Mitosis and Meiosis: Chromosome Simulation Carolina Distance Learning Investigation Manual

Transcription

1 Mitosis and Meiosis: Chromosome Simulation Carolina Distance Learning Investigation Manual World-Class Support for Science & Math

2 Table of Contents OVERVIEW... 3 OBJECTIVES... 3 TIME REQUIREMENTS... 3 BACKGROUND... 4 MATERIALS... 5 SAFETY... 5 PREPARATION... 5 ACTIVITY 1: MITOSIS ACTIVITY... 6 ACTIVITY 2: MEIOSIS ACTIVITY ACTIVITY 3: CHROMOSOME MISREGULATION

3 OVERVIEW Cellular events are modeled in this chromosome-simulation activity. Popbeads are used to visualize and understand the organization and segregation of chromosomes during cell division. The regulated steps of mitosis, meiosis, and crossing-over are explored in the first part of the series, and then abnormalities, such as nondisjunction and aneuploidy, are examined. OBJECTIVES Model mitosis and meiosis. List the steps of mitosis and meiosis. Identify chromosome abnormalities. Describe how genetic diversity is increased through the process of crossing-over. TIME REQUIREMENTS Preparation Meiosis Activity Mitosis Activity Chromosome Misregulation Activity 10 minutes 10 minutes 10 minutes 20 minutes The activities are designed to be conducted in order. Time for answering the Prelaboratory Questions is not included in the time estimates listed above. 3

4 BACKGROUND There are two leading theories about how cellular and genetic abnormalities lead to the development of cancer. One involves gene mutations and the other involves chromosome misregulation. Understanding chromosome structure and regulation allows researchers to investigate disorders related to cell division by mitosis and meiosis. Mitosis and meiosis are the means by which genetic information, i.e., DNA, which is packaged in threadlike structures called chromosomes, is passed from one generation of cells to the next. In mitosis, the nucleus of a diploid cell (containing replicated chromosomes) divides. The result of mitosis is two cells that are genetically identical, with the same (diploid) number of chromosomes as the parent cell. In meiosis, the nucleus of a diploid cell, containing a complete set of chromosomes, divides twice. Genetic information is exchanged between homologous chromosomes. The result of meiosis is four genetically diverse haploid cells, called gametes, each possessing half the number of chromosomes as those in the parent cell. In terms of the life of a cell, reproduction is a crucial, but relatively brief, part of an ongoing process known as the cell cycle. The longer period between cell divisions is called interphase. During interphase, the cell grows, replicates its chromosomes, and produces and assembles the cellular structures needed for cell division. Figure 1. The somatic cell cycle Mitosis is the division of the nuclear material within a somatic cell (any cell not designed for sexual reproduction) by which one parent cell divides creating two genetically identical daughter cells. This form of cellular reproduction ensures that the daughter cells produced by the parent cell contain genetic material identical to that of the parent cell. The names of the stages of meiosis and some of the processes involved are similar to those in mitosis, but meiosis has a dramatically different purpose. Meiosis occurs in gametes, cells designed for sexual reproduction, and contains half the number of chromosomes as that of the original cell. 4

5 MATERIALS Included in the materials kit: Red popbeads 42 Yellow popbeads 42 Magnetic centromeres 4 Plastic centriole beads 4 Embroidery thread 1 Needed from the equipment kit: Wax pencil or marker Needed, but not supplied: Transparent adhesive tape Masking tape Meter stick, yard stick, or ruler Scissors Digital camera or cell phone camera Reorder Information: A replacement kit for Mitosis and Meiosis: Chromosome Simulation, item number , can be ordered from Carolina Biological Supply Company. SAFETY Read all instructions for this laboratory activity before beginning. Follow the instructions closely and observe established laboratory safety practices, including use of appropriate personal protective equipment (PPE) described in the Safety and Procedure section. This kit contains small plastic parts that could be a choking hazard. Keep away from children. PREPARATION 1. Clear sufficient space on a desk, counter, or floor so that there is a flat surface measuring approximately 1m x 1m to conduct the modeling. 2. Read the Student Guide thoroughly and become familiar with the kit materials and activities before beginning. 3. Gather all additional supplies that are needed but not supplied. 5

