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Questions Why are you bigger now than you were born? Why do cells divide instead of growing? What needs to happen for you to grow? Prokaryotes x Eukaryotes: How do you think their cells divide?
Prokaryotes x Eukaryotes Prokaryotes contain 1 single DNA molecule (chromosome) They divide by binary fission: DNA duplicates +cell splits in 2 Eukaryotes have more DNA molecules (chromosomes They are enclosed in the nucleus A complete set is necessary for the cell to function properly Cell division more complex: mitosis
What is the purpose of cell division? Growth (example: baby adult) Reproduction (asexual reproduction in single celled organisms) Replacement of dead/damaged/infected (example: skin/red blood cells/bone cells) Gamete formation in multi-cellular organisms (a special cell division process called meiosis)
Why are cells small? Surface area to volume ratio limits cell size Rate of heat production/waste/resource consumption – volume Rate or exchange material/energy – surface area As cell size increases, the surface area to volume ratio decreases Metabolic rates increase faster than the surface area’s ability to exchange nutrients, hence a maximum size is reached. Cell size, therefore, remains small
Chromosome Structure In eukaryotes: made up of DNA and proteins At different times, proteins cause the DNA to: be spread out like spaghetti in a bowl be tightly condensed into the X-shaped (these we can see in the microscope) Region where 2 molecules are attached: centromere that serves as an attachment point for the spindle fibers during mitosis.
Interphase = period between 2 cell divisions. Cell increases in size, but the chromosomes are invisible (long and thin)
Mitosis Mitosis is the division of the eukaryote nucleus, which goes on throughout life in all parts of the body. Organelles can be randomly separated into the daughter cells but chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA. Every human cell has the same 46 chromosomes Mitosis is usually divided into 4 phases: Prophase (P) Metaphase (M) Anaphase (A) Telophase (T) PMAT
End of Telophase: Cytokinesis The organelles get divided up into the 2 daughter cells passively: they go with whichever cell they find themselves in. Plant x Animal cells: Plants: a new cell wall made of cellulose forms between the 2 new nuclei (cell plate) Animals: a ring of actin fibers (microfilaments) forms around the cell equator, pinching the cell in half.
Summary of Mitosis Prophase: Chromosomes condense Nuclear envelope disappears Centrosomes move to opposite sides of the cell Spindle fibers form and attach to centromeres on the chromosomes Metaphase Chromosomes are lined up on equator of cell Centrioles are at opposite ends of cell Anaphase Centromeres divide: each 2-chromatid chromosome becomes two 1-chromatid chromosomes Chromatids pulled to opposite poles by the spindle fibers Telophase Chromosomes decondense Nuclear envelope reappears Cytokinesis: the cytoplasm is divided into 2 cells http://biology.uoregon.edu/reference/ort_mitosis/CellCycle.html
What happens when there is no control of cell division? Tumors = result of uncontrolled cell division Tumors can occur in any organ or tissue, though are most common after exposure to carcinogens (e.g. tobacco smoke) or in particularly active tissues (e.g. breast, skin) Angiogenesis: tumor recruits blood vessels and grows larger Metastasis: part of the tumor invades the blood vessel, travels through the blood and starts to forma a tumor in another part of the body
Most common treatments: surgery, radiation or chemotherapy
It is hard to remove all the tumor cells. Tumors often lack sharp boundaries for easy removal, and metastatic tumors can be very small and anywhere in the body.
Radiation and chemotherapy are aimed at killing actively dividing cells, but killing all dividing cells is lethal: you must make new blood cells, skin cells, etc. So treatment must be carefully balanced to avoid killing the patient.
Chemotherapy also has the problem of natural selection within the tumor. If any of the tumor cells are resistant to the chemical, they will survive and multiply. The cancer seems to have disappeared, but it comes back a few years later in a form that is resistant to chemotherapy. Using multiple drugs can decrease the risk of relapse: it’s hard for a cell to develop resistance to several drugs at the same time.