• Prophase: Chromosomes condense, spindle apparatus forms.

• Prometaphase: Nuclear envelope breakdown (NEBD), spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate (spindle equator).

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform around separated chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Cell Cycle Checkpoints

Checkpoints are control mechanisms that ensure the fidelity of cell division.

• G1/S Checkpoint: Checks for cell size, nutrients, and DNA integrity before DNA replication.

• G2/M Checkpoint: Ensures DNA replication is complete and checks for DNA damage before mitosis.

• Spindle Checkpoint: Ensures all chromosomes are properly attached to the spindle before anaphase.

Failure of Cell Cycle Control and Cancer

Loss of proper cell cycle control can lead to uncontrolled cell proliferation, a hallmark of cancer.

• Proto-oncogenes: Normal genes that promote cell division; when mutated, they become oncogenes and can drive cancer (gain-of-function mutation).

• Tumor Suppressor Genes: Genes that inhibit cell division or detect DNA damage; loss or inactivation removes the "brake" on cell division (loss-of-function mutation).

• Metastasis: The spread of cancer cells to other parts of the body.

Example: HeLa Cells

HeLa cells are a famous example of human cells that have been dividing continuously since 1951, derived from a cervical cancer tumor of Henrietta Lacks. These cells are widely used in research.

Summary Table: Cell Cycle Checkpoints

Key Equations

• Number of chromosomes in diploid cells (humans):

• Number of chromosomes in haploid cells (humans):

Additional info: Expanded explanations of cell cycle phases, checkpoint functions, and cancer genetics were added for academic completeness.