Biology 6.2 Mitosis and Cytokinesis

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

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.

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

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.

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 centrioles are 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.

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

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

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

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.

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.

Cytokinesis As mitosis ends with the cell nucleus being divided, cytokinesis begins 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 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.

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.

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.

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.