4/14/13Biol 1002 - Spring 2013 Cell cycle, Mitosis, Meiosis Cell cycle, Mitosis, Meiosis Signed in as Nora Trejos Help Close Resources Cell cycle, Mitosis, Meiosis Due: 3:00pm on Monday, April 15, 2013 Note: You will receive no credit for late submissions. To learn more, read your instructor's Grading Policy Chapter 12 Pre-Test Question 1 Part A Why do some species employ both mitosis and meiosis, whereas other species use only mitosis? Hint 1. Consider the kinds of cell reproduction different species need. ANSWER: They need only mitosis to make large numbers of cells such as sperm. They need only meiosis if they produce egg cells. They need both if they are producing animal gametes. A single-celled organism only needs mitosis. They need meiosis if the cells are producing organs such as ovaries. Correct The form of cell division that produces animal gametes is meiosis, not mitosis. The production of gametes in plants is a function of both mitosis and meiosis (Ch. 38). Chapter 12 Pre-Test Question 3 Part A Why is it difficult to observe individual chromosomes with a light microscope during interphase? Hint 1. Consider the events of prophase. ANSWER: Sister chromatids do not pair up until division starts. They have uncoiled to form long, thin strands. They leave the nucleus and are dispersed to other parts of the cell. The spindle must move them to the metaphase plate before they become visible. The DNA has not been replicated yet. Correct Except during the M phase, the DNA is extended, allowing its genes to be transcribed for protein synthesis. Chapter 12 Question 2 Part A If there are 20 chromatids in a cell in G1 phase, how many centromeres are there? ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 1/24 4/14/13 40 20 10 30 80 Cell cycle, Mitosis, Meiosis Correct Mitosis (1 of 3): Mitosis and the Cell Cycle (BioFlix tutorial) Mitosis is the part of eukaryotic cell division in which the nucleus divides and distributes the chromosomes to the two daughter nuclei, ensuring that each daughter cell receives chromosomes identical to those of the parent cell. Mitosis is just one part of the cell cycle, the overall sequence of events from the formation of a cell by cell division to the cell’s own division into two new daughter cells. In this tutorial, you will review key terms associated with mitosis and the cell cycle, cellular processes that occur during the main phases of the cell cycle, and changes in DNA structure during the cell cycle. Before beginning this tutorial, watch the Mitosis animation. Pay particular attention to the structural changes that occur in the DNA before and during cell division. Part A - Mitosis and cell cycle terminology As in most areas of biology, the study of mitosis and the cell cycle involves a lot of new terminology. Knowing what the different terms mean is essential to understanding and describing the processes occurring in the cell. Drag the terms on the left to correctly complete these sentences. Not all the terms will be used. Hint 1. What is a sister chromatid? During interphase, the cell’s DNA replicates, producing an identical copy of each DNA molecule. By the end of mitosis, one copy of each DNA molecule has been moved to each daughter cell. Drag each label to the appropriate target. Not all labels will be used. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 2/24 4/14/13 Hint 2. The main stages of the cell cycle Cell cycle, Mitosis, Meiosis The cell cycle can be divided into two principle stages: interphase and the mitotic (M) phase. Interphase consists of three subphases: G1, S, and G2. During interphase, the cell grows and copies its chromosomes in preparation for cell division. The mitotic phase includes mitosis (division of the nucleus, which itself is divided into further subphases) and cytokinesis (division of the cytoplasm). Hint 3. What cellular structures are involved in the separation of chromosomes during mitosis? A number of cellular structures are involved in physically separating the duplicated chromosomes into two daughter nuclei during mitosis. Which of the following statements does not correctly describe the role of a cellular structure in mitosis? ANSWER: During mitosis, the attachment of the sister chromatids to each other at the centromere is broken, permitting the chromatids to separate. Every chromosome is attached to the mitotic spindle by two sets of microtubules, one extending from each pole of the cell. The kinetochore is the structure that holds the sister chromatids together. The centrosomes are the organizing centers for the formation of the mitotic spindle in animal cells. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 3/24 4/14/13 Cell cycle, Mitosis, Meiosis Correct The key structures involved in mitosis are labeled in this diagram of an animal cell that shows the two sister chromatids of each duplicated chromosome beginning to attach to the mitotic spindle by means of their kinetochores. The centrosomes anchor the mitotic spindle at opposite ends of the cell. Part B - Phases of the cell cycle The cell cycle represents the coordinated sequence of events in the life of a cell from its formation to its division into two daughter cells. Most of the key events of the cell cycle are restricted to a specific time within the cycle. In this exercise, you will identify when various events occur during the cell cycle. Recall that interphase consists of the G1, S, and G2 subphases, and that the M phase consists of mitosis and cytokinesis. Drag each label to the appropriate target. Hint 1. Role of checkpoints in the cell cycle Checkpoints are control points in the cell cycle where “stop and go” signals regulate whether or not a cell continues to the next part of the cycle. For example, For example, cells that pass through the G1 checkpoint usually complete the cell cycle and divide. If a cell does not pass through the G1 checkpoint, it exits the cell cycle and enters a nondividing state called the G0 phase. Hint 2. What distinguishes the G2 phase from the S phase? Once the cell passes through the G1 checkpoint, it enters the S phase, followed by the G2 phase. Which two statements correctly describe the processes that occur in the S and G2 phases? ANSWER: DNA replication occurs in the G2 phase. DNA replication occurs in the S phase. Centrosome replication occurs in the G2 phase. Centrosome replication occurs in the S phase. Hint 3. What processes occur during the M phase of the cell cycle? The M (mitotic) phase begins when the cell passes through the G2 checkpoint. It is typically the shortest, but most complex, phase of the cell cycle. Which three of the following processes occur during the M phase of the cell cycle? ANSWER: cytokinesis centrosome replication formation of the mitotic spindle separation of sister chromatids DNA replication ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 4/24 4/14/13 Cell cycle, Mitosis, Meiosis Correct Many organisms contain cells that do not normally divide. These cells exit the cell cycle before the G1 checkpoint. Once a cell passes the G1 checkpoint, it usually completes the cell cycle--that is, it divides. The first step in preparing for division is to replicate the cell’s DNA in the S phase. In the G2 phase, the centrosome replicates. In early M phase, the centrosomes move away from each other toward the poles of the cell, in the process organizing the formation of the mitotic spindle. At the end of the M phase when mitosis is complete, the cell divides (cytokinesis), forming two genetically identical daughter cells. Part C - Changes in DNA structure during the cell cycle As the chromosomes of a parent cell are duplicated and distributed to the two daughter cells during cell division, the structure of the chromosomes changes. Answer the three questions for each phase of the cell cycle by dragging the yes and no labels to the appropriate locations in the table. Note: Assume that by the end of the M phase, the parent cell has not yet divided to form two daughter cells. Hint 1. When are sister chromatids present? Sister chromatids play an essential role in ensuring that each daughter cell receives genetic material that is identical to that which was present in the original parent cell. Which statement about sister chromatids is correct? ANSWER: Sister chromatids first form in the G1 phase, and are present until they separate in early anaphase. Sister chromatids first form in the S phase, and are present until they separate in early anaphase. Sister chromatids first form during prophase, and are present until they separate in telophase. Hint 2. The condensation state of DNA throughout the cell cycle As you can observe in micrographs of cells undergoing mitosis, the chromosomes condense at the beginning of the M phase. Because the chromosomes are at least a thousand times longer than the cell, condensation greatly facilitates separation of the sister chromatids into the daughter cells during mitosis. The chromosomes remain condensed until telophase, when they begin to de-condense. But why doesn’t the DNA remain condensed throughout interphase too? One important event that occurs during the S phase of interphase is the replication of the DNA. Replication requires that the DNA be in its uncondensed form. In addition, transcription of genes for protein synthesis also requires that the DNA be in an uncondensed form. Hint 3. Changes in DNA content during the cell cycle Once a cell passes the G1 checkpoint, its DNA is replicated during the S phase of interphase. Replication means that an exact copy of the DNA in each chromosome is made, thus doubling the cell’s DNA content. Only once cytokinesis is completed at the end of telophase does the cell’s DNA content return to the level found in G1 cells. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 5/24 4/14/13 ANSWER: Cell cycle, Mitosis, Meiosis Correct Sister chromatids form when DNA replicates in the S phase. The sister chromatids become individual chromosomes once they separate in early anaphase. Similarly, the cellular DNA content doubles in the S phase when the DNA replicates. However, the cell’s DNA content does not return to its normal (undoubled) levels until after cytokinesis is complete and two daughter cells have formed. The condensation state of the DNA is not related to the presence or absence of sister chromatids. The DNA condenses in prophase and remains condensed until after the sister chromatids separate and the new daughter cells begin to form. In late telophase/cytokinesis, the emphasis shifts to cell growth and DNA replication for the next cell cycle. For these processes to occur, the DNA needs to be de-condensed so it is accessible to the cellular machinery involved in transcription. Mitosis (2 of 3): Mechanism of Mitosis (BioFlix tutorial) During mitosis, duplicated chromosomes (each consisting of a pair of sister chromatids) that formed during interphase separate within the parent cell and are distributed to two identical daughter nuclei. The mechanism of mitosis involves coordinated changes in chromosome structure, the mitotic spindle, and the nuclear envelope. In this tutorial, you will identify the stages of mitosis shown in micrographs of dividing animal cells and review the interactions between chromosomes and the mitotic spindle that lead to the separation of each pair of sister chromatids. Before beginning this tutorial, you may want to review the stages of mitosis depicted in the Mitosis animation. Part A - Identifying the stages of mitosis The stages of mitosis were originally defined by cellular features observable through a light microscope. The six micrographs below show animal cells (lung cells from a newt) during the five stages of mitosis, plus cytokinesis. (Note that interphase is not represented in these micrographs.) In these images, the chromosomes have been stained blue, microtubules green, and microfilaments red. Drag each micrograph to the target that indicates the stage of mitosis or cytokinesis it shows. Hint 1. How can you distinguish between prophase and prometaphase? Prophase and prometaphase are sequential stages in mitosis that set up the cell for nuclear division. Sort the following events according to whether they occur in prophase, in prometaphase, or in neither of these stages. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 6/24 4/14/13 Cell cycle, Mitosis, Meiosis Hint 2. Chromosome position in metaphase Metaphase is characterized by the alignment of the chromosomes along the metaphase plate, a plane equidistant from the poles of the spindle. Although metaphase is often illustrated with all the chromosomes neatly aligned along the metaphase plate, this is not always the case. In real cells, the arms of each chromosome may extend some distance away from the metaphase plate, and only the centromere of each chromosome may be aligned on the metaphase plate. Hint 3. What are the key characteristics of anaphase, telophase, and cytokinesis? In anaphase and telophase, the chromosomes separate and two daughter nuclei are formed. In cytokinesis, the cytoplasm of the cell divides. Sort the following processes into the appropriate bin to indicate the earliest stage in which the process can be observed in an animal cell. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 7/24 4/14/13 Cell cycle, Mitosis, Meiosis ANSWER: Correct As these six micrographs demonstrate, cellular events observable by light microscopy can be used to define the six stages of mitosis and cytokinesis. However, deciphering which stage is which in real cells can be much more challenging than in the drawings of idealized cells you see in your textbook. Thus, it is important to carefully observe the completeness of the mitotic spindle and the location of the chromosomes, as well as how condensed the chromosomes are. Part B - Mechanisms underlying the events of mitosis Mitosis unfolds through a sequence of stages marked by specific events in the cell. The structural changes in the cell are brought about by a series of tightly coordinated underlying mechanisms. Sort each process into the appropriate bin to indicate the stage of mitosis in which it occurs. If a process occurs in more than one stage, sort it to the stage when it first occurs. Hint 1. Metaphase: dividing mitosis into two groups of cellular events In metaphase, the centromeres of the duplicated chromosomes align along the metaphase plate, midway between the poles of the cell. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 8/24 4/14/13 Cell cycle, Mitosis, Meiosis To help you sort the processes in this question, consider that many of the cellular events that precede metaphase are reversed after metaphase. For example, prior to metaphase, the DNA condenses in preparation for chromosome separation. After metaphase, the DNA de-condenses once chromosome separation is complete. Hint 2. Which proteins are involved in changes to cellular structures during mitosis? Many of the changes in structure that occur in stages of mitosis prior to metaphase prepare the cell for the separation of the chromosomes. These changes are reversed in the stages following metaphase, after the chromosomes have separated. Several proteins are involved in these changes. Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Not all terms will be used. ANSWER: Hint 3. During mitosis, what is the sequence of events involving kinetochores? Kinetochores are assemblies of proteins that function in attaching the chromosomes to the mitotic spindle. By the end of prometaphase, each sister chromatid of a duplicated chromosome has a kinetochore located at its centromere. In some types of cells, the kinetochore contains motor proteins that are responsible for moving chromosomes along the spindle microtubules (see diagram below). In other types of cells, the chromosomes are pulled toward the poles by motor proteins associated with the centrosomes. In these cells, the kinetochore only attaches the chromosome to the mitotic spindle. Consider a cell in which chromosome movement along the mitotic spindle is accomplished by motor proteins in the kinetochores. Drag the labels onto the flowchart to show the correct sequence of events in kinetochore function during mitosis. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 9/24 4/14/13 ANSWER: Cell cycle, Mitosis, Meiosis ANSWER: Correct The micrographs in Part A show some of the cellular processes that occur during the stages of mitosis. In prophase, the microtubules of the spindle apparatus begin to assemble from individual tubulin subunits. As the identical chromatids of each pair of sister chromatids condense during this stage, they are held together by cohesin proteins. Prometaphase is marked by fragmentation of the nuclear envelope, expansion of the spindle into the nuclear region, and attachment of some spindle fibers to the chromosomes via the kinetochores. Metaphase, marked by the alignment of chromsomes along the metaphase plate, is brought about by kinetochores aligning and then remaining motionless relative to the poles of the cell. In anaphase, the cohesin proteins are cleaved, and the kinetochores move toward the poles of the cell, separating the sister chromatids. As telophase proceeds, the kinetochore microtubules of the spindle disassemble. As the chromosomes reach the poles of the cell, the nuclear envelopes of the two new daughter nuclei form. Part C - The mitotic spindle The mitotic spindle consists of two types of microtubules: kinetochore microtubules and nonkinetochore microtubules. In animal cells, these two types of microtubules function differently in the stages of mitosis. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 10/24 4/14/13 Cell cycle, Mitosis, Meiosis Complete the sentences by dragging the labels to the appropriate locations. Labels may be used once, more than once, or not at all. Hint 1. Construction of the mitotic spindle in the early stages of mitosis The centrosomes are the organizing centers for the mitotic spindle. The centrosomes replicate during G2 of interphase; at the beginning of mitosis, the centrosomes are adjacent to one another. As prophase progresses, the centrosomes move apart and toward the poles of the cell as the microtubules extending from each centrosome lengthen. This lengthening occurs in the region of overlap between the microtubules and forces the centrosomes further apart. Hint 2. How do kinetochore and nonkinetochore microtubules differ? Both kinetochore and nonkinetochore microtubules are components of the mitotic spindle, but they serve different functions during mitosis. Can you identify the roles of these two types of microtubules in animal cells? Drag the terms on the left to the appropriate blanks on the right to complete the sentences. Terms can be used once, more than once, or not at all. ANSWER: Hint 3. How do nonkinetochore microtubules function during anaphase? In animal cells, while the chromosomes move toward the poles of the cell during anaphase, the nonkinetochore microtubules cause the cell to elongate. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 11/24 4/14/13 Cell cycle, Mitosis, Meiosis The nonkinetochore microtubules form pairs (one from each centrosome), which overlap and are in contact with each other near the metaphase plate. Which two statements correctly describe how the nonkinetochore microtubules cause an animal cell to elongate during anaphase? Select the two statements that apply. ANSWER: The nonkinetochore microtubules move past each other in the region where they overlap. Each pair of nonkinetochore microtubules lengthens at the centrosome. Each pair of nonkinetochore microtubules lengthens at the overlapping ends near the metaphase plate. ANSWER: Correct The mitotic spindle is the machinery that guides the separation of chromosomes in anaphase. Prior to metaphase, the mitotic spindle is constructed by lengthening microtubules that extend from each centrosome. In metaphase, the kinetochore microtubules have attached each pair of sister chromatids, and the nonkinetochore microtubules overlap extensively at the metaphase plate. During anaphase, the kinetochore microtubules shorten as the chromosomes move toward the poles of the cell. At the same time, the nonkinetochore microtubules lengthen and push past each other, elongating the cell. By the end of telophase, all the microtubules associated with the mitotic spindle have disassembled. Part D - Roles of the mitotic spindle Consider an animal cell in which motor proteins in the kinetochores normally pull the chromosomes along the kinetochore microtubules during mitosis. Suppose, however, that during metaphase, this cell was treated with an inhibitor that blocks the function of the motor proteins in the kinetochore, but allows the kinetochore to remain attached to the spindle. The inhibitor has no effect on any other mitotic process, including the function of the nonkinetochore microtubules. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 12/24 4/14/13 Cell cycle, Mitosis, Meiosis Consider these three questions concerning this animal cell that has been treated with the inhibitor: 1. Will this cell elongate during mitosis? (yes or no?) 2. Will the sister chromatids separate from each other? (yes or no?) 3. Will the chromosomes move to the poles of the cell? (yes or no?) Type in the answer to each of the three questions (either yes or no) separated by commas. For example, if the answer to all three questions is yes, you should enter yes,yes,yes. Hint 1. How to approach question 1 -- Will the cell elongate? Part C of this tutorial reviewed the processes that lead to cell elongation during mitosis in animal cells. The cellular structures most closely associated with cell elongation during mitosis are the nonkinetochore microtubules. Does the inhibitor affect the function of the nonkinetochore microtubules? Hint 2. How to approach question 2 -- Will the sister chromatids separate? The cellular structures that are most important in separation of the sister chromatids (after they have become disconnected from each other) are the kinetochore microtubules. Read carefully about the effect of the inhibitor and think through the following questions: Does the inhibitor block the function of the kinetochores in chromosome movement? Does the inhibitor cause the chromosomes to disconnect from the kinetochore microtubules? Are the chromosomes connected to the centrosomes by the kinetochore microtubules? Are there other processes going on in the cell that are causing the centrosomes to move farther apart? Will these processes also affect the position of the chromosomes? Hint 3. How to approach question 3 -- Will the chromosomes move to the poles of the cell? In this type of cell, motor proteins in the kinetochore are responsible for moving the chromosomes along the kinetochore microtubules. Is any component of this machinery affected by the inhibitor treatment? If so, will the chromosomes be able to move to the poles of the cell? ANSWER: yes,yes,no Correct The inhibitor does not affect the cleavage of cohesins (the proteins that hold the sister chromatids together), the attachment of the chromosomes to the kinetochore microtubules, or the elongation of the cell due to the nonkinetochore microtubules. The inhibitor only affects the motor protein that pulls the chromosome along the kinetochore microtubule in anaphase. Thus, in the treated cell, the sister chromatids can still separate at the beginning of anaphase due to the fact that the cell is elongating (the centrosomes at the poles of the cell are moving farther apart) and the kinetochore microtubules still connect the chromosomes to the centrosomes. However, because the chromosomes cannot move along the kinetochore microtubules, they will never reach the poles of the cell. Activity: The Cell Cycle Click here to complete this activity. Then answer the questions. Part A Which of these phases encompasses all of the stages of mitosis? session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 13/24 4/14/13 Cell cycle, Mitosis, Meiosis ANSWER: B D E C A Correct This is mitosis. Part B During _____ both the contents of the nucleus and the cytoplasm are divided. ANSWER: the mitotic phase G1 S mitosis G2 Correct The mitotic phase encompasses both mitosis and cytokinesis. Part C During _____ the cell grows and replicates both its organelles and its chromosomes. ANSWER: interphase mitosis G1 S cytokinesis Correct These are the events of interphase. Activity: Four Phases of the Cell Cycle session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 14/24 4/14/13 Cell cycle, Mitosis, Meiosis Click here to view this animation. Then answer the questions. Part A Which of the following is a function of the S phase in the cell cycle? Hint 1. What does the S in S phase represent? ANSWER: The sorting of chromosomes The synchronization of organelle division The synthesis of sister chromatids The separation of sister chromatids Correct DNA replication occurs during S phase and results in two sister chromatids for each original chromosome. Part B The copying of chromosomes occurs during which of the following phases of the cell cycle? Hint 1. Consider what each phase name stands for. ANSWER: G2 phase S phase G1 phase M phase Correct Chromosomes must undergo replication before mitosis can occur; this copying occurs during the S phase. Part C For the first several divisions of early frog embryos, cells proceed directly from the M phase to the S phase and back to M without gaps. Which of the following is likely to be true about dividing cells in early frog embryos? Hint 1. What normally occurs during gap phases? ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 15/24 4/14/13 Cell cycle, Mitosis, Meiosis The number of chromosomes decreases with each generation. Frog embryos do not have DNA. The cells get smaller with each generation. The cells do not divide by mitosis. Correct Without gap phases, the cells have no opportunity to grow. Part D True or false? The M phase is characterized by the replication and division of a cell's chromosomes. Hint 1. Cells prepare for DNA synthesis during the G1 phase. ANSWER: True False Correct Correct: Cells replicate their chromosomes during the S phase and divide partition their chromosomes during the M phase. Part E If an organism normally has 34 chromosomes, how many molecules of DNA should there be in the G1 phase of the cell cycle? Hint 1. Review the cell cycle, the timing of DNA replication, and the timing of segregation of chromosomes during mitosis. ANSWER: 17 136 34 68 Correct There is one DNA molecule per chromosome. Part F Which of the following events would cause the cell cycle to arrest? Hint 1. Consider the assessments that must be made at each of the three checkpoints in the cell cycle. ANSWER: Poor nutrient conditions Complete chromosome replication Chromosomes lined up along the metaphase plate Synthesis of cyclins by growth factors and activation of a cyclin-Cdk complex session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 16/24 4/14/13 Cell cycle, Mitosis, Meiosis Correct Poor nutrient conditions would arrest the cell cycle at the G1 checkpoint. Mitosis (3 of 3): Comparing Cell Division in Animals, Plants, and Bacteria (BioFlix tutorial) The division of any cell, prokaryotic or eukaryotic, requires that the genetic information in each of the parent cell’s chromosomes be faithfully copied and distributed into each daughter cell. Only then does the cell physically divide. Because of their divergent evolutionary paths, bacteria, plants, and animals have somewhat different mechanisms of chromosome separation and cell division. Part A - Comparing chromosome separation in bacteria and eukaryotes In all cells, separation of replicated chromosomes is a prerequisite for cell division. However, the mechanism of chromosome separation in bacteria is distinct from that in eukaryotes in several ways. Sort the following statements into the appropriate bin. Hint 1. Review of changes to chromosomes during cell division in eukaryotes In eukaryotic cells, replication of each chromosome is accompanied by complex changes in the structure of the chromosomes. These changes begin during S phase of interphase and result in the formation of a pair of fully condensed sister chromatids in prometaphase of mitosis. Watch this segment of the Mitosis animation to see these structural changes that occur in a eukaryotic cell. Hint 2. What structures in bacterial cells are likely involved in chromosome separation? A bacterial cell contains a single, circular chromosome. As in a eukaryotic cell, after replication of the DNA, the chromosomes move to opposite ends of the bacterial cell prior to cell division by a mechanism that is not fully understood. In order for the chromosomes to separate, they must attach to something in the cell. Based on what is known about how bacterial chromosomes separate, which of the following structural features of a bacterial cell likely play a role in chromosome separation? Select the two that apply. ANSWER: plasma membrane nuclear envelope spindle microtubules origin of replication (where DNA replication starts on the chromosome) Hint 3. How do structural properties of bacterial chromosomes influence what happens to them during cell division? Although all chromosomes consist of DNA, bacterial chromosomes are structurally distinct from their eukaryotic cousins. How do these differences affect what happens to bacterial chromosomes during cell division? Which of the following statements are correct? Select the two that apply. ANSWER: Following replication of the DNA, bacterial chromosomes remain physically associated with each other until just before cell division occurs. A bacterial chromosome consists of double-stranded DNA, and thus replication of the DNA before cell division is not necessary. A bacterial chromosome is highly folded and coiled, but lacks the fully condensed structure of a eukaryotic chromosome during cell division. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 17/24 4/14/13 Cell cycle, Mitosis, Meiosis Correct Although the processes of chromosome separation in bacteria and eukaryotes have a common evolutionary origin, the actual mechanisms are different. Structurally, bacterial cells contain a single chromosome that is much shorter than those in eukaryotic cells, and bacterial cells lack a mitotic spindle. The bacterial chromosome does not fully condense before separation. However, the physical separation of the replicated bacterial chromosomes still involves attachment to some structure in the cell: possibly the plasma membrane at the origins of replication. Part B - Cytokinesis in plant cells Cytokinesis in animal cells is accomplished by constriction of the cell along the plane of cell division (formation of a cleavage furrow). In plant cells, which have cell walls, a completely different mechanism of cytokinesis has evolved. Which of the following statements are true of cytokinesis in plant cells? Select the two that apply. Hint 1. Formation of the cell plate During cytokinesis in a plant cell, the cell plate forms as vesicles derived from the Golgi apparatus fuse along the plane of cell division. As more vesicles fuse with the cell plate, it grows outward until it fuses with the plasma membrane of the parent cell. Hint 2. What role, if any, do either microtubules or microfilaments play in cytokinesis in plant cells? Both microtubules and microfilaments are parts of the cytoskeleton and are important in many cellular processes, including motion and determination of cell shape. Which of the following statements correctly describes the role of these cytoskeletal elements in plant cytokinesis? ANSWER: Microfilaments cause constriction (cleavage) of the plasma membrane. Neither microtubules nor microfilaments play a role in plant cell cytokinesis. Both microtubules and microfilaments move the cell plate to the center of the cell, where it fuses with the plasma membrane. Microtubules guide Golgi-derived vesicles to the middle of the cell where they form the cell plate. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 18/24 4/14/13 Cell cycle, Mitosis, Meiosis The cell plate consists of the plasma membrane and cell wall that will eventually separate the two daughter cells. Vesicles from the Golgi apparatus move along microtubules, coalesce at the plane of cell division, and form a cell plate. The plasma membrane of the parent cell grows inward, eventually joining with the cell plate. After chromosome separation is complete, a network of microfilaments forms near the plane of cell division that will separate the two new cells. Correct In plant cell division, after chromosome separation, the microtubules of the mitotic spindle reorganize into a network that guides vesicles derived from the Golgi apparatus to the plane of cell division. These vesicles begin to fuse, forming the cell plate. As more vesicles are added to the cell plate, it grows outward, eventually fusing with the parent cell plasma membrane. Membrane from the vesicles forms the new plasma membrane for each daughter cell. At the same time, materials that were enclosed in the vesicles form the new cell wall between the new plasma membranes of the daughter cells. Part C - The roles of proteins in bacterial cell division and eukaryotic cytokinesis The division of a bacterial cell into two daughter cells (called binary fission) is accomplished by a protein called FtsZ. FtsZ is very similar to the tubulin subunits that form microtubules in eukaryotes. After the replicated bacterial chromosomes have moved to opposite ends of the cell, a ring of FtsZ proteins forms inside the plasma membrane in the region where the cell will divide. As the FtsZ ring constricts, the plasma membrane and bacterial cell wall fold in and eventually separate into two cells. Plant and animal cells also require cytoskeletal proteins for cytokinesis, although the roles these proteins play differ among bacteria, plants, and animals. For each space in the table, drag the appropriate label to indicate if the statement is true or false for each group of organisms. Hint 1. Comparison of cell walls in plants and bacteria Nearly all plant cells and many bacteria are surrounded by a cell wall. However, the roles these walls play in cell division differ dramatically. In plants, the wall is sufficiently rigid so that in most cells, constriction of a ring of proteins could not cause the wall to fold in and eventually pinch off. Rather, a new plant cell wall is produced between the daughter cells by the formation of a cell plate, which eventually joins with the existing cell wall. In contrast, in bacteria, the cell wall is sufficiently flexible to fold in during constriction of the FtsZ protein ring. Hint 2. What roles do microfilaments and microtubules or related proteins play in cell division in bacteria, plants, and animals? Bacteria lack the distinct microfilaments and microtubules that dominate the cytoskeletons of plant and animal cells. However, bacteria contain proteins that are similar to actin (the building block of microfilaments) and tubulin (the building block of microtubules). The tubulin-like proteins in bacteria function in the formation of two daughter cells through binary fission. Drag the terms on the left to the appropriate blanks on the right to complete the sentences. