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Chapter 10 Notes Cell Growth Cell Growth Living things grow by producing more cells. Adult animal cells are no larger than those of a young animal, there are just more of them. Limits to Cell Growth There are two main reasons why cells divide rather than continuing to grow indefinitely: 1.)The larger a cell becomes, the more demands the cell place on its DNA. If a cell were to grow without limit, an “information crisis” would occur. Limits to Cell Growth 2.)The larger a cell becomes the cell has more trouble moving enough nutrients and wastes across the cell membrane. Food, water, and waste products enter and leave a cell. The rate at which this exchange takes place depends on the surface area and volume of the cell. Limits to Cell Growth As the length of a cell increases, its volume increases faster than its surface area. If a cell got too large, it would be more difficult to get sufficient amounts of nutrients in or out. This is one reason why cells do not grow much larger even if the organism does. Division of the Cell Before it becomes too large, a growing cell divides forming two “daughter” cells. (cell division) Before cell division occurs, the cell replicates (copies) all of its DNA. Each daughter cell receives it own genetic information Section 10-2 Notes Cell Division Cell Division Every cell must copy its genetic information before cell division begins Each daughter cell then gets a complete copy of that information The rest of cell division simply involves dividing cell contents Cell Division Eukaryotes have two main stages of cell division 1. Mitosis – the division of the nucleus 2. Cytokinesis – the division of the cytoplasm Chromosomes The genetic info is carried by chromosomes Chromosomes are only visible during cell division Before cell division they are duplicated resulting in two “sister” chromatids Each pair of chromatids is attached at an area called the centromere The Cell Cycle The cell cycle is a series of events cells go through as they grow and divide During the cell cycle the cell: Grows Prepares for division Divides to form two daughter cells The Cell Cycle The phases of the cell cycle include interphase and cell division Interphase is divided into 3 phases 1. G1 – cells increase in size and make new proteins and organelles 2. S – replication of chromosomes takes place 3. G2 – many of the organelles and molecules needed for cell division are produced The Cell Cycle Cell Division (aka M phase) includes mitosis and cytokinesis Mitosis is divided into 4 phases 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase The Cell Cycle 1.Prophase – The chromosomes condense and become visible The centrioles separate and move to opposite sides of the nucleus The chromosomes attach to fibers in the spindle which is a structure that helps move chromosomes apart At the end, the nuclear envelope breaks down The Cell Cycle 2. Metaphase – The chromosomes line up across the center of the cell The centromere of each chromosome attaches to the spindle The Cell Cycle 3. Anaphase – The centromeres joining the sister chromatids split The sister chromatids become individual chromosomes The two sets of chromosomes move apart The Cell Cycle 4. Telophase – The chromosomes move to opposite ends of the cell They lose their distinct shapes Two new nuclear envelopes form The Cell Cycle Cytokinesis Occurs the same time as telophase Animal cells - the cell membrane pinches the cytoplasm into two nearly equal parts Plant cells – a cell plate forms midway between the divided nuclei, where the cell wall begins to form Section 10-3 Notes Regulating the Cell Cycle Controls on Cell Division Not all cells move through the cell cycle at the same rate. Most muscle and nerve cells do not divide at all once they have developed. The cells of the skin, digestive tract and bone marrow grow and divide rapidly throughout life. Controls on Cell Division When a group of cells are placed in an enclosed dish they will continue growing and dividing. This process stops when the cells come in contact with other cells When an injury such as a cut in the skin or a break in a bone occurs, cells at the edges of the injury are stimulated to divide rapidly. this action produces new cells, starting the process of healing. Cell Cycle Regulators For many years, biologists searched for a substance that might regulate the cell cycle. In the early 1980’s two biologists found the substance they called cyclin. Cell Cycle Regulators There are two types of regulatory proteins: Those that occur inside the cell (internal) Those that occur outside the cell (external) Internal Regulators Internal regulators are proteins that respond to events inside the cell. They allow the cell cycle to proceed only when certain processes have happened inside the cell. External Regulators External regulators are proteins that respond to events outside the cell. They direct cells to speed up or slow down the cell cycle. Growth factors are among the most important external regulators. Uncontrolled Cell Growth Cell growth is regulated carefully because the consequences of uncontrolled growth are very severe. Cancer cells do not respond to the signals that regulate the growth of most cells. They divide uncontrollably and form masses of cells called tumors. Uncontrolled Cell Growth Most cancer cells have a defect in a gene called p53. This gene causes the cell to lose the information needed to respond to signals that would normally control their growth.
