Eukaryotic Cell Cycle and Cancer Exploration Worksheet Answer KEY

The Eukaryotic Cell Cycle and Cancer In Depth INTRODUCTION This handout complements the Click & Learn The Eukaryotic Cell Cycle and Cancer and is intended as an indepth examination of the cell cycle and the protein players involved. For a more general overview, please see the overview version. PROCEDURE Follow the instructions as you proceed through the Click & Learn and answer the questions in the spaces below. Click on the “Background” tab on the right side. 1. Compare and contrast the reasons cell division is important for unicellular (single-celled) and multicellular organisms. Unicellular – reproduce Multicellular – grow, and to replace dead or damaged cells 2. Provide an example of why cell division remains important to an adult organism even after it is fully developed. Turnover of cells/tissues (e.g. – in the skin or gut) Growth and/or repair of injured tissues (e.g. – your muscles after a hard workout or a broken bone) 3. What is the role of growth factors? They serve as signalling molecules that instruct cells to divide. *review Dr.Pettit’s lecture slides for a additional details about growth factors and how they regulate the cell cycle* 4. Cells divide, differentiate, or die. What is differentiation? Differentiation is when cells stop dividing and adopt a specialized function. 5. What is apoptosis? Explain its purpose. Apoptosis = programed cell death. It eliminates unnecessary cells during development and removes unhealthy or damaged cells in the mature organism. Note – it does this without activating the immune system 6. Organisms maintain the right number of cells by regulating the cell cycle. What are “cell cycle regulators?” Cell cycle regulators are molecular signals that may stimulate or halt cell division, instruct cells to differentiate, or initiate cell death. The Eukaryotic Cell Cycle and Cancer – In Depth 7. Watch the video clip of cells in the small intestine. Name the general location along the villus where the following processes occur: Crypt Cell Division: Cell Differentiation: Along its journey from the crypt to the top of the villus (the lumen). Tip of the villi Apoptosis: 8. Name one harmless result of too little cell division. Hair loss 9. Name one harmless result of too much cell division. Wart or benign tumor Click on the section of the circle labeled “Cell Cycle Phases” in the center purple circle on the right and use the “Overview” information in the window on the left to answer the questions below. 10. List, in order, the four events we collectively call the “cell cycle.” Next to each event, write the correlating cell cycle phase name. a. Growth – G1 b. DNA replication - S c. preparation to divide – G2 d Division - M 11. In general, what is the purpose of a checkpoint in the cell cycle? Strictly regulate the progression of the cell from one stage to the next to avoid errors in the process. 12. What is one potential outcome when errors occur in this highly regulated cell cycle process? Cancer Click on “Cell Cycle Regulators and Cancer” in the center purple circle on the right. Use the information under “Regulators Overview” in the window on the left to answer the questions below. 13. What type of protein that regulates the cell cycle is encoded by proto-oncogenes? Proto-oncogenes encode proteins that stimulate/promote the progression of a cell through the cell cycle. The Eukaryotic Cell Cycle and Cancer – In Depth 14. What type of protein that regulates the cell cycle is encoded by tumor suppressor genes? Tumor suppressor genes encode proteins that inhibit the progression of a cell through the cell cycle. 15. The most important cell cycle regulators are the ______cyclin dependent kinases (CDKs)_______________ . 16. What is a kinase, and what does it do? A kinase is an enzyme that adds a phosphate to another molecule. 17. When are CDKs present inside the cell during the cell cycle? When are they active? CDKs are always present in the cell; however, they are only active when bound to a cyclin protein. *review the complete stepwise procedure of how this occurs in Dr.Pettit’s lecture slides* 18. When are cyclins present inside the cell during the cell cycle? The concentration of individual cyclins fluctuates and they are only present at certain stages of the cell cycle. 19. CDKs form molecular complexes with cyclins. What do activated CDK-cyclin complexes do? Activated CDK-cyclin complexes regulate the progression of a cell through the stages of the cell cycle. Using the cell cycle diagram on the right and both links in the center purple circle (‘cell cycle phases’ and ‘cell cycle regulators and cancer’), complete the table below for each phase. Use bullet points and focus on major events that occur during each phase, checkpoint, and regulatory process. Complete the entire row before moving on to the next phase. PHASE PHASE EVENTS CHECKPOINT EVENTS - confirm that there is no DNA damage - the cell increases in size G1 - prepares to replicate its DNA - confirm that there are sufficient resources to proceed through cell division REGULATORY PROCESSES 1) Growth factors promote G1phase cyclin expression. 