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Transcript
General Biology lab Mitosis and the Cell Cycle Objectives • To prepare slides of onion root tips demonstrating the stages of mitosis (somatic cell division) and identification of cells in the various stages of mitosis. • The cell division cycle : – is a sequence of events in a eukaryotic cell between one mitotic division and another. • It includes : 1. Interphase (G1 phase, S phase, and G2 phase) • Chromatin appears dispersed, DNA replication occurs. (Chromatin is a mass of uncoiled DNA and associated proteins called histones). 2. M phase. In the M-phase both the nucleus and the cytoplasm divide (mitosis and cytokinesis). Duration of the cell cycle in Human cells (in hours) It takes about 16 hours Interphase Mitosis G1 S G2 M 5 7 3 1 Vary among cell types Consistent among cell types Duration of phases of Mitosis: Prophase: 36 minutes Metaphase: 3 minutes Anaphase: 3 minutes Telophase: 18 minutes Mitosis Mitosis : • Produces two new daughter cells with the same number and kind of chromosomes as the parent cell. • Mitosis, or nucleus division, is the first part of M-phase and in consists of four stages : 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase 1. Prophase: • • • • Chromatin condenses chromosomes become visible nuclear envelope and nucleoli disappear spindle starts to form attach to the kinetochore ( a portion of the centromere). 2. Metaphase All chromosomes align in one plane at the center of the cell called the equatorial plane (also referred to as the metaphase plate). Anaphase: Spindle fibers shorten the kinetochores separate the sister chromatids (daughter chromosomes) are pulled apart and begin moving to the cell poles. Telophase: the last stage of division The chromosomes gradually de-condense to form the chromatin seen in interphase. Formation of a new nuclear envelope around each group of chromosomes. The nucleoli reappear. Cytokinesis ( cytoplasmic division) Usually occurs at the end of telophase. In plant cells cytokinesis is accomplished by the formation of a cell plate. Animal cells separate by forming a cleavage furrow. Preparing An Onion Root Tip Squash Onion bulbs have been rooted in water. Growth of new roots is due to the production and elongation of new cells. Mitotic divisions are usually confined to the cells near the tip of the root. Why use onion roots for viewing mitosis? 1. The roots are easy to grow in large numbers. 2. The cells at the tip of the roots are actively dividing. 3. Because each cell divides independently of the others, a root tip contains cells at different stages of the cell cycle. 4. The chromosomes can be stained to make them more easily. Materials • • • • • • Onion bulb with roots Crystal violet and other stains. Fixative solution (1 part glacial acetic acid to 3 parts ethanol). 1M HCl Clean slide & cover Forceps Procedure 1. Cut 2-3 mm of onion root 2. Use forceps to transfer an onion root tip into the cup of HCl and leave for 4 minutes at 60ºC. 3. Transfer the root tip to the cup containing fixative (Carnoy) and leave it for 4 minutes. 4. Then place the root tip on a slide. 5. Cover the root tip with a few drops of stain for 2 minutes 6. Put a cover slip over the root, put a paper towel or other absorbent paper and with your thumb firmly press on the cover slip. 7. Observe your preparation under the microscope Interphase Prophase Metaphase Anaphase Telophase