Chapter 2: Mitosis & Meiosis

Eukaryotic cells organize their genetic material, DNA, into chromosomes, which condense from diffuse chromatin fibers to become visible during division. In diploid (2n) organisms, chromosomes exist as homologous pairs—one member derived from the maternal parent and one from the paternal parent—that share the same gene locations (loci). The entire sequence of growth and division is defined by the cell cycle, which includes interphase stages G1, S (where DNA replication occurs), and G2. Mitosis leads to the production of two diploid daughter cells that are genetically identical to the parent cell. During mitosis, replicated chromosomes, each composed of two genetically identical sister chromatids joined at a common centromere, align at the metaphase plate. The critical separation of sister chromatids, termed disjunction, occurs during anaphase. This precise movement is regulated by the protein complex cohesin, which holds the chromatids together, the protective protein shugoshin, and the enzyme separase, which cleaves cohesin, allowing the separation and migration of daughter chromosomes toward opposite poles via spindle fibers. The process of cell division is vigilantly controlled by cell-cycle checkpoints, which employ regulatory molecules, including cyclin-dependent kinases and cyclins, to prevent errors that could potentially lead to malignancy. Conversely, meiosis is a specialized two-step division that reduces the chromosome number by precisely half, yielding haploid (n) gametes or spores. This is achieved through the reductional division (Meiosis I) and the equational division (Meiosis II). During Prophase I, homologous chromosomes undergo synapsis, forming structures called tetrads. Within these tetrads, non-sister chromatids exchange genetic material through crossing over at visible sites called chiasmata, which is the second major source of genetic variation (alongside random alignment of homologs). Meiosis I separates the homologous pairs (dyads), and Meiosis II separates the sister chromatids (monads). In gametogenesis, the cytoplasmic partitioning is unequal in females (oogenesis), resulting in one large ovum and nonviable polar bodies, but equal in males (spermatogenesis), yielding four functional sperm cells.