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Cell Cycle and Cell Division The cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA to produce two daughter cells. This process is essential for growth, development, and repair in multicellular organisms. 1. The Cell Cycle The cell cycle is divided into two major phases: Interphase and M Phase (Mitotic Phase). A. Interphase Often mistakenly called the "resting phase," this is actually a period of intense metabolic activity and preparation for division. It occupies about 95% of the duration of the cycle. * G;, Phase (Gap 1): The cell grows, carries out normal metabolic functions, and synthesizes RNA and proteins. Organelles replicate. * S Phase (Synthesis): DNA replication occurs. The amount of DNA in the cell doubles, though the number of chromosomes remains the same. * G»> Phase (Gap 2): Further growth and protein synthesis (e.g., tubulin for spindle fibers) occur. The cell prepares for mitosis. * Gp Phase (Quiescent Stage): Cells that do not divide further exit the cycle and enter this inactive stage. Anaphase ¢ Centromeres split; chromatids separate and move toward opposite poles. ¢ Separated chromatids are now called daughter chromosomes. Telophase ¢ Chromosomes cluster at opposite poles and de-condense. ¢ Nuclear envelope reforms around each cluster; nucleolus reappears. Cytokinesis ¢ Animal Cells: A cleavage furrow forms in the plasma membrane, deepening inward to divide the cytoplasm. ¢ Plant Cells: A cell plate forms at the center, growing outward to meet the existing lateral walls. 3. Meiosis (Reductional Division) Meiosis occurs in germ cells, producing four haploid daughter cells (2n — x). It involves two successive nuclear divisions: Meiosis | and Meiosis II. Meiosis | * Prophase I: The most complex stage. Subdivided into: ¢ Leptotene: Chromosomes become visible. * Zygotene: Pairing of homologous chromosomes (Synapsis). * Pachytene: Crossing over occurs between non-sister chromatids of homologous chromosomes, mediated by recombinase. * Diplotene: Homologous chromosomes separate except at crossover points (Chiasmata). Meiosis Il This phase is similar to mitosis. It involves the separation of sister chromatids. ¢ Prophase II, Metaphase II, Anaphase II, Telophase II result in the formation of four haploid cells, each with half the original number of chromosomes. Significance * Mitosis: Maintains genetic stability, ensures growth, and enables asexual reproduction and tissue repair. * Meiosis: Ensures genetic variation through crossing over and maintains constant chromosome numbers across generations by producing gametes. 
