Mitosis and Cell Division: Structure, Process, and Key Terms

Mitosis and Cell Division

Introduction to Mitosis

Mitosis is a fundamental process by which eukaryotic cells divide to produce two genetically identical daughter cells. This process ensures the maintenance of chromosome number across generations of somatic cells, supporting growth, repair, and asexual reproduction in multicellular organisms.

• Role of Mitosis: Maintains genetic continuity by equally distributing duplicated chromosomes to daughter cells.

• Chromosome Number: Each daughter cell receives the same number of chromosomes as the parent cell.

• Somatic Cells: All body cells except gametes (sperm and egg) divide by mitosis.

Key Terms and Structures

• Chromosome: A DNA molecule with associated proteins, carrying genetic information.

• Duplicated Chromosome: A chromosome that has been copied during the S phase, consisting of two sister chromatids joined at a centromere.

• Unduplicated Chromosome: A single chromatid, present before DNA replication or after sister chromatids separate.

• Sister Chromatids: Two identical copies of a chromosome, attached at the centromere, formed during DNA replication.

• Centromere: The constricted region of a chromosome where sister chromatids are joined and where spindle fibers attach via the kinetochore.

• Kinetochore: A protein complex assembled on the centromere that links each chromatid to spindle microtubules.

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

The Spindle Apparatus and Associated Structures

• Spindle Apparatus: A structure made of microtubules that segregates chromosomes during mitosis.

• Spindle Fibers: Microtubules that attach to chromosomes at the kinetochore and help pull sister chromatids apart.

• Kinetochore Microtubules: Attach to the kinetochore and move chromosomes.

• Polar (Non-kinetochore) Microtubules: Overlap at the cell center and help elongate the cell during division.

• Centrosomes: Organelles that serve as the main microtubule-organizing centers in animal cells; duplicate before mitosis.

• Centrioles: Cylindrical structures within centrosomes (in animal cells) that help organize spindle fibers.

Phases of the Cell Cycle

• Interphase: The period of cell growth and DNA replication, consisting of three subphases:

  • G1 Phase: Cell grows and carries out normal functions.

  • S (Synthesis) Phase: DNA is replicated, chromosomes become duplicated.

  • G2 Phase: Cell prepares for mitosis; organelles and molecules required for division are produced.

• M (Mitosis) Phase: The phase where the nucleus divides, followed by cytokinesis (division of the cytoplasm).

Phases of Mitosis

Mitosis is divided into several distinct phases, each with characteristic events and structures:

1. Prophase (Early and Late):

   • Chromatin condenses into visible chromosomes.

   • Sister chromatids are joined at the centromere.

   • Mitotic spindle begins to form from centrosomes, which move to opposite poles.

   • In late prophase (prometaphase), the nuclear envelope breaks down and spindle fibers attach to kinetochores.

2. Metaphase:

   • Chromosomes align at the metaphase plate (cell equator).

   • Each sister chromatid is attached to spindle fibers from opposite poles.

3. Anaphase:

   • Sister chromatids separate at the centromere and are pulled toward opposite poles by spindle fibers.

   • Each chromatid is now considered an individual chromosome.

4. Telophase:

   • Chromosomes arrive at the poles and begin to decondense back into chromatin.

   • Nuclear envelopes reform around each set of chromosomes.

   • Spindle apparatus disassembles.

5. Cytokinesis:

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

   • In animal cells, a cleavage furrow forms and pinches the cell in two.

   • In plant cells, a cell plate forms, which develops into a new cell wall.

Comparison of Cytokinesis in Animal and Plant Cells

• Animal Cells: Cytokinesis occurs via the formation of a cleavage furrow, which contracts to divide the cell.

• Plant Cells: Cytokinesis occurs by the formation of a cell plate, which becomes the new cell wall separating the daughter cells.

Key Regulatory Molecules

• Cyclins: Proteins that regulate the progression of cells through the cell cycle by activating cyclin-dependent kinases (CDKs).

Summary Table: Key Terms in Mitosis

Term	Definition
Mitosis	Division of the nucleus resulting in two genetically identical daughter cells
Cytokinesis	Division of the cytoplasm following mitosis
Centromere	Region where sister chromatids are joined
Kinetochore	Protein complex on the centromere where spindle fibers attach
Spindle Apparatus	Microtubule structure that separates chromosomes
Centrosome	Microtubule-organizing center in animal cells
Chromatid	One of two identical halves of a duplicated chromosome
Sister Chromatids	Identical copies of a chromosome connected at the centromere
Metaphase Plate	Imaginary plane where chromosomes align during metaphase
Cleavage Furrow	Indentation that begins the process of cytokinesis in animal cells
Cell Plate	Structure that forms during cytokinesis in plant cells

Example: Mitosis in Human Cells

• Human somatic cells have 46 chromosomes. After DNA replication, each cell has 46 duplicated chromosomes (92 chromatids).

• During mitosis, sister chromatids separate, ensuring each daughter cell receives 46 chromosomes.

Equations and Diagrams

• Chromosome Number Conservation:

  • Let n be the number of chromosomes in a somatic cell.

  • After mitosis: $n_{parent} = n_{daughter}$

Additional info:

• Prometaphase is sometimes considered a subphase of prophase, marked by the breakdown of the nuclear envelope and attachment of spindle fibers to kinetochores.

• Microtubules are dynamic structures composed of tubulin proteins, essential for chromosome movement.

• Cell cycle checkpoints (regulated by cyclins and CDKs) ensure proper division and prevent errors.