Unit 5 Section 1 Evidence 1: Why aren t all organisms made of just one cell? Why do cells divide, instead of simply growing larger? Cell Theory states the following 1. All organisms are made of cells 2. All existing cells are produced by other living cells. (cells come from other living cells) 3. The cell is the basic unit of life. Evidence 2: Before the cell theory was developed, many people claimed that spontaneous generation was possible. In other words, that cells arose from nonliving cells, such as dust or rotting meat. Respond to that claim using the principles of cell theory. All cells come from existing cells. Some can reproduce in 20 minutes and repeatedly divide while others divide at a slower rate. Evidence 3: Describe a specific situation in which an organism s cells would need to divide. Just as all species have life cycles, so do cells. Cell cycle = the regular pattern of growth, DNA duplication, and cell division that occurs in eukaryotic cells. 4 main stages of the cell cycle 1. Gap 1 2. Synthesis 3. Gap 2 4. Mitosis 5. Cytokinesis (discussed in section 2) Originally there were only 2 stages of the cell cycle due to the fact that there was only activity being done in one stage. The first is called Interphase which includes Gap 1, Synthesis, and Gap 2 and the second was Mitosis. Gap 1 (G1) First stage of the cell cycle Normal functions of the cell are carried out Cells increase in size and organelles increase in number here Most of the life of the cell is spent here Checkpoint at the end of the time here to see if a cell can continue to the next stage Synthesis (S) Second stage of the cell cycle Synthesis means the combining of parts to make a whole During the S stage, the cell makes copy of its nuclear DNA
By the end of this stage there are 2 complete sets of DNA Evidence 4: Why is it important that DNA is copied before the cell divides? Gap 2 (G2) Third stage of the cell cycle Normal functions of the cell are carried out Additional growth occurs Checkpoint at the end of the time here to see if the cell is adequate in size and contains undamaged DNA Evidence 5: If a cell has damaged DNA, what do you think happens during the G2 checkpoint? Mitosis (M) Fourth stage of the cell cycle Includes 2 processes o Mitosis The division of the cell nucleus and its contents Result is two identical nuclei o Cytokinesis The process that divides the cytoplasm Result is two cells Parent cell mitosis cytokinesis daughter cells Checkpoints in the cell cycle Checkpoint 1 o End of gap 1 o A cell checks the DNA for any damage before copying begins and makes sure conditions are right for division Checkpoint 2 o End of gap 2 o Both DNA and the centrosome need to have been successfully copied o Those parts of the cytoskeleton that help to pull the two halves apart must be in place Checkpoint 3
o Final checkpoint that occurs halfway through Mitosis o Checks that the two copies of DNA have been properly attached to the cytoskeleton that will separate them Cells divide at different rates Cell division is linked to the bodies need for those cells Human cells takes around 12 hours Division is greater in embryo s and children than in adults but the cell cycle is shorter Evidence 6: Why does a skin cell need to divide at a faster rate than a liver cell? Cell size is limited Cells have upper and lower size limits If cells were to small they could not contain all of the necessary organelles and molecules If cells were to large they would have a large ratio to surface area volume would be effected and in cell division the daughter cells would not be the proper size and therefore would be too large for life. Ratio of surface area to volume in cells As cells grow, its volume increases more rapidly than its surface area. When the surface area to volume ratio is too small, the cell cannot move materials into and out of the cell at a sufficient rate or in sufficient quantities. Evidence 7: Make a claim for why cells must divide, rather than grow larger. Explain how surface area and volume, as well as transport across the cell membrane, are related to cell size and homeostasis.
Both internal and external factors regulate the cell cycle External Factors Comes from outside the cell Includes messages from other cells or parts of the body Includes both chemical and physical signals o Physical signals include cell to cell contact o Chemical signals could come from proteins Internal Factors Can be triggered by external factors 2 most important internal factors are o Kinases An enzyme that when activated, transfers a phosphate group from one molecule to another This increases energy and makes it change shape o Cyclines A group of proteins that are rapidly made and destroyed at certain points in the cell cycle Apoptosis Programmed cell death Occurs when internal or external signals activate genes that produce self-destructive enzymes o Example is webbed fingers and toes in embryo but not in newborn Cells collapse inward without spilling their contents Cell necrosis Cancer Cells that die through injury The common name for a class of disease characterized by uncontrolled cell division They divide more often than healthy cells do They form disorganized clumps called tumors These cells do not perform a specific job o If in the lung they do not exchange gases like the lung cells do Tumors 2 types of tumors Benign o Cells remain clustered together o Harmless and can be cured by removing it Malignant o Some of the cancer cells break away from the cluster o Breaking away from the cluster is called metastasize o The cells that broke off are carried in the blood system or lymph system
o These cells then form more tumors o Difficult to rid the body of these tumors once they have metastasized Carcinogens are substances known to produce or promote the development of cancer Evidence 8: Look at the image on page227 in figure 7 and describe the differences in the normal cells and the cancerous cells shown.