Final Bio 202.02 DB Clarification Post 5 Sp19 kms PDF

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Bio 202.02 Spring 2019 Discussion Clarification Post 5 Chapter 12 Chapter 12: Mitosis, Cell Cycle, Intro to Cancer 1. Nuclear Envelope The nuclear envelope breaks down during mitosis because A. the mitotic spindle has formed B. the chromosomes have condensed *C. specific nuclear proteins have become phosphorylated D. the centrosomes have duplicated E. the sister chromatids have separated Response: To answer this question, you must understand the timing and interactions between cell structures and molecules during mitosis. You are asked to identify the answer that best explains why the nuclear envelope breaks down during mitosis. You should be able to draw on what you learned in Chapter 6 about the structure and contents of the nuclear envelope, and Chapter 11 about cell signals and phosphorylation performed by protein kinases and apply it to cell division. The nuclear membrane is a porous double-membrane that is reinforced by the nuclear lamina and nuclear matrix. The nuclear membrane houses the nucleus, which is where chromosomes are found. Kinases are enzymes that add phosphate groups to proteins, also known as phosphorylation. The phosphorylation of proteins changes their structure and function. The nuclear envelope must break down so kinetochore microtubules can attach to and separate sister chromatids. The formation of the mitotic spindle itself does not break down the nuclear envelope, but the movement of the spindle is aided by the breakdown of the nuclear envelope. This falsifies choice A. Chromosomes must condense to allow the kinetochore microtubules to attach and safely separate sister chromatids. It is true that chromosomes have condensed when the nuclear envelope fragments. The timing of all events that occur during mitosis is critical for successful cell division and so it is highly regulated. Choice B can be falsified because cell structures and organelles are not sentient. They do not watch and wait for their turn in the cell cycle. These changes are the result of biochemical reactions. The breakdown of the nuclear envelope is regulated by cyclin-cdk complex called MPF. MPF phosphorylates proteins that break down the nuclear envelope and allows the daughter chromosomes to migrate to opposite ends of the mitotic spindle. This supports choice C. The falsification provided for choice B can be applied to Choices D, and E as well but it is useful to look for more than one way an answer choice can be falsified. Centrosomes are duplicated during G2, before a cell even enters mitosis. If the nuclear envelope broke down before the cell entered M phase, the chromosomes would not have condensed which could lead to serious problems for the cell. This falsifies choice D. The separation of sister chromatids would not be possible if the nuclear membrane is still intact because kinetochore microtubules would not be able to reach them. This falsifies choice E and confirms choice C is the best answer. Clarification of Confusion Specific to the Spring 2019 Discussions: Posts to this question typically did not provide enough detail to falsify the incorrect answers or

did not identify the best answer. Specific points of confusion are clarified below. 1. Almost all posts to this question thread did not relate each answer choice back to what the question is specifically asking. For example, it is useful to know when in the cell cycle the nuclear envelope breaks down, but the question is asking what causes it to break down. Falsifications must be both factually correct and logical arguments for why an answer is not the best choice. In other words, they must address the science of the answer within the context of the science of the question. 2. Several posts incorrectly identified choice A as the best answer. These posts provided a lot of information about the purpose of the mitotic spindle and how it forms but did not address how this could cause the nuclear envelope to fragment. Possible reasons for this confusion are not double checking your understanding of what the question is asking, not relating the answer back to the question, looking for the correct answer instead of using the process of falsification, or assuming previous posts are reliable references and repeating them without checking their facts or reasoning. 3. MPF phosphorylates several different proteins. Different proteins have different functions; therefore, MPF can have different effects on mitosis and the cell cycle. If you are unsure of what MPF is, what it does, or how it does it read pages 244-247 of the Campbell Biology 11th edition textbook and ask Prof. Erster, Instructional Support, or TAs questions. Suggestion: Check your arguments and confirm they are logical with respect to the answer choice and the question. Do the same when reading your colleague’s posts. Do you agree with their reasoning? If so, what new scientific support can you add to the discussion? If not, provide scientific support for why you think they are wrong. Questions to Consider: 1. What allows cells to pass through the G2 checkpoint? 2. What type of enzyme removes phosphate groups from proteins? 2. Spindle Microtubules Once researchers understood that chromosomes are moved by the spindle microtubules, the next question they wanted to answer is how the microtubules function to bring about this process. They used fluorescent labels to make the microtubular structures fluoresce. When anaphase began, they photobleached a section of microtubules. As chromosomes moved toward the poles of the daughter cells, the photobleached sections of the microtubules remained stationary. This result suggests that _____. A. the microtubules shorten at the centrosome end B. the microtubules are of constant length; centrosomes move farther apart to separate chromosomes *C. the microtubules shorten at their kinetochore end D. the microtubules overlap, and slide with respect to one another, effectively shortening the microtubules without depolymerizing the actual fiber

