Get Ready for This Chapter: Chapter 12 Question 1 In this chapter, you will learn about the process by which cells divide and the implications of cell division as related to cancer. Before beginning this chapter, you should be able to describe the structure of the nucleus and the chromosomes in it (see Concept 6.3 and Figure 6.9). The following question provides a quick check of your basic knowledge in this area. Part A What is the best description of the structure of a chromosome in the nucleus of a non-dividing cell? ANSWER: One long DNA molecule, with no proteins. Two long DNA molecules wound around each other, associated with many proteins. One long DNA molecule, associated with many proteins. Two long DNA molecules wound around each other, with no proteins. Correct If you had trouble with this question, review the following material: Within the nucleus of a non-dividing cell, the DNA is organized into discrete units called chromosomes, structures that carry the genetic information. Each chromosome contains one long DNA molecule associated with many proteins. Get Ready for This Chapter: Chapter 12 Question 2 In this chapter, you will learn about the process by which cells divide and the implications of cell division as related to cancer. Before beginning this chapter, you should be able to describe the structure and function of microtubules and microfilaments (see Concept 6.6, Table 6.1, and Figures 6.21 and 6.26). The following question provides a quick check of your basic knowledge in this area. Part A Which of the following statements best describes microtubules? ANSWER: Microtubules are permanent cellular structures that help the cell to move from place to place. Microtubules are made up of subunits of actin, and function in muscle contraction and changes in cell shape. Microtubules are fibrous proteins coiled into cables that maintain cell shape and anchor the nucleus. Microtubules are made up of subunits of tubulin, and are structures along which substances are transported in the cell. Correct If you had trouble with this question, review the following material: Microtubules—hollow rods constructed from globular proteins called tubulins—shape and support the cell and serve as tracks along which organelles equipped with motor proteins can move. Microfilaments are made up of subunits of actin, and function in muscle contraction and changes in cell shape. Intermediate filaments are fibrous proteins coiled into cables that maintain cell shape and anchor the nucleus. 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: Reset replicated chromosome pair of chromosomes sister chromatid unreplicated chromosome Hint 2. The main stages of the cell cycle Help 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: 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. 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. ANSWER: Reset Help 1. DNA replication produces two identical DNA molecules, called sister chromatid(s) , which separate during mitosis. 2. After chromosomes condense, the centromere(s) is the region where the identical DNA molecules are most tightly attached to each other. 3. During mitosis, microtubules attach to chromosomes at the kinetochore(s) . chromosome(s) 4. In dividing cells, most of the cell's growth occurs during interphase . 5. The mitotic spindle(s) is a cell structure consisting of microtubules, which forms during early mitosis and plays a role in cell division. 6. During interphase, most of the nucleus is filled with a complex of DNA and protein in a dispersed form called chromatin . 7. In most eukaryotes, division of the nucleus is followed by cytokinesis , when the rest of the cell divides. 8. The centrosome(s) are the organizing centers for microtubules involved in separating chromosomes during mitosis. 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, 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 formation of the mitotic spindle separation of sister chromatids DNA replication centrosome replication ANSWER: Reset Help At this point, cell commits to go through the cycle. Non-dividing cells exit cell cycle. DNA replicates. Cell divides, forming two daughter cells. Two centrosomes have formed. Mitotic spindle begins to form. 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 during prophase, and are present until they separate in telophase. Sister chromatids first form in the S phase, and are present until they separate in early anaphase. Sister chromatids first form in the G1 phase, and are present until they separate in early anaphase. 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. ANSWER: Reset yes yes yes no no no yes yes yes yes yes yes Help yes no 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. Misconception Question 57 Part A Which statement provides the best description of the interphase portion of the cell cycle? ANSWER: During interphase, a cell is metabolically active. Interphase is a brief period between mitosis and chromosome duplication. 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. 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. Prophase. 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 (Prophase, Metaphase, Anaphase, and Telophase and Cytokinesis). Metaphase. Anaphase. Telophase and Cytokinesis. 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: Reset Centrosomes begin to move apart. Spindle begins to form. Help Spindle fibers penetrate nuclear region. Chromosomes align midway between poles of cell. Centrosomes are on opposite sides of nucleus. 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: Reset Sister chromatids separate. Nuclear envelopes begin to form. Cell elongates between poles. Chromosomes begin de-condensing. Two separate daughter cells form. ANSWER: Help Reset Help 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. 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: Reset motor protein(s) Help 1. In a stage prior to metaphase, cohesin(s) are involved in holding sister chromatids together. In a stage after metaphase, cleavage of these proteins permits the sister chromatids to separate. actin(s) 2. In a stage prior to metaphase, the mitotic spindle is assembled from individual tubulin(s) molecules. At a stage after metaphase, the mitotic spindle disassembles. 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. ANSWER: Reset Motor proteins in kinetochores move sister chromatids to metaphase plate. ANSWER: Kinetochores remain motionless while sister chromatids detach from each other. Motor proteins in kinetochores move individual chromosomes to poles of cell. Help Reset Tubulins assemble into spindle microtubules. Cohesins join sister chromatids of duplicated chromosome. Microtubules attach to kinetochores. Kinetochores are motionless in relation to poles of cell. Kinetochores move toward poles of cell. Help Spindle microtubules disassemble. Cohesins break down. 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.