Biology 6.2 Mitosis and Cytokinesis

1. Biology 6.2 Mitosis and Cytokinesis Mitosis and Cytokinesis

2. Objectives • Students will. . . . • Describe the structure and function of the spindle during mitosis • Summarize the events of the four stages of mitosis • Differentiate cytokinesis in animal and plant cells

3. Chromatid Separation in Mitosis • Every second about 2 million new blood cells are produced in your body by cell divisions occurring in the bone marrow. • These cells have received the signal to divide. • The cells advance beyond the G2 phase and enter into the last two phases; mitosis and cytokinesis.

4. Chromatid Separation in Mitosis • During mitosis, the nuclei divide to form two nuclei, each containing a complete set of the cell’s chromosomes.

5. Chromatid Separation in Mitosis • During mitosis, the chromatids on each chromosome are physically moved to opposite sides of the dividing cell with the help of the spindle. • Spindles are cell structures made up of both centrioles and individual microtubule fibers that are involved in moving chromosomes during cell division.

6. Forming the Spindle • At each of the cell’s poles lies a centrosome. • The centrosome is an organelle that organizes the assembly of the spindle. • In animal cells, a pair of centriolesare found inside each centrosome. • Centrioles and spindle fibers are both made of hollow tubes of protein called microtubules. • Each spindle fiber is made of an individual microtubule. • Each centriole however, is made of nine triplets of microtubules arranged in a circle.

7. Mitosis and Cytokinesis • Although mitosis is a continuous process, biologists traditionally divide it into four stages • Step 1: Prophase • Step 2: Metaphase • Step 3: Anaphase • Step 4: Telophase

8. Mitosis and Cytokinesis Step 1: Prophase • Chromosomes coil up and become visible during prophase. • The nuclear envelope dissolves and a spindle forms.

9. Mitosis and Cytokinesis • Step 2: Metaphase • During metaphase, the 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.

10. Mitosis and Cytokinesis • Step 3: Anaphase • Centromeres divide during anaphase. • The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten.

11. Mitosis and Cytokinesis • Step 4: Telophase • A nuclear envelope forms around the chromosomes at each pole. • Chromosomes, now at opposite poles, uncoil and the spindle dissolves. • The spindle fibers break down and disappear. Mitosis is complete.

12. Cytokinesis • As mitosis ends with the cell nucleus being divided, cytokinesisbegins. • During cytokinesis, the cytoplasm of the cell is divided in half, and the cell membrane grows to enclose each cell, forming two separate cells as a result. • The end result of mitosis and cytokinesis is two genetically identical cells where only one cell existed before. Cytokinesis: the cytoplasm divides

13. Cytokinesis • In plant cells and other cells that have rigid cell walls, the cytoplasm is divided in a different way. • In plant cells, vesicles formed by the Golgi Apperatus fuse at the midline of the dividing cell and form a cell plate. Cell plate

14. Cytokinesis • A cell plate is a membrane bound cell wall that forms across the middle of the plant cell. • A new cell wall than forms on both sides of the cell plate. • When complete, the cell plate separates the plant cell into two new plant cells. Cell plate

15. Key Concepts. . . . • During mitosis, spindle fibers drag the chromatids to opposite poles of the cell. • A nuclear envelope forms. • Each resulting nucleus contains a copy of the original cell’s chromosomes • Cytokinesis in animal cells occurs when a belt of protein threads pinches the cell membrane in half. • Cytokinesis in plant cells occurs when vesicles from the Golgi Apparatus fuse to form a cell plate.

16. Computer Lab • Go to the computer lab. • Today’s assignment is to research the “Cell Cycle”, including mitosis and cytokinesis, and sequence the steps in order demonstrating the order of cell division. • Create a written 1 page with all the steps in order with a brief explanation of each step. • Match this one written page to a diagram you have located online or created yourself demonstrating the complete cycle.