Response: Your understanding of Dr. Borisy’s experiment on the movement of the mitotic spindle is tested by this question. By experimentally marking a section of the spindle microtubules that connect the spindle poles to the kinetochores, it’s possible to identify from where the tubulin subunits are being removed (e.g. from the pole end, the kinetochore end, or the center). Borisy and his team observed that the photobleached section of microtubules remained stationary. If the microtubules shortened at the centrosome end, the photobleached section would appear to move towards the centrosome. Choice A can be falsified because the question states that in this experiment, the photobleached region of microtubules did not appear to move at all. Choice B can be falsified based on what you learned about the dynamic instability of microtubules in the Chapter 6 lectures. Microtubules can be lengthened or shortened as needed by the cell via polymerization or depolymerization. Because the photobleached sections of the microtubules remained stationary, the tubulin subunits were likely being removed from the kinetochore end. This supports choice C. If microtubules overlapped without depolymerizing, the tubulin subunits in the photobleached region would shift and the mitotic spindle would remain in the cell after anaphase which could affect the re-formation of the nuclear envelope. This falsifies choice D and confirms choice C as the best answer.

3. Correct Sequence Which is the correct sequence of events for a cell entering mitosis?

A. E A D C B B. D C B E A *C. B E A D C D. B A E D C Response: You must identify the stages of mitosis depicted in the figure to answer this question. Mitosis begins with prophase, which is when duplicated chromosomes start appearing, and nucleoli start disappearing. The centrosomes can be visualized, and they start to move away to the opposite ends of the cell. Image B matches this description. You can see chromosomes have been replicated and the nucleolus is not present. This falsifies choices A and B. The next step in mitosis is prometaphase, which is when the nuclear envelope fragments and microtubules start to extend from the centrosomes. Some of these microtubules will associate with chromosomes, while others will associate with microtubules from the opposite pole. After

this step, the cell enters metaphase, which is when the chromosomes all associate with the microtubules via their kinetochores and line up at the metaphase plate. This description matches Image E, which falsifies choice D. By the process of elimination, choice C can be identified as the best answer, however the rest of the images should be checked make sure nothing was missed. After metaphase, the dividing cell enters anaphase, which is distinguished by the separation of the sister chromatids to the opposite poles. Image A depicts this. After anaphase comes telophase, which is when the two daughter nuclei form in the cell. Image D portrays the cell that is undergoing this stage of mitosis. Image C shows one of the daughter cells after mitosis is complete. The chromosomes are decondensing and the nucleolus is present. This confirms choice C as the best answer. Clarification of Confusion Specific to the Spring 2019 Discussion Boards: Many posts demonstrated difficulty identifying the correct answer to this question. The main areas of confusion are addressed below. 1. The most common incorrect answer identified as the best answer was choice B. Some of these posts correctly recognize the drawing in figure D as telophase but identify that as the start of mitosis which not correct. 2. It is important to remember that the names of mitotic stages are just labels used to break-down and describe one complex cellular process. Mitosis is one phase in the overall cell cycle. The phases of mitosis are all part of the same phase of the cell cycle – M phase. Similarly, G1, S, and G2 are all part of the same phase – interphase. 3. At the end of mitosis, healthy cells do not start mitosis again. They enter G1 or G0. Cells must pass the G1and G2 checkpoints before they can enter mitosis. Suggestion: Study the cell cycle and mitosis by sketching and labeling the stages from memory and checking your work afterwards. Be sure to include checkpoints and what is required to pass them. It is also important to understand why there are different stages of the cell cycle, and what can go wrong if they are skipped by the cell. Questions to Consider: 1. What happens at the M checkpoint? 2. Are the phases of mitosis identical in every eukaryotic organism? Why or why not? 3. What are some of the differences between mitosis and binary fission? 4. Nuclear Membrane in Mitosis The nuclear membrane is absent in which of the following phases of mitosis? I. Anaphase II. Telophase III. Metaphase A. I B. I and II *C. I and III D. II and III Response: Your understanding of the timing of mitotic events is tested by this question. The nuclear membrane is degraded by the end of prophase to allow microtubule attachment and separation of

