Eukaryotic Cell

Cells must reproduce in order for the organism to grow or repair damage. For single-celled organisms, cellular reproduction creates a new organism. Each new cell must get a complete set of chromosomes, which therefore must be duplicated and evenly divided between the two daughter cells.

The orderly series of events involving cell growth and division is termed the cell cycle. Immediately following a division, the cell grows by taking up and metabolizing nutrients, and by synthesizing the many proteins, lipids, nucleic acids, sugars, and other molecules it needs. DNA replication occurs next, making duplicate chromosomes, followed by a short period in which the cell synthesizes the numerous proteins specific for cell division itself.

Cell division includes two linked processes: mitosis, or chromosome division, and cytokinesis, or cytoplasm division. Triggered by specific protein changes, the chromosomes begin to coil up tightly and become visible under the microscope. Cytoskeleton fibers attach to them, and position the chromosomes in pairs along the cell's imaginary equator. At the same time, the nuclear envelope breaks down into numerous small vesicles. The cytoskeleton fibers (termed the spindle) pull the chromosome duplicates apart, segregating one member of each pair to opposite sides of the cell. Other cytoskeleton proteins pinch the membrane along the equator (in animal cells) or build a wall across it (in plant cells) to separate the two cell halves, ultimately forming two daughter cells. Finally, the nuclear envelope re-forms and the chromosomes uncoil, starting a new round of the cell cycle.