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Transcript
The Cell Cycle Most cells undergo a regular sequence of growth and division known as the cell cycle. The Cell Cycle consists of three stages. Stage 1: Interphase - The stage where a cell grows and the DNA is duplicated in order to get ready for cell division. Stage 2: Mitosis - The stage where a cell seperates its DNA into two identical sets. Mitosis occurs in 4 steps. Stage 3: Cytokinesis - The stage where a cell divides into two new daughter cells that are identical to the parent. Stage 1 - Interphase Most of a cells life (90%) is spent in interphase. During this time, the cell grows & functions normally and prepares for cell division by copying its DNA in a process called replication. In this photograph, a cell from a fish is shown. The circle in the center of the cell is the nucleus. The black stringy material in the nucleus is the DNA. This DNA is copied during Interphase, and when this is finished the cell has two complete sets of DNA. Inter is a prefix that means “between.” Stage 2 - Mitosis Mitosis is the process of asexual cell division used by most cells. (Cells incvolved in sexual reproduction use the other method, called meiosis.) Mitosis occurs in 4 steps. Prophase Metaphase Anaphase Telophase During these 4 steps, the DNA that was duplicated during Interphase seperates to form 2 new nuclei that each have a single set of complete DNA. Stage 2A: Prophase Prophase is the first step of mitosis. In this step, the DNA gathers together and forms chromosomes in order to get ready to divide. Tiny “threads” of DNA coil up to form chromatids that join to form chromosomes. The nuclear membrane dissolves, and two points (called centrioles) form at the opposite ends of the cell. Later the daughter cells will form around these centrioles. Pro is a prefix that means “before or primary” (first). Chromosomes Each chromosome is made of two identical strands of coiled DNA (called chromatids) that are joined in the center by a centromere. These two strands later separate to make an identical copy for each of the two daughter cells. Strand 1 Strand 2 Strand 1 Strand 2 Stage 2B: Metaphase Metaphase is the second stage in mitosis. During this step, the chromosomes line up in the center of the cell (between the centrioles at each end), and get ready to divide. Spindle fibers attach each chromosome to the centrioles like small strings. In this photograph, the chromosomes have lined up in the center of the cell. The centrioles are visible at each end and spindle fibers attach the chromosomes to them. Meta is a prefix that means “after or behind.” Stage 2C: Anaphase Anaphase is the third stage of mitosis. In this step, the chromosomes separate and then begin to pull apart and move to opposite ends of the cell. In this photograph, the chromosomes have separated and are being pulled to the opposite ends of the cell by spindle fibers attached to each centriole. Ana is a prefix that means “upward, back, or again.” Stage 2D: Telophase Telophase is the fourth stage of mitosis. In this step, the two sets of chromosomes form two new nuclei, and the cell divides. In this photograph, the chromosmes have separated into two distinct groups. A new nuclear membrane forms around each, creating two nuclei - one for each daughter cell. Telo is a prefix that means “end.” Stage 3: Cytokinesis Cytokinesis is the last stage of the cell cycle. In this step, the cell membrane “pinches in” between the two new nuclei, and forms two separate daughter cells. During this phase the cytoplasm and organelles of the cell are divided between the two daughter cells. In this picture, the cell has divided in half and Mitosis is complete. Each daughter cell has its own set of DNA which is identical to the original cell.