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Mitosis and the Cell Cycle Cell reproduction Biology is the only subject in which multiplication is the same thing as division… S1 Why do cells divide… One-celled organisms 1. for reproduction amoeba Multi-celled organisms 1. for growth & development 2. from fertilized egg to adult for repair & replacement replace cells that die from normal wear & tear or from injury S6 starfish The Cell Cycle---life cycle of a cell The sequence of growth and division of a cell. The cell cycle occurs in a series of stages, or phases 95% of cell cycle is in interphase 5% of cell cycle is in mitosis and cytokinesis Interphase Stage between cell division Most of the cell’s life takes place in Interphase Chromosome replication (their number doubles) Interphase continued— the chromosmes become a pair of chromatids and are held together by a centromere Centromere This stage ends in preparation for mitosis(dividing of the cell ) Stage 2 Mitosis Happens in all eukaryotic cells Cell division process 4 major stages or steps REMEMBER PMAT Prophase Prepare for cell division Mitosis begins (cell begins to divide) 1.Chromatin condenses to form X like structures 2.Spindle fibers form between the poles 3.Nuclear membrane breaks down duplicated chromosomes (prophase) cell nucleus S16 Metaphase Meet in the Middle Chromosomes line up across the middle of the cell with sister chromatid at equator Spindle fibers attach to centromere metaphase duplicated chromosomes lined up in middle of cell Anaphase AWAY– the chromatids are pulled away from each other. Centromeres break Each chromatid is now a separate chromosome Each chromosome is pulled to each pole of the cell anaphase chromosomes split & move to opposite ends S18 Telophase Tearing apart into Two cells. Cell begins to pull in two Two new nuclear envelopes form and surround each set of chromosomes Chromosomes unwind into long strands Last stage of mitosis telophase S19 3rd Stage Cytokinesis Cell membrane pinches down and divides the cytoplasm Two new identical daughter cells are formed In cells that have a cell wall a cell plate forms between the new cells and then becomes the cell wall Summary Cell Cycle 1.Interphase—growing stage 2.Mitosis—dividing stage 4 Stages a.Prophase—Preparing b.Metaphase—Meeting the Middle c.Anaphase—attachment changes d.Telophase—Tearing into two 3.Final Process Cytokinesis-2 new daughter cells