6 ACTIVITY 1: MITOSIS ACTIVITY Mitosis is a continual process in which the phases progress from one to the next gradually. However, for the purposes of this discussion and to facilitate modeling, events in the process of mitosis are designated as separate phases. During this activity, lab materials will be manipulated in order to visualize the events that occur during each phase. For each step of the Procedure, read the description of the events in the phase, and then follow the directions to model the step using the model materials provided. Prepare photos of each stage of the mitotic cycle for use in answering the Laboratory Questions and for future review. PROCEDURE 1. BUILDING TWO CHROMOSOMES (a homologous pair) a. Make four strands containing 10 yellow beads each, and four strands containing 10 red beads each. b. Connect two red strands to one magnetic centromere to create a 20-bead strand with the centromere in the center. Each of these two strands is referred to as a chromosome arm. c. Repeat Step 1.b. with the remaining strands of red beads. d. Connect two yellow strands to one magnetic centromere to create a 20-bead strand with the centromere in the center. e. Repeat Step 1.d. with the remaining strands of yellow beads. f. Place the two long red strands together, and allow them to join at the magnetic centromeres. This entire combined structure represents one chromosome. The materials representing chromatids in the activities described herein are only attracted to each other at the centromere, but in the cell, chromatids are associated throughout their length. g. Allow the two long yellow strands to come together and join them at the magnetic centromeres. This entire structure represents another chromosome, in this case, the homolog of the red chromosome. Together, the models represent a homologous pair of chromosomes. 6

7 2. BUIILD A NUCLEAR MEMBRANE a. Acquire six lengths of masking tape, each approximately 30cm (1 ft.) long. b. Use the pieces of tape to create a closed circle on the surface of the desk or on the floor. The tape ring represents the nuclear membrane of the cell. The entire workspace represents the area inside the cell. c. Place the two chromosomes inside the nuclear membrane. 3. INTERPHASE: Interphase occurs before mitosis as a preparation for nuclear division. It is the longest stage of the cell cycle. During this time, DNA is replicated and the cell rapidly grows and produces organelles. The nuclear membrane and nucleolus are distinct. The nucleolus is made of nucleic acids and proteins. a. Position two plastic centriole beads outside the nuclear membrane ring. Remember that each bead represents a pair of centrioles. b. Position the two chromosomes in a pile inside the nuclear membrane. c. Create a label using the tape and a wax pencil or marker for the stage in the mitotic cycle that reads Interphase (Before Mitosis) and position the label next to the nuclear membrane. d. Take a photo of the model, including the label, with a digital camera or smartphone. This image will be used later for answering the Laboratory Questions, and the photo can also be used as a reminder for future test review. A cell viewed under a microscope during interphase would not have distinguishable chromosomes because they are so long and thin. Chromosomes take on their characteristic X shape only when they are condensed. 7

8 4. PROPHASE: Prophase is the first phase of mitosis. During this phase, replicated DNA condenses into chromosomes that consist of two identical sister chromatids bound by a centromere. The nuclear membrane begins to disappear as it breaks down. Centrioles are located on opposite sides of the cell. In later phases, microtubules radiating from the centrioles will attach to each side of the centromere and pull the sister chromatids apart. a. Move the centrioles so that they are on opposite sides of the nucleus, approximately 20 cm from the outside edge of the nuclear membrane. b. Use a piece of transparent tape to secure each centriole bead to the work surface so that the opening of the bead faces the area that was the nucleus. c. Break down the nuclear membrane by pulling pieces of masking tape from the work surface. If possible, keep the masking tape in strips on the side of the workspace to reuse later as new nuclear membrane. d. Arrange the chromosomes so that the arms are outstretched to demonstrate that they are condensed. Then, twist the arms of the sister chromatids so they are laying over one another. e. Create a label for the stage in the mitotic cycle that reads Mitosis Prophase, and take a photo of the labeled model. Figure 2. Metaphase of mitosis 8

