1. According to the cell theory, where do all cells come from? • Cells divide to make a new identical cell. • At this moment, cells in your body are growing, dividing, and dying. • Old cells on your scalp, palms of your hands, and soles of your feet are shedding and being replaced. • Cuts and bruises are healing • Your intestines are producing millions of new cells each second. • Early biologists noticed that just before cells divide, several short structures suddenly appeared in the nucleus. • Then they noticed that after cellular division that these structures would vanish. WAIT TO ANSWER: Where is DNA found in a eukaryotic cell? Nucleus • For most of a cell’s life, DNA exists in long strands called chromatin. CELL REPRODUCTION • Before a cell divides, chromatin coils up into tightly packed structures called chromosomes. Chromosome CELL REPRODUCTION • Chromosomes are the carriers of genetic material that is copied and passed from generation to generation of cells. CELL REPRODUCTION Before a cell can divide- a copy of all the chromosomes needs to be made so that the new “daughter” cell has the exact DNA as the “parent” cell. “Parent Cell” Chromosomes “Daughter Cell” Chromosomes TALK TO YOUR NEIGHBOR: (I will randomly call on someone in a few seconds) DNA can be very stringy and called _______ When a cell is about to divide, it coils up tightly and is now a “fatter” structure called _________ TALK TO YOUR NEIGHBOR: (I will randomly call on someone in a few seconds) Why does a cell make a copy of it’s chromosomes before it divides and makes another cell? The Cell Cycle • The cell cycle is the sequence of growth and division of a cell. (life cycle) • The cell cycle consists of Interphase, Mitosis, and Cytokinesis The Cell Cycle • The majority of a cell’s life is spent in the growth period known as interphase. The Cell Cycle Interphase • During Interphase, under a microscope, there doesn’t appear to be much going on. • BUT, Interphase is the busiest phase of the cell cycle • During this time the cell is growing, making more organelles, and chromatin (DNA) is replicating. Chromatin Replicated Chromatin The Cell Cycle Mitosis • Following interphase, a cell enters its period of nuclear division called mitosis. • There are four phases in mitosis… PROPHASE: 1. The membrane around the nucleus and nucleolus disintegrate (disappear) 2. (In animal cells) centrioles begin to move to opposite ends of the cell and spindle fibers begin to form Nucleus Centrioles Spindle Fiber 3. Chromatin coils up to form visible chromosomes Replicated Chromatin Replicated Chromosome • The two halves of the doubled structure are called sister chromatids. • Sister chromatids are identical to each other. Sister chromatids • Sister chromatids are held together by a structure called a centromere, which plays a role in chromosome movement during mitosis. Centromere PROPHASE METAPHASE: 1. The chromosomes move to the middle (equator) of the cell 2. Spindle fibers on attach to the centromeres on either side of the chromosome Centromere Spindle fibers METAPHASE ANAPHASE: 1. The spindle fibers pull on the centromeres, break them, and pull the sister chromatids apart to either side of the cell- toward the centrioles. ANAPHASE TELOPHASE: 1. Two complete cells are forming. 2. Chromosomes unravel back into chromatin 3. Nuclear membrane begins to re-appear around the chromatin TELOPHASE The Cell Cycle Cytokinesis • Following telophase, the cell’s cytoplasm completely divides in a process called cytokinesis. • This is the final step in the cell cycle. The Cell Cycle Cytokinesis • Cytokinesis differs between plants and animals. • Toward the end of telophase in ANIMAL cells, the plasma membrane pinches in along the equator. The Cell Cycle Cytokinesis Cytokinesis • Plant cells have a rigid cell wall, so the plasma membrane does not pinch in. • A structure known as the cell plate is laid down across the cell’s equator. Result of Mitosis • When mitosis is complete, unicellular organisms remain as single cells. Result of Mitosis • Following mitosis, in multi-cellular organisms, cell growth and reproduction result in groups of cells that work together as tissue to perform a specific function. Control of Cell Cycle • Occasionally , cells lose control of the cell cycle. • Cancer is a growth resulting from uncontrolled cell division. • This loss of control may be caused by environmental factors or certain inheritances. Control of Cell Cycle • There are enzymes (proteins) that control the rate of the cell’s life cycle. • Scientists believe that if something goes wrong with these enzymes, they cause cells to skip a long interphase and divide a lot faster than they should. CANCER • A bunch of cancer cells together form masses of tissue called tumors that deprive normal cells of nutrients. CANCER • The causes of cancer are difficult to pinpoint because both genetic and environmental factors are involved. • Cancer may also be caused by viral infections that damage the genes.