Pre-lab Homework Lab 2: Mitosis and the Cell Cycle Name: Date/Lab time: 1. Label the figure with the following phases of the cell cycle (note the position of interphase and mitosis): G 1 G 2 S Anaphase Metaphase Telophase Prophase Cytokinesis 2. Briefly describe the main occurrences in the four phases of mitosis. Include what happens to the nucleus, spindle fibers and chromosomes. Prophase: Metaphase: Anaphase: Telophase: 3. On the back of this sheet, briefly describe how we will estimate the length of time a growing onion root tip cell spends in each of the phases of the cell cycle (hint: read lab exercise 3). 1
2
Name: Date/Lab time: Lab 2: Mitosis and the Cell Cycle LAB SYNOPSIS: You will model the process of the cell cycle using pop-beads. You will identify the phase of the cell cycle in micrographs of plant and animal cells. You will estimate the amount of time cells spend in each of the phases of the cell cycle. OBJECTIVES: After successfully completing this lab, a student will be able to: Describe the major events of the cell cycle. Illustrate the process of nuclear division with models, words, and diagrams. Recognize cells in the various phases of the cell cycle given micrographs or descriptions. Overview: The cell cycle is the process in which one parent cell becomes two new daughter cells. The resulting two cells are genetically identical to each other and genetically identical to the parent cell. Most of your contain 46 chromosomes, 23 inherited from your mom, 23 inherited from your dad. The goal of the cell cycle is to make exact copies of each of these chromosomes. Exercise 1: Modeling the Cell Cycle with Pop Beads To understand the mechanism of the cell cycle, we are going to use pop beads to model the each of the phases. Procedure: Constructing Chromosomes Using Pop Beads. 1. Using figure 1 as a guide, construct chromosomes for a pretend cell containing 4 chromosomes (2 red chromosomes originally inherited from its mother and 2 yellow chromosomes from its father). This is a diploid cell. Diploid cell (2n)- A cell containing 2 sets of chromosomes. One set from mom, one set from dad. Diploid cells contain homologous pairs of chromosomes. Homologous chromosomes- are similar in size, the order of genes and centromere position (represented in fig. 1 by the white magnetic links). Homologous chromosomes are not however identical because one was inherited from mom and the other from dad. Figure 1. Pop bead chromosomes Sets and Pairs For figure 1 understand; Which set of chromosomes were inherited from the father? Which set of chromosomes were inherited from the mother? Which chromosomes are homologous pairs? Confirm your understanding of this with your instructor before continuing. 3
The goal of this exercise is to go through the phases of the cell cycle to make two new cells that are an exact copy of the cell in figure 1. The Cell Cycle- The sequence of phases that occurs as a cell divides (one parent cell becomes two genetically identical daughter cells). Fig 2. The phases of the cell cycle were first identified by how they appear under the light microscope. Three major events are noted under the microscope; interphase, mitosis & cytokinesis. Figure 2. The Cell Cycle (one cell becomes two) Interphase- Phase of the cell cycle when the nucleus of the cell does not appear to be doing much. Interphase is divided into 3 main parts; Gap 1, Synthesis Phase & Gap 2. For this activity we will focus on what happens to the chromosomes during the Synthesis Phase. See your textbook and lecture for details of the Gap 1 and Gap 2 phases. Gap 1 (G 1 )- Phase in which the cell grows and functions normally. If the cell does not divide, it enters what is called the G 0 phase, otherwise it will continue into the synthesis phase. Synthesis Phase (S-Phase)- Phase in which the cell duplicates its DNA. During the S-phase of interphase each chromosome goes through the process of DNA replication. Following DNA replication, each chromosome is composed of 2 identical sister chromatids attached at the centromere. Procedure cont. 2. Although in real cells you cannot see individual chromosomes, to demonstrate the results of the S-phase, construct exact copies of each of your chromosomes and link them via their magnets. Note: Following the S-phase, you still have 4 chromosomes, but each chromosome is made up of two sister chromatids. Fig. 3. Figure 3. After DNA Replication Label each of the following in figure 3. Chromosome, Sister Chromatid & Centromere Gap 2 (G 2 )- Phase in which the cell resumes its growth in preparation for mitosis. 4
Mitosis- Phase of the cell cycle that separates the sister chromatids (nuclear division). We will focus on what is happening to the chromosomes. See your textbook for additional details. Mitosis is separated into 4 parts; prophase, metaphase, anaphase and telophase. Prophase- chromosomes become visible under the light microscope. Prior to prophase, chromosomes are not visible. During prophase the chromosome s DNA begins to super-coil or condense into visible chromosomes. Also during prophase the nucleolus disappears and the nuclear envelope breaks down. Microtubules extend from centrioles at either pole of the cell forming the spindle apparatus. 3. Use figure 4 to guide you through the process. Fig. 4A represents prophase. Your moving of the chromosomes with your hands is equivalent to the work the spindle microtubules are doing. Metaphase- chromosomes line up in the middle of the cell (along the metaphase plate). 4. Line chromosomes up single-file so sister chromatids are facing either side of the cell (Fig. 4B). Anaphase- sister chromatids begin to separate and move to either side of the cell. 5. Separate sister chromatids and pull them to either side of the cell (Fig. 4C). Telophase- sister chromatids, now considered individual chromosomes, are pulled all the way to ether side of the cell. The nuclear envelope reforms around the chromosomes, the chromosomes de-condense and the nucleolus reforms. 6. Pull chromosomes further away from one another (Fig. 4D). ----------------------------------------------------------- Cytokinesis- This is the process of cell division, dividing one cell into two new daughter cells. 7. Pull chromosomes yet further away from each other. Note: each new cell should have identical chromosomes to each other and to the original parent cell (Fig. 4E). Figure 4. Phase of the Cell Cycle Practice the phases of the cell cycle again without the aid of the figures. If this were a human skin cell; How many total chromosomes would there be prior to the cell cycle? How many sets of chromosomes would there be? How many chromosomes would there be in each set? 5
Exercise 2: Identifying Cells in Mitosis The phases of the cell cycle (interphase, mitotic phases and cytokinesis) can be identified from micrographs (photos of microscope images). Use the provided micrographs (onion root tip and/or white fish blastula cells) and identify the phases of the cell cycle. In these micrographs, plant cells tend to be square, while animal cells tend to be circular. Procedure: 1. Nucleus: The nucleus is often the most obvious structure within a cell. Identify the nucleus in the provided micrographs. Changes to the structure of the nucleus will help you distinguish cells undergoing mitosis from those still in interphase. 2. Interphase: Find a cell that has a nucleus with no visible chromosomes (visible chromosomes would appear as thick strands, almost wormlike). Not being able to see the chromosomes is characteristic of a cell s nucleus in interphase. Sketch a cell in the interphase box on the following table. Label any visible structures, and list the main events happening at this phase. 3. Prophase: Find a cell whose nucleus appears to be in prophase. Review the phases from your modeling activity above with the pop beads. Sketch a cell in prophase, label any structures you can see, and list the main things that are happening in this phase. 4. Continue on finding cells in metaphase, anaphase, and telophase. Sketch each cell, label your sketch, and list what is happening during each phase. Labeled sketch of cell Interphase Main events of phase Prophase Metaphase 6
Anaphase Telophase Exