Mitosis and Meiosis: Cell Division Processes Explained
Mitosis
Prophase: Chromatin condenses into visible chromosomes, each consisting of two chromatids joined at the centromere. Centrioles move to opposite cell poles, forming mitotic spindle fibers. The nuclear membrane and nucleolus disappear, dispersing chromosomes into the cytoplasm. Kinetochores form at chromosome centromeres, attaching to microtubules.
Metaphase: Chromosomes reach maximum condensation. The spindle fully forms, extending between the poles. Kinetochore microtubules gradually align chromosomes along the cell’s midplane (metaphase plate). Centromeres position perpendicularly to the axis of the centrioles, orienting each chromosome’s chromatids toward opposite poles.
Anaphase: Chromatids separate and move to opposite poles, initiated at the centromere. Polar microtubules elongate, separating the spindle poles. Anaphase concludes when chromosomes reach the poles.
Telophase: Nucleoli reappear, and chromosomes decondense. Nuclear membranes reform around each chromosome group from the endoplasmic reticulum.
Meiosis
Prophase I:
- Leptotene: Chromosomes condense and become visible, each with two chromatids. Chromosome ends attach to the nuclear envelope.
- Zygotene: Homologous chromosomes pair up (synapsis) via the synaptonemal complex.
- Pachytene: Crossing-over occurs (genetic material exchange between homologous chromatids).
- Diplotene: Homologous chromosomes begin separating, remaining connected at chiasmata (crossing-over points).
- Diakinesis: Chromosomes condense further. Sister chromatids remain joined at the centromere, while homologous chromosomes are connected by chiasmata. The nucleolus and nuclear membrane disappear, and the spindle starts forming.
Metaphase I: Similar to mitotic metaphase, tetrads align at the equatorial plate, joined by chiasmata. Centromeres of homologous pairs are on opposite sides, with kinetochores attached and oriented.
Metaphase II: Spindle fibers attach to chromosome kinetochores. Chromosomes align along the cell’s equatorial plane. Metaphase I and II differ: chromosomes are in tetrads (groups of four) during Metaphase I and in pairs during Metaphase II.
Anaphase II: Chromatids separate at their centromeres, with one set moving to each pole. Each chromatid is now a chromosome.
Telophase II: One member of each homologous pair is at each pole. Nuclear envelopes reassemble, the spindle disappears, chromosomes elongate into chromatin threads, and cytokinesis occurs. Nucleoli reform, and cell division completes, producing two daughter cells. Meiosis results in four haploid nuclei, each with a unique combination of genes due to chromosome shuffling and crossing-over.