sister chromatids. Kinetochore microtubules of the mitotic spindle start attaching to chromatids in prometaphase, and they are aligned in the center of the spindle in metaphase. The nuclear membrane would block microtubules from reaching and attaching to the kinetochores of chromatids. This supports item III and the answer choices that include it – choices C and D. Chromatids are separated in anaphase which requires cleavage of cohesins along their arms and motor proteins moving them on the spindle. The use of the mitotic spindle in anaphase supports item I and answer choices that include item I – choice C. The spindle couldn’t move in the presence of the nuclear membrane. In telophase the nuclear membrane reforms from fragments of the parent cell’s nuclear membrane. This falsifies any answer choice that includes option II – choices A and B and D and confirms choice C as the best answer.

5. Chromatid Separation What provides the force to pull chromatids apart from each other in mitosis? *A. Motor proteins on microtubules in the kinetochore B. Motor proteins on microtubules in the spindle poles C. Removal of tubulin subunits from the center of microtubules shortens them D. Addition of tubulin subunits to non-kinetochore microtubules lengthens them, forcing chromatids apart from each other. Response: This question tests your understanding of the dynamics of the mitotic spindle and chromatid movement in mitosis. The mitotic spindle is composed of microtubules. Microtubules are made longer or shorter by the polymerization and depolymerization of tubulin subunits. Motor proteins such as dynein, that you learned about in Chapter 6, power the movement of microtubules. Motor proteins help the microtubule to move the sister chromatids apart to different sides of the cell. Motor proteins on the kinetochore end walking chromatids along the microtubules is sometimes referred to as the “Pac-man” method of microtubule shortening. The microtubules depolymerize after the motor proteins pass them. This supports choice A. In some cases, motor proteins at the spindle poles will “reel-in” chromatids in from the metaphase plate. The “reelingin” method exists, but the motor proteins are at the spindle pole end, not inside the spindle poles. This falsifies Choice B. The removal of tubulin subunits is part of the dynamic instability of microtubules. The energy released by the hydrolysis of GTP to lengthen microtubules can be coupled to shorten them as well, but it does not provide the force to pull chromatids apart. This falsifies choice C. The addition of tubulin subunits to non-kinetochore microtubules lengthens them and the cell. It does not affect the length of kinetochore microtubules. This falsifies choice D and confirms choice A as the best answer.

6. Plant Cell Walls How is the new cell wall between two daughter cells formed after division of plant cells? A. Cell wall is pulled inwards into the cell by a belt of microfilaments that pinches the cell in half. B. Cell wall is pulled inwards into the cell by microtubules that pinch the cell in half C. Cell wall material is deposited onto the outside of vesicles that line up in the center of the cell *D. Cell wall material inside vesicles that line up in the center of the cell forms the new cell wall after the vesicles fuse Response: This question tests your understanding of the differences in the mechanisms used to perform cytokinesis in plant and animal cells. Microtubules prevent tension from causing damage to cells and so are not ideal for pinching animal or plant cells in half. This falsifies choice A. Vesicles do carry cell wall material to the center of a dividing plant cell to build a cell plate. The material is carried inside the vesicles and the vesicles fuse with one another to make a continuous plate. This falsifies choice B and supports choice C. The cell plate then fuses with the plasma membrane, giving rise to two daughter cells. This falsifies choice D and confirms choice C as the best answer. The existing cell wall is not extended, vesicles carrying cell wall material fuse with the plasma membrane of the cell.