9 5. Metaphase: Metaphase is the shortest phase of mitosis. During this phase, the nuclear membrane has completely disappeared. Microtubules radiating from opposite sides of the cell have attached to the centromeres of the chromosomes. Chromosomes line up along the equator of the cell. a. Cut four lengths of thread, each approximately 1 meter long. b. Using the four pieces of thread, tie a loop around each magnetic centromere. When finished, each chromatid should have a length of thread tied to it. c. The threads represent microtubules that function to move the chromosomes around in the cell. Pass the loose end of each thread through the hole of a centriole bead, such that if the threads were pulled through the centriole beads, the two parts (chromatids) of the current chromosome would separate. (See Figure 2.) d. Gently pull the threads from one chromosome through the centrioles so that the chromosome moves to the center, but the chromatids remain attached. e. Pull the threads from the second chromosome to move it to the center also. e. Create a label for this stage in the mitotic cycle that reads Mitosis Metaphase, and take a photo of the labeled model. The two chromosomes should be roughly aligned between the centrioles. This action represents the chromosomes lining up along the metaphase plate. 6. Anaphase: During Anaphase, the microtubules pull sister chromatids toward opposite sides of the cell. Each chromatid is now properly referred to as a chromosome. The cell now has twice the original number of chromosomes. This segregation results in two regions of compact chromosomes that are genetically identical. a. Pull the threads through the centriole beads until the chromatids separate. b. Continue to separate the chromatids until they are adjacent to the centrioles. Each side of the cell should have one red and one yellow chromosome. c. Create a label for this stage in the mitotic cycle that reads Mitosis Anaphase, and take a photo of the labeled model. 9

10 7. Telophase: A visible nuclear membrane begins to form around each set of chromosomes. The chromosomes decondense. The nucleoli form in each nucleus, and the microtubules break down. a. Remove the thread from each chromosome, and set aside the threads. b. To represent decondensing, pile the chromosomes beside their closest centriole. c. Use the masking tape previously set aside to create a new nuclear membrane around each set of chromosomes (or obtain more tape if necessary). This concludes the modeling of mitosis. d. Create a label for this stage in the mitotic cycle that reads Mitosis Telophase, and take a photo of the labeled model. e. Save the pieces of thread and the chromosome structures for the next activity. Throughout the cell cycle, the centrioles are paired; the double centriole replicates as part of telophase. However, in these activities, each double centriole is represented by a single bead. Cytokinesis is the division of the cytoplasm and its contents into two daughter cells. This stage of the cell cycle may occur at the same time as telophase of mitosis. Cytokinesis in plant and animal cells occurs by different mechanisms. Plant cells develop a cell plate along the equatorial plane outward to the cell wall to divide the cell; new cell walls form along the cell plate. In animal cells, microfilaments form around the cell and contract, forming an indentation called a cleavage furrow. Eventually the parent cell is pinched into two daughter cells. 10