2) G1-phase cyclins accumulate activating the CDKcyclin, allowing it to phosphorylate target proteins that ultimately promotes the cell’s transition to S-phase if the DNA is damaged, p53 will inhibit the G1 CDK-cyclin complex preventing the cell from leaving G1 until the damage is repaired or it dies The Eukaryotic Cell Cycle and Cancer – In Depth In the absence of growth factors, Rb prevents cells from entering S-phase *a few more details on these mechanisms are outlined in Dr. Pettit’s lecture slides* - replicate the DNA - confirm that there are no errors in the replicated DNA 1) Growth factors promote Sphase cyclin expression. 2) S-phase cyclins accumulate activating the Sphase CDK-cyclin complex that promotes DNA replication S If DNA damage is identified, specific proteins (e.g. – tumor suppressor genes ataxia telangiectasia mutated (ATM) and Breast Cancer 1 (BRCA1)) recognize and inhibit cell cycle progression - continued cellular growth in preparation for division - confirm that the DNA is not damaged - confirm that there are 2 complete sets of chromosomes - confirm that there are enough cellular components for 2 daughter cells 1) M-phase cyclin expression. 2) M phase cyclins accumulate activating the M-phase CDKcyclin complex that promotes DNA replication G2 If DNA is damaged, tumor suppressor genes (such as p53) will inhibit the CDK-cyclin complex preventing the G2-M phase transition until the damage is repaired or the cell dies. The Eukaryotic Cell Cycle and Cancer – In Depth - stop growing - divide into 2 complete daughter cells - confirm that the sister chromatids are attached to the mitotic spindle M (mitosis) M-phase CDK-cyclins activate the anaphasepromoting complex/cyclosome (APC/C) once all of the chromosomes are attached to the mitotic spindle. Once active, the APC/C allows the proteins holding the sister chromatids together to be degraded and the chromatids to be separated to opposite sides of the cell during anaphase. If the chromosomes are not properly attached to the mitotic spindle, the mitotic arrest deficient (MAD) protein inhibits the APC/C preventing transition to anaphase 20. Go to “Cell Cycle Phases” and click on “Interphase.” The interphase alternates with mitosis. What happens during interphase and what phases does it include? This is when the cell grows and replicates its DNA in preparation for division. Interphase includes G1, S, and G2. 21. Go to “Cell Cycle Phases” and click on “G0.” The G0 phase is a resting or nondividing stage. What three factors determine if a cell enters G0? 1) the organism’s stage in development 2) the type of cell 3) the available resources 22. Provide an example of a fully differentiated cell that is (a) permanently in G0 and (b) one that can leave G0 to progress through the cell cycle and divide again. a. neuron OR muscle cell (myocyte) b liver cell (hepatocyte) The Eukaryotic Cell Cycle and Cancer – In Depth Click on “Cell Cycle Regulators and Cancer” in the center purple circle on the right. Then click on the “Cancer Overview” tab in the window to the left (right tab). 23. Cancer is an improperly regulated cell cycle. Name two reasons why cells can form tumors. 1) Cells divide too much 2) Not enough cells are eliminated (too little cell death). 24. What causes uncontrolled cell division at the genetic level? Mutations in the proteins that normally regulate the cell cycle (both positive and negative regulators). 25. Watch the video clip. At the cellular level in this example, explain what occurs if the APC gene is mutated. These cells fail to differentiate and pile up rather than exiting the crypt resulting in tumor formation. 26. Normally, proto-oncogenes stimulate the cell cycle. What do mutated proto-oncogenes (i.e., oncogenes) cause? Oncogenes stimulate cell division causing uncontrolled cell division. 27. Normally, tumor suppressor genes inhibit the cell cycle. What do mutated tumor suppressor genes cause? The cell cycle is no longer inhibited causing uncontrolled cell division. 28. To cause cancer, proto-oncogenes require _✓__ 1 OR 2 allele(s) (circle the correct answer) to be mutated and are therefore considered _✓__ dominant OR recessive (circle the correct answer). This results in a ____gain_______ of function. 29. To cause cancer, tumor suppressor genes require ___ 1 (or) _✓__ 2 allele(s) (circle the correct answer) to be mutated and are therefore considered ___ dominant (or) _✓__ recessive (circle the correct answer). This results in a ______loss_____ of function. 