Mitosis and Meiosis: Cell Division Processes
Mitosis
Across the dividing cell, mitosis is characterized by two key components: the chromatic and achromatic apparatus. The chromatic apparatus consists of chromosomes and the nucleolus. The achromatic apparatus is formed by centrioles, asters, and the spindle.
Prophase
Between G2 and prophase, the cytoskeleton reorganizes. In plant cells, the cell wall remains rigid.
Metaphase
During metaphase, microtubules move chromosomes to opposite poles. Longer kinetochore microtubules exert a stronger pull. Chromosomes align along the cell’s equatorial axis.
Anaphase
Anaphase begins with centromere division, separating chromatids into independent chromosomes. Kinetochores move chromosomes poleward along microtubules, which simultaneously disassemble.
The efficiency of chromosome segregation is consistent regardless of chromosome number or ploidy (haploid, diploid, polyploid).
Telophase
In telophase, the spindle disintegrates, nuclear membranes reform, chromosomes decondense, and the nucleolus reappears. Cytokinesis typically occurs during telophase, creating independent daughter cells.
Meiosis
Gametes, or sex cells (ova in females, sperm in males), unite during sexual reproduction. Fertilization is the union of egg and sperm, resulting in a zygote.
Meiosis is a cell division process that halves the chromosome number to produce gametes. Two successive divisions follow a single chromosome duplication, resulting in four haploid daughter cells.
Interphase before meiosis resembles interphase before mitosis, involving protein synthesis, energy storage, and material exchange.
Stages of Meiosis
Meiosis comprises two divisions: Meiosis I and Meiosis II.
Meiosis I
Prophase I
Chromatin condenses, chromosomes become visible, and the nuclear membrane and nucleolus break down. The meiotic spindle forms. Homologous chromosomes align and intertwine (synapsis), forming tetrads (four chromatids). Crossing over, the exchange of chromatid fragments, may occur.
Metaphase I
Tetrads align along the cell’s equator, perpendicular to spindle fibers.
Anaphase I
Homologous chromosome pairs separate, moving to opposite poles. Each chromosome still consists of two chromatids.
Telophase I
Cytoplasm divides, forming two haploid cells. Nuclear membranes reform.
Interkinesis
A brief interphase-like stage without chromosome duplication.
Meiosis II
Prophase II
Nuclear membrane and nucleolus break down, chromosomes condense.
Metaphase II
Chromatids align along the cell’s equator.
Anaphase II
Chromatids separate, moving to opposite poles.
Telophase II
Cytoplasm divides, forming four haploid cells. Nuclear membranes reform.