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 19/24 4/14/13 Cell cycle, Mitosis, Meiosis Hint 3. Comparison of cytokinesis in plant and animal cells and bacterial cell division Proteins associated with the cytoskeleton are essential to cytokinesis in plants and animals. Similarly, cytoskeleton-like proteins are essential to cell division in bacteria. The bacterial origin of plants and animals might suggest that these proteins and mechanisms of division are all related; however, evolution is not always that straightforward. Bacterial and animal cells divide in a very similar manner but accomplish division using different proteins. In contrast, plants and bacteria divide in a very different manner but accomplish division using very similar proteins. ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 20/24 4/14/13 Cell cycle, Mitosis, Meiosis Correct The physical division of one cell into two during cell division is common to all types of cells. In all cases, proteins related to the cytoskeleton play some critical role. However, the mechanism by which division occurs depends on whether a rigid cell wall is present. In bacteria and animal cells, which do not have a rigid cell wall, division occurs by constriction of a ring of proteins (microtubule-like proteins in bacteria and microfilaments in animal cells) that pinches the cell in two. In plants, which do have a rigid cell wall, microtubules guide the aggregation of Golgi-derived vesicles to form the cell plate, which eventually forms the new cell wall and plasma membrane between the daughter cells. Chapter 12 Pre-Test Question 9 Part A Cells will usually divide if they receive the proper signal at a checkpoint in which phase of the cell cycle? Hint 1. This is the first phase after the M phase. ANSWER: G1 cytokinesis S M G2 Correct In mammalian cells, this checkpoint is termed the restriction point. Chapter 12 Pre-Test Question 5 Part A Which of the following correctly matches a phase of the cell cycle with its description? Hint 1. The S stage occurs between G1 and G2. ANSWER: S: immediately precedes cell division M: duplication of DNA G1: follows cell division G2: cell division All of the above are correctly matched. Correct After mitosis and cytokinesis, the newly formed daughter cells enter the G1 phase. Chapter 12 Pre-Test Question 7 Part A Which of the following is found in binary fission but not in mitosis? Hint 1. Consider each of these processes. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 21/24 4/14/13 ANSWER: Duplicated chromosomes attach to the plasma membrane. Replicated strands of DNA separate. The result produces 2 nuclei. Following the process, a membrane separates the 2 copies. Replication of DNA begins at an origin. Cell cycle, Mitosis, Meiosis Correct This is not true of mitosis. Chapter 12 Question 4 Part A If there are 20 centromeres in a cell at anaphase, how many chromosomes are there in each daughter cell following cytokinesis? ANSWER: 40 30 10 80 20 Correct Chapter 12 Question 9 Part A Measurements of the amount of DNA per nucleus were taken on a large number of cells from a growing fungus. The measured DNA levels ranged from 3 to 6 picograms per nucleus. In which stage of the cell cycle did the nucleus contain 6 picograms of DNA? ANSWER: G1 M G0 S G2 Correct Chapter 12 Question 14 Part A Using which of the following techniques would enable your lab group to distinguish between a cell in G2 and a cell from the same organism in G1? ANSWER: session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 22/24 4/14/13 radioactive-labeled nucleotides fluorescence microscopy labeled kinetochore proteins electron microscopy spectrophotometry Cell cycle, Mitosis, Meiosis Correct Misconception Question 56 Part A If a eukaryotic cell is in the G1 phase of the cell cycle, which statement about the cell’s chromosomes must be correct? ANSWER: The chromosomes are preparing for DNA synthesis. Each chromosome consists of two identical chromatids. Each chromosome is made of a complex of DNA and associated proteins. Correct Eukaryotic cells are made of chromatin, a complex of DNA and associated protein molecules. The proteins maintain the structure of the chromosome and help control gene activity. Misconception Question 57 Part A Which statement provides the best description of the interphase portion of the cell cycle? ANSWER: Interphase is a brief period between mitosis and chromosome duplication. During interphase, a cell is metabolically active. Interphase is a resting stage prior to cell division. Correct Interphase accounts for about 90% of the cell cycle and includes many key events critical to the cell cycle. It is not a resting stage; the cell is metabolically active during this phase. Misconception Question 59 Part A How do cancer cells differ from normal cells? ANSWER: Cultured cancer cells exhibit anchorage dependence. Cancer cells may be immortal. Cancer cells trigger chromosomal changes in surrounding cells. Correct Cancer cells can go on dividing indefinitely in culture if they are given a continual supply of nutrients; in essence, they are “immortal.” Read about HeLa cells. session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 23/24 4/14/13 Score Summary: Your score on this assignment is 81.5%. You received 13.85 out of a possible total of 17 points. Cell cycle, Mitosis, Meiosis session.masteringbiology.com/myct/assignmentPrintView?assignmentID=1937214 24/24