30. Watch the video clip. a. Using the gas pedal analogy, explain the impact on the cell cycle of a proto-oncogene versus an oncogene. A proto-oncogene is always serving as a ‘go’ signal pushing on the gas to promote the cell cycle. When even a single copy/allele for that gene is mutated becoming an oncogene, the gas pedal is depressed further increasing the rate of the cell cycle. b. Using the brake pedal analogy, explain the impact on the cell cycle of one mutated tumor suppressor gene allele versus two mutated tumor suppressor alleles. When one copy/allele for such a gene is lost, the brake pedal remains depressed by the single functioning copy. However, when both alleles are lost, the brake is no longer depressed, and only then can the cell cycle can proceed. APPLICATION/EXTENSION QUESTIONS Now that you have finished the Click & Learn, use your knowledge to answer the following questions. 31. p53 is a protein that is encoded by a tumor suppressor gene, and some scientists refer to it as “the guardian of the genome.” a. Explain its normal role and why scientists would regard it as the “guardian of the genome.” P53 prevents cell cycle progression when DNA damage is detected (in both G1 and G2) by either allowing time for repair to occur or the cell to die. This prevents a cell from passing on a damaged genome to daughter cells, ‘guarding’ it. b. Explain what happens to the cell cycle if both alleles of the gene encoding p53 are mutated. When both allele for p53 are mutated, the cell no longer has the ability to inhibit the cell cycle when its DNA is damaged, this dramatically increasing an individual’s risk of cancer. 32. Explain why people who inherit one mutated allele of the BRCA1 gene have a higher likelihood of developing cancer. BRCA1 mediates DNA repair/death once DNA damage has been identified by proteins such as ATM. If it is required to promote/activate repair or death, loss of even some of its activity will reduce the cell’s ability to respond in an appropriate/sufficient way to DNA damage, increasing cancer risk. 33. Predict a potential outcome of a mutated mitotic arrest deficient (MAD) protein. Loss of MAD activity - Cells will proceed into anaphase even if the mitotic spindles are not appropriately attached to all chromosomes. Chromosomes will be segregated unevenly into the daughter cells leading to an increased risk of cancer. Increased MAD activity - Cells will be unable to activate the APC/C complex and become stalled in metaphase, impairing cell division and the process it is playing a part of. The Eukaryotic Cell Cycle and Cancer – In Depth 34. Use the model illustrated in the figure below to answer the accompanying questions. a. The human gene EGFR located on chromosome 7 is a proto-oncogene that codes for a growth factor cell surface receptor. The binding of growth factors to this receptor can lead to cell proliferation. Hypothesize what potential impact a mutated EGFR allele will have on a cell. Give one possible impact and explain your answer. Loss of EGFR function – Cells will not receive the stimulatory message promoting expression of the G1-cyclin, the G1 CDK-cyclin complex will not form to activate the CDK, and exit from G1 into S phase will be inhibited. Gain of EGFR function – Cells will receive the stimulatory message more (possibility even continually) promoting expression of the G1-cyclins, activation of the G1 CDK-cyclin complex, and progression through the cell cycle. This could lead to uncontrolled cell division and cancer. b. RAS is a G protein that is activated when a growth factor attaches to EGFR. Its activation results in the exchange of GTP for GDP. Once activated, the GTP cannot be hydrolyzed and RAS cannot be deactivated. What is one potential outcome of a mutation in one of the two copies of RAS? Mutation that increases RAS activity (one allele) - Overstimulation of the cell cycle leading to uncontrolled cell division and cancer. Mutation that reduced/prevents RAS activity (one allele) – No change in the cell cycle as the other allele and functional protein produced from it, allows the cell cycle to proceed normally. c. Mutations in the genes that code for proteins in this pathway have been linked to various types of cancer (i.e., RAS: pancreatic, BRAF: colorectal, MEK: melanoma, EGFR: lung). If you were developing a Dr. Pettit BIO1140 Winter 2021 Adapted from Adapted from the HHMI BioInteractive ‘The Eukaryotic Cell Cycle and Cancer In -Depth Medicine’ Worksheet The Eukaryotic Cell Cycle and Cancer – In Depth new cancer drug, what would be an appropriate target protein for the new drug therapy? Justify your answer. It would depend on which type of cancer I was aiming to treat. I would select the specific protein that was impaired/mutated in that particular cancer (E.g - RAS: pancreatic, BRAF: colorectal, MEK: melanoma, EGFR: lung) and attempt to resort its normal function. However, if I was seeking to develop a more general purpose chemotherapeutic agent against many types of cancer, it might be possible to target a common downstream target (e.g. MEK or even other proteins it activates to produce the cellular response of proliferation) to prevent proliferation of the tumor cells.