Cell Cycle & Mitosis

Cell Cycle The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. A cell spends 90% of its time in interphase (the first three phases of the cell cycle). The cell will only move into the 4th and 5th phase if it is going to divide.

Phases of the Cell Cycle: First Growth (G1) phase: Cell grows rapidly and carries out its routine functions Occupies major portion of cell’s life Cells not dividing remain in this phase

Phases of the Cell Cycle: Synthesis (S) phase: Cell’s DNA is copied during this phase At the end of this phase, each chromosome consists of two chromatids attached at the centromere.

Phases of the Cell Cycle: Second Growth (G2) phase: Preparations are made for the nucleus to divide Mitosis: Process in cell division where the nucleus is divided into two nuclei Cytokinesis: Cytoplasm divides, producing two identical cells

Control of the Cell Cycle: Cell Growth (G1) checkpoint: Decides whether or not the cell will divide DNA Synthesis (G2) checkpoint: DNA replication is checked at this point by DNA repair enzymes If passed, mitosis is triggered Mitosis checkpoint: Triggers exit from mitosis Signals the beginning of the G1 phase Cancer, the uncontrolled growth of cells, may result due to problems with the cell checkpoints or the cell division process.

Diagram: Turn to page 10 in your packet.

Mitosis: During mitosis, the nucleus of the cell divides into two identical nuclei.

Stages of Mitosis: PMAT Prophase: Chromosomes coil up and become visible. The nuclear envelope dissolves and a spindle forms. Metaphase: Chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids of each chromosome to opposite poles.

Stages of Mitosis: PMAT Anaphase: Centromeres divide. The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten. Telophase: A nuclear envelope forms around the chromosomes at each pole. Chromosomes, now at opposite poles, uncoil and the spindle dissolves. Spindle fibers break down and disappear.

Cytokinesis: As mitosis ends, cytokinesis begins. During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell. There are now two genetically identical cells where before there was only one.