Prophase
Prophase is the first and typically longest stage of mitosis, during which chromatin condenses into distinct, visible chromosomes and the mitotic spindle begins to assemble. The nuclear envelope breaks down toward the end of prophase, and each chromosome is revealed as two sister chromatids joined at the centromere. In meiosis, prophase I is particularly significant because homologous chromosomes pair up (synapsis) and exchange genetic material through crossing over.
Key Events During Prophase
| Event | Mitotic Prophase | Meiotic Prophase I |
|---|---|---|
| Chromatin condensation | Yes | Yes |
| Nuclear envelope breakdown | Yes (late prophase) | Yes |
| Spindle formation | Begins | Begins |
| Homolog pairing (synapsis) | No | Yes |
| Crossing over | No | Yes |
| Substages | None | Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis |
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Related Terms
Metaphase
Metaphase is the stage of cell division in which chromosomes reach maximum condensation and align along the cell's equatorial plane, known as the metaphase plate, pulled by spindle fibers attached to each chromosome's centromere. This precise alignment ensures that when sister chromatids separate, each daughter cell receives exactly one copy of every chromosome. Metaphase is used in karyotyping because chromosomes are most visible and identifiable at this stage.
Mitosis
Mitosis is the process of nuclear division in eukaryotic cells that produces two genetically identical daughter nuclei, each containing the same chromosome number as the parent cell. It is the primary mechanism of growth, tissue repair, and asexual reproduction in multicellular organisms. Mitosis proceeds through four sequential stages — prophase, metaphase, anaphase, and telophase — before the cytoplasm divides in cytokinesis.
Meiosis
Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four genetically unique haploid daughter cells from a single diploid parent cell. It is essential for sexual reproduction, generating gametes (sperm and egg cells) in animals and spores in plants and fungi. Genetic diversity is introduced through two key events: crossing over during prophase I and independent assortment of homologous chromosomes.
From Greek "pro" meaning before and "phasis" meaning appearance; coined by Walther Flemming in 1882 to denote the preparatory stage before chromosome movement.