Cell Division and Growth PDF

Cell Growth Why do cells divide rather than continue to grow for life? a. Because its more difficult for larger cells to move enough nutrients and waste across the cell membrane. Rate of this exchange depends on the surface area to volume ratio. b. DNA meets the needs of the cell when the cell is sm...

Cell Growth Why do cells divide rather than continue to grow for life? a. Because its more difficult for larger cells to move enough nutrients and waste across the cell membrane. Rate of this exchange depends on the surface area to volume ratio. b. DNA meets the needs of the cell when the cell is small, however as the cell increases in size, the DNA cannot meet its needs anymore. Surface Area to Volume Ratio 1. To obtain the ratio of surface area to volume, divide the surface area by the volume.  b. Rate of this exchange depends on the surface area to volume ratio II. Surface Area to Volume Ratio To obtain the ratio of surface area to volume, divide the surface area by the volume. 1. Notice that volume increases much more rapidly than surface area. This causes the surface area to volume ratio decrease, which is a serious problem for the cell. As cells get too larger it makes it more difficult to get sufficient amounts of oxygen and nutrients in and waste products out.  Cell Sizes Surface Area 54 cm2 lengthxwidthx6 2 24 cm2 6 cm 27cm3 Volume 3 1 cm3 8 cm Lengthxwidthxheight Ratio of surface 6:1 3:1 2:1 area to volume  Division of the Cell Before the cell gets too large it divides into two “ daughter” cells by cell division. Cell division solves the problem of cells getting too large by increasing size while reducing volume while (more small cells instead of one large cell).  Cell Division In eukaryotes the division is more complex it occurs in two stages: a. Mitosis: division of the nucleus b. Cytokinesis: division of the cytoplasm b. Chromosomes are not visible except during cell division. (known as chromatin when not dividing) c. Replication (copying) of the DNA occurred before cell division, therefore every chromosome is actually two identical “sister chromatids d. Each pair of chromatids is connected to each other at an area called the centromere, usually located near the center of the chromosome. centromere Sister chromatids  The Cell Cycle 1.Cell Cycle: the series of events the cell goes through as they grow and divide separated by periods of “in-between” time called interphase. 2. During the cell cycle the cell grow , prepares for division, and divides to form two daughter cells, each of which then begins the cycle again. 3. Four phases of the cell cycle  Events of the Cell Cycle a. G1 phase cell grow, increase in size and synthesize new proteins and organelles. b. S phase: chromosome replication (synthesis) c. G2 phase: organelles and molecules required for cell division are produced. d. M Phase: mitosis and G1, S, and G2 are all taking place during cytokinesis: interphase the phase between divisions.  G1 M-phase s G2 Mitosis ▪ The mitotic phase can be sub-divided into four phases prophase, metaphase, anaphase, and telophase. Mitosis results in two “daughter cells”, which are identical to each other, and is used for growth. ▪ Mitosis is followed by cytoplasmic division (cytokinesis) to complete cell division. ▪ Interphase stage between division is when the cell is not dividing, but is carrying out its normal cellular function. In this case chromatin is not visible and replication of cell organelle like mitochondria, occurs in the cytoplasm. G1, S and G2 phases are occurring.  chromosomes condense and become visible. Due to DNA replication during interphase, each chromosome consists of two identical sister Prophase chromatids connected at the Beginning centromere. of mitosis centrioles move to opposite (nuclear division) poles of cell Nucleolus disappears. Spindle fibers begin to form and attach to chromosomes near the centromere Phase ends with the breakdown of the nuclear membrane.  Spindle fibers connect Metaphase centriole to chromosomes (middle) Chromosomes align along equator of cell. Centromeres split, allowing sister chromatids Anaphase to separate. chromosome Chromatids move separate towards opposite poles, centromeres first, creating a “V” shape Telophase New nuclei Spindle fibers disperse form nuclear membrane form around each set of chromatids Nucleoli reform End of nuclear division a cleavage furrow forms, which splits New the cell in two daughter cells form  Centrioles Centrioles Centromere Nuclear Chromatin Centrioles envelope Chromosomes _______________ (paired chromatids) Daughter Cells Spindle Centrioles Individual Chromosomes Nuclear envelope reforms Regulating the Cell Cycle Multucellular organisms control cell growth and division very carefully (way to increase number of cells and size of organism). This provides the replacement of cells that wear out or broken down. Cells in a petri dish will continue to divide until they come into contact with other cells. Then the cells stop dividing. So, the controls for cell growth and division can be turned on and off. The same thing happen in our own bodies.  3. 1. 5. 2. 4. Question: What happens when you cut your finger or break your bone? Answer: The cells bordering the injury will begin dividing to fill in the gap in the tissues that have been torn or broken. This is the process known as healing. Cell Cycle Regulators Cells in mitosis contained a protein named cyclins that when injected into a cell would cause the formation of spindle fibers and regulates the cell cycle. Cyclins rise and fall in time with the cell cycle. Two main groups of protein regulators: 1. Internal Regulators: respond to events inside the cell. 2. External Regulators: respond to events outside the cell. Ex. healing Uncontrolled Cell Growth (cancer) 1. Cell Growth is so controlled because when it is not controlled things go very, very wrong. 2. Cancer is a disorder in which some of the body’s own cells lose the ability to control growth. 3. Cancer cells do not respond to the signals that regulate the growth of most cells. 4. When cells divide uncontrolled, they form masses of cells called tumors that can damage the surrounding tissue. 5. Cancer cells can break off and spread throughout the body disrupting normal activities and causing serious medical problems or even death. B. Causes of Cancer 1. Smoking (tobacco). 2. Radiation exposure 3. Viral infection. 4. Genetic defect C. Cancer is a disease of the cell cycle. Breast Cancer Cell