Cell Division, DNA and Protein Synthesis

The Cell Cycle and the Stages of Mitosis

Before mitosis or meiosis I, an animal or plant cell typically passes through interphase; meiosis II follows meiosis I without another full interphase or round of DNA replication. In a typical cell cycle, interphase precedes mitosis or meiosis I and includes G1, S, and G2; meiosis II is not preceded by another full interphase. During the S phase of interphase, chromosomal DNA is replicated. This replication ensures that a complete set of genetic instructions is ready before the cell enters either mitosis or meiosis I. Everyday tissue maintenance depends on this careful preparation, ensuring that new cells receive proper genetic material.

Mitosis functions in somatic cell growth, tissue repair, and asexual reproduction. Mitosis consists of four sequential stages: prophase, metaphase, anaphase, and telophase. During prophase, diffuse chromatin condenses into visible chromosomes, and the nuclear envelope breaks down in late prophase or prometaphase. Spindle fibers attach to the kinetochores of these condensed chromosomes to direct their movement. During metaphase, the spindle fibers guide the chromosomes so that they align along the equator or metaphase plate of the cell.

During anaphase, sister chromatids separate and are pulled toward opposite cell poles. During telophase, nuclear envelopes reform around the separated daughter chromosomes, and the chromatin decondenses back into an uncoiled state. Nuclear division is accompanied by cytokinesis, which is the physical division of the cytoplasm. Cytokinesis often begins before nuclear division is complete. In animal cells, cytokinesis occurs through the constriction of a cleavage furrow, pinching the parent cell into two genetically identical daughter cells that maintain the same chromosome number as the parent cell.