11 ACTIVITY 2: MEIOSIS ACTIVITY Meiosis occurs in gamete cells (sex cells) and involves two sequential divisions, called meiosis I and meiosis II. Each nucleus resulting from meiotic division is haploid, meaning that it contains half the number of chromosomes as the parent. Like mitosis, meiosis is a continual process in which the phases progress smoothly from one to the next, with no distinct point separating them. For the purposes of this discussion and to facilitate modeling, the events in the process of meiosis are designated as separate phases. During this activity, modeling materials will be manipulated in order to visualize the events that occur during each phase. For each step of the Procedure, read the description of the events in the phase, and then follow the directions to model the step using the model materials provided. Prepare photos of each stage of meiosis for use in answering the Laboratory Questions and for future review. PROCEDURE 1. BUILD A NUCLEAR MEMBRANE a. Use the saved tape to create a closed circle or hexagon shape on the surface of the desk or on a clear space on the floor. If necessary, obtain additional masking tape. b. Allow the two red chromatids (from the previous activity) to come together at the magnetic centromeres. Repeat with the two yellow chromatids. c. Place the red and yellow chromosomes inside the nuclear membrane. The tape ring represents the nuclear membrane of the cell. The entire workspace represents the area inside the cell 2. INTERPHASE: Interphase, the longest stage of the process, occurs before meiosis, as a preparation for nuclear division. During this time, DNA is replicated, and the cell rapidly grows and produces organelles. The nuclear membrane and nucleolus are distinct. The nucleolus consists of nucleic acids and proteins. a. Position two plastic centrioles outside the nuclear membrane ring. b. Position the chromosomes in a pile inside the nuclear membrane. c. Create a label for the stage in the meiotic cycle that reads Interphase (Before Meiosis) and position the label next to the nuclear membrane. d. Take a photo of the labeled model with a digital camera or smartphone. This image will be used later for answering the Laboratory Questions and can also be used for future test review. Individual chromosomes would not be visible during interphase in a cell viewed under a microscope because the chromosomes are so long and thin. Only when condensed are the chromosomes visible under the microscope, assuming their characteristic X shape. 11

12 3. PROPHASE I: The chromosomes condense and the nuclear membrane breaks down and disappears. Microtubules attach to the chromosomes, pulling homologs together. The homologs can exchange genetic information by way of crossing-over. Crossing-over occurs when sections of DNA are traded between non-sister chromatids of a homologous pair. This allows for greater genetic variation between daughter cells. a. Move the two centrioles so that they are on opposite sides of the nucleus, approximately 5 cm inside the nuclear membrane. b. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of the bead faces the area that was the nucleus. c. Break down the nuclear membrane by pulling pieces of tape from the work surface. If possible, keep the tape in strips on the side of the workspace to reuse later as a new nuclear membrane. d. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. Then, twist the arms of sister chromatids together. e. Bring the homologous chromosomes together while the arms of each chromosome are still twisted. The red chromosome (the two red chromatids) will be connected to the yellow chromosome (the two yellow chromatids) at the centromeres. f. To simulate crossing-over, detach a few beads from one homologous chromosome and exchange them in the corresponding location with its homologous pair. The traded sections must be an equal number of beads. g. Create a label for the stage in the meiotic cycle that reads Meiosis Prophase I and take a photo of the labeled model. 4. METAPHASE I: The nuclear membrane has completely disappeared. Microtubules radiating from opposite sides of the cell line up homologous pairs along the equator of the cell. The way that the homologs of a pair line up along the equator is random. The importance of this random assortment is that it generates greater genetic variation between daughter cells. For simplicity, model only one homologous pair of chromosomes. Chromosome number varies by species. a. Using two pieces of thread, tie a loop around each pair of magnetic centromeres. When finished, each chromosome (consisting of two chromatids) should have a length of thread tied to it. b. Pass one loose end of the thread through one of the centrioles, and pass the other loose end through the other centriole. c. Position the chromosomes near the midpoint between the centrioles and at a right angle to the threads, as shown in Figure 3. d. Create a label for the stage in the meiotic cycle and take a photo of the labeled model. 12

13 Figure 3: Metaphase I of Meiosis Figure 3 does not reflect the crossing over step and therefore will differ slightly from the models. 5. ANAPHASE I: Homologous chromosomes are pulled toward opposite sides of the cell by microtubules. Note that sister chromatids are still held together by a centromere, which is not the case in anaphase of mitosis. a. Gently pull the ends of the threads through the centrioles so that the homologous chromosomes separate and move toward opposite sides of the cell and adjacent to a centriole. b. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. 6. TELOPHASE I: A visible nuclear membrane begins to form around each set of chromosomes. The chromosomes decondense, and the microtubules break down. These processes may vary somewhat by species. a. Remove the thread from each chromosome and set aside the threads. Save the pieces of thread for the next activity. b. To represent decondensing, pile the chromosomes beside their closest centriole. c. Use the masking tape previously set aside, along with additional masking tape, to create a new nuclear membrane around each set of chromosomes. Each new nuclear membrane should be about the same size as the original. This concludes the modeling of meiosis I. d. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. e. Cytokinesis will occur at the same time as telophase. Each of the two cells has a haploid set of chromosomes, but these are still sister chromatids. 13