7. Microtubule Organization Where do the microtubules of the spindle originate during mitosis in animal cells? *A. centrosome B. centromere C. desmosomes D. kinetochore Response: Microtubules were introduced in the Chapter 6 content about the cytoskeleton. This is an opportunity to review and build on that information. As this question describes, spindle microtubules are present during M phase of the cell cycle. They are composed of the same α and β tubulin dimers as cytoskeletal microtubules but specialize in aligning and separating paired chromosomes. The centrosome is an organelle found near the nucleus of animal cells. During G1, cells have one centrosome that later is replicated during G2 phase of the cell cycle. Within a centrosome is a pair of centrioles – microtubule structures in a 9+0 arrangement. Centrosomes are part of the mitotic spindle that organizes and produces microtubules. This supports choice A. Centromeres are the region of a chromosome that appears narrower than the rest of the chromosome. This is the region where sister chromatids are most closely attached to each other by cohesins and is the location where kinetochore proteins bind. Knowing that chromosomes are molecules of DNA falsifies choice B. Desmosomes are the junctions that fasten cells together. They do not contain tubulin or play a

role in the production of microtubules, falsifying choice C. Kinetochores are proteins that attach to centromeres. During prometaphase, spindle microtubules bind to the kinetochores on chromosomes to line them up for metaphase. This falsifies choice D and confirms choice A as the best answer. Spindle microtubules originate from centrosomes and attach to kinetochores. 8. Mitotic Prophase In a cell with a diploid number of 12, how many chromatids are present at prophase of mitosis? A. 6 B. 48 C. 12 *D. 24 Response: The diploid number refers to the number of chromosomes found in a somatic cell. For example, the diploid number for human cells is 46. All mitotic phases occur after DNA has been replicated during synthesis (S phase). A cell that contains 12 chromosomes when it is not dividing, will contain 24 sister chromatids, which is still considered 12 chromosomes, after S phase and before cytokinesis. Clarification of Confusion Specific to the Spring 2019 Discussion Boards: This question revealed confusion about what makes a cell diploid versus haploid and the conceptual differences between chromosomes and chromatids. These main points of confusion are clarified below. 1. If you find the distinction between chromosomes, chromatids, and how they are counted in terms of haploid or diploid number, you are not alone. This concept trips up pretty much everyone at first. Ploidy describes the number of sets of chromosomes found in a cell. A set of chromosomes is the number of different chromosomes an organism has. Humans, for example have a set of 23 chromosomes and since we inherit one set from each of our parents, we are diploid organisms and our diploid number is 46. The diploid number of the cell in this question is 12 which means it must have two sets of 6 chromosomes. 2. Chromatid is the word biologists use to refer to chromosomes that have been duplicated. They are still chromosomes, but they are identical copies of each other during mitosis. You will see the exception to this when you learn about recombination during meiosis. Since the question is asking for the number of chromatids present at prophase, the replication of the 12 chromosomes has already occurred during S phase. This gives 24 chromatids, which are still considered 12 chromosomes. Suggestion: By asking you to perform a basic mathematical calculation, this question tests your understanding of specific biological terms. Use these questions to confirm understanding and identify those “unknown unknowns” that we all have. These are things we take for granted because we think we know them but don’t. For example, we may have memorized the definition

of a diploid cell, but if we do not understand how that relates to sets of chromosomes and you may apply it incorrectly when thinking about replication. Questions to Consider: How many sets of chromosomes are found in a triploid cell? How many pairs of chromosomes are found in a cell with a diploid number of 12? 9. Mitosis In normal eukaryotic cells, mitosis will not begin until the entire genome is replicated. If this inhibition is removed so that mitosis begins during S-phase, which one of the following would occur? A. The cells would grow more quickly. *B. The genome would become fragmented and incomplete. C. The cells would display unregulated, cancerous growth. D. The genome would be temporarily incomplete in each daughter cell, but DNA repair will fill in the missing gaps. Response: This question requires you to apply what you learned about cell cycle control and predict the most likely outcome for a cell that begins dividing at the same time chromosomes are duplicating. Cells grow during the G1 and G2 stages of the cell cycle. Choice A can be falsified because cells do not grow in either mitosis or S phase. The gen...