14 7. PROPHASE II: Meiosis II begins after a brief interphase. DNA was not replicated, but the centrioles do duplicate before prophase II. The chromosomes of each cell condense, and the nuclear membranes break down. Note that two cells are now being modeled. Model the same event in each cell, side-by-side. a. Add two more centrioles. b. Move each pair of centrioles so that they are on opposite sides of the nucleus, approximately 20 cm from the outer edge of the nuclear membrane. c. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of the bead faces the area that was the nucleus of the cell. d. Break down the nuclear membranes by pulling pieces of tape from the work surface. If possible, keep the masking tape in strips on the side of the workspace to reuse later as a new nuclear membrane. e. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. Then, twist the arms of sister chromatids together. f. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. The goal of meiosis is to create a haploid gamete cell; therefore there is a very brief interphase II separating meiosis I and meiosis II. This stage exists to only to reform structures dissolved in telophase I and not to replicate DNA. 8. METAPHASE II: The chromosomes line up along the equator of each cell. Model the same event in both cells, side-by-side. a. Using four pieces of thread, tie a loop around each magnetic centromere. When finished, each chromatid should have a length of thread tied to it. b. Pass one of the loose ends of the thread through one of the centrioles, and pass the other loose end through the remaining centriole. c. Position the chromosome near the midpoint between the centrioles and at a right angle to the threads, as shown in Figure 4. d. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. 14

15 Figure 4. Anaphase II of meiosis Figure 4 does not reflect the crossing over step and therefore will differ slightly from the models. 9. ANAPHASE II: Microtubules pull sister chromatids toward opposite sides of the cell. Each chromatid is now properly referred to as a chromosome. a. Gently pull the ends of the threads through the centrioles so that the sister chromatids separate and move toward opposite sides of the cell. b. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. 10. TELOPHASE II: A visible nuclear membrane begins to form around each set of chromosomes. The chromosomes decondense and the microtubules break down. a. Remove the thread from each chromosome and set it aside. b. To represent decondensing, pile the chromosomes beside their closest centriole. c. Use the masking tape previously set aside, along with additional masking tape, to create a new nuclear membrane around each of the four nuclei. This concludes the modeling of meiosis. d. Create a label for this stage in the meiotic cycle, and take a photo of the labeled model. e. Cytokinesis occurs during Telophase II separating the two cells into four haploid cells, but those events are not portrayed in the model. 11. Save the chromosome structures for the next activity. Undo any bead changes that were made to the chromosomes in the crossing-over step. Each structure should be entirely red or entirely yellow. 15

16 ACTIVITY 3: CHROMOSOME MISREGULATION Normally, mechanisms are in place to regulate the distribution of chromosomes in new cells as they divide. However, in such a complex process, errors can and do occur. Abnormalities in chromosomes number and structure result in a number of genetic disorders, and some current theories link chromosomal abnormalities to cancer. Each somatic cell has 46 chromosomes in humans. The improper separation of chromosomes during meiosis, called nondisjunction, results in gametes that possess an unusual number of chromosomes. These cells are called aneuploid. Ploidy refers to the number of chromosomes in a cell. Down syndrome, for example, is a result of possessing an extra chromosome 21. Down syndrome is also called Trisomy 21, indicating that there are three copies of chromosome 21. In this activity, two mechanisms of nondisjunction during meiosis are modeled. Then, the progression of events when an aneuploid gamete fuses with another gamete in fertilization preparing to begin mitosis is observed. Nondisjunction of Homologous Chromosomes During Meiosis I Procedure Using the materials from the Meiosis Activity, build two chromosomes, one of each color. Make sure that each chromosome is made of two sister chromatids. Each sister chromatid should be made of 20 beads with a magnetic centromere in the center. 1. Build a nuclear membrane out of masking tape, and place the two chromosomes inside the nuclear membrane. 2. Interphase (normal): a. Position two plastic centrioles outside the nuclear membrane ring. b. Place the chromosomes in a pile inside the nuclear membrane. 3. PROPHASE I (normal): a. Move the centrioles so that they are on opposite sides of the nucleus, approximately 20 cm from the outer edge of the nuclear membrane. b. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of the bead faces the area that was the nucleus. c. Break down the nuclear membrane by pulling pieces of tape from the work surface. If possible, keep the tape in strips on the side of the workspace to reuse later as a new nuclear membrane. d. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. Then, twist the arms of sister chromatids together. e. Bring the homologous chromosomes together while the arms of each chromosome are still twisted. Normally, crossing-over would occur at this point, but for this modeling activity, crossing-over will be skipped to make it is easier to track each chromosome. 16

17 4. METAPHASE I (misregulation occurs): a. Using a piece of thread, tie one loop around both pairs of magnetic centromeres. b. Pass the loose end of the thread through one centriole. c. Position the chromosomes near the midpoint between the centrioles and at a right angle to the thread. 5. ANAPHASE I (misregulation continues): Gently pull the ends of the thread through the centriole. The microtubules (thread) from one side did not successfully attach to one of the pairs, or the centromeres did not detach from the homologous pairs. The cause is not entirely clear, but the result is that both homologous pairs end up on the same side of the cell. 6. TELOPHASE I (misregulation continues): a. Remove the thread from the chromosomes and set it aside. b. To represent decondensing, pile the chromosomes beside their closest centriole. c. Cytokinesis will occur as the same time as telophase. Due to the misregulation, one cell has an extra chromosome, and one cell lacks a chromosome. Remember that this model only describes the behavior of one homologous chromosome pair. Imagine there are other homologous chromosomes that were successfully separated. Use the masking tape previously set aside, along with additional masking tape, to create a new nuclear membrane around the area where a new nuclear membrane would form. Because of the misregulation, one membrane will have both homologous chromosomes (that is, 4 chromatids) and the other membrane will have no chromosomes at all. In a typical cell there are other chromosomes not represented by this model; therefore, think of the empty membrane as missing a chromosome, rather than lacking chromosomes completely. 17

18 7. PROPHASE II (misregulation continues): a. Add two more centrioles. b. Move each centriole so that they are on opposite sides of the nucleus, approximately 20 cm from the outer edge of the nuclear membrane. c. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of the bead faces the area that was the nucleus of the cell. d. Break down the nuclear membranes by pulling pieces of tape from the work surface. e. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. 8. METAPHASE II (misregulation continues): The chromosomes line up along the equator of each cell. The homologous pair should be separated. a. Using four pieces of thread, tie a loop around each magnetic centromere. When finished, each chromatid should have a long thread tied to it. b. For each chromosome, thread one loose end of the thread through one of the centrioles and the other loose end through the opposite centriole. c. Gently pull the threads to position the chromosomes near the midpoint between the centrioles and at a right angle to the threads. Sister chromatids are still attached. 9. ANAPHASE II (misregulation continues): Microtubules pull sister chromatids toward opposite sides of the cell. Each chromatid is now properly referred to as a chromosome. Gently pull the ends of the threads through the centrioles so that the sister chromatids separate and move toward opposite sides of the cell until they are adjacent to the centrioles. 18

19 11. TELOPHASE II (misregulation continues): A visible nuclear membrane begins to form around each set of chromosomes. The chromosomes decondense, and the microtubules break down. a. Remove the thread from each chromosome and set it aside. b. To represent decondensing, pile the chromosomes beside their closest centriole. a. Use the masking tape previously set aside, along with additional masking tape, to create a new nuclear membrane around each of the four nuclei. Two of the nuclei will have no chromosomes, and the other two will have two chromosomes each one red and one yellow. Another way of describing this is that two cells have n + 1 chromosomes, and two have n Nondisjunction of Sister Chromatids During Meiosis II Procedure a. Using the materials from the Meiosis Activity, build two chromosomes. Each chromatid should be made of 20 beads of the same color with a magnetic centromere in the center. b. Build a nuclear membrane out of masking tape, and place the two chromosomes inside the nuclear membrane. 13. INTERPHASE (normal): a. Position plastic centrioles, one on each side 20 cm outside the nuclear membrane ring. b. Place the chromosomes in a pile inside the nuclear membrane. 14. PROPHASE I (normal): a. Move the centrioles so that they are on opposite sides of the nucleus, approximately 20 cm from the edge of the nuclear membrane. b. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of one of the beads of each pair faces the area that was the nucleus. c. Break down the nuclear membrane by pulling pieces of tape from the work surface. If possible, keep the tape in strips on the side of the workspace to reuse it later as a new nuclear membrane. d. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. Then, twist the arms of sister chromatids together. e. Bring the homologous chromosomes together while the arms of each chromosome are still twisted. Normally, crossing-over would occur at this point, but is omitted for this modeling activity, so it is easier to keep track of each chromosome. 19

20 15. METAPHASE I (normal): a. Using a piece of thread, tie a loop around each pair of magnetic centromeres. The red chromatids are together and the yellow chromatids are together. b. Pass the loose ends of the thread through the centrioles. The thread from the red chromosome will be passed through one centriole and the thread from the yellow chromosome will be passed through the opposite centriole. c. Position the chromosomes near the midpoint between the centrioles and at a right angle to the thread. 16. ANAPHASE I (normal): Gently pull the ends of the thread through the centrioles so that the homologous pairs separate and move toward opposite sides of the cell. 17. Telophase I (normal): a. Remove the thread from the chromosomes and set it aside. b. To represent decondensing, pile the chromosomes beside their closest centriole c. Cytokinesis will occur as the same time as telophase. The model now represents two cells, each possessing one chromosome. Remember that this model only describes the behavior of one homologous pair of chromosomes. Use the masking tape set aside, along with additional masking tape, to create a new nuclear membrane in the area where a new nuclear membrane normally would form. 18. PROPHASE II (normal): a. Add two more centrioles. b. Move each centriole so that they are on opposite sides of the nucleus, approximately 20 cm from the outside edge of the nuclear membrane. c. Use a piece of transparent tape to secure each centriole to the work surface so that the opening of the bead faces the area that was the nucleus of the cell. d. Break down the nuclear membranes by pulling pieces of tape from the work surface. e. Arrange the chromosomes with arms outstretched to demonstrate that they are condensed. 20

21 19. METAPHASE II (misregulation occurs): a. Using four pieces of thread, tie a loop around each magnetic centromere. When finished, each chromatid should have a length of thread tied to it. b. For one cell, pass the loose end of a thread through one of the centrioles. Thread the other loose end through the opposite centriole. Demonstrate misregulation in the second cell by only connecting one thread to a centriole. Leave the other thread unattached. c. Position the chromosomes near the midpoint between the centrioles and at a right angle to the threads. 20. ANAPHASE II (misregulation continues): For one cell, gently pull the ends of the threads through the centrioles so that the sister chromatids separate and move toward opposite sides of the cell. Demonstrate misregulation in the second cell by only pulling the thread attached to the centriole, leaving the sister chromatids attached. 21. Telophase II (misregulation continues): a. Remove the thread from each chromosome and set it aside. b. To represent decondensing, pile the chromosomes beside their closest centriole. c. Use the masking tape previously set aside, along with additional masking tape, to create a new nuclear membrane around each of the four nuclei. Two of the nuclei will have one chromosome each (n). One will have chromosomes two (n + 1), and one will have no chromosomes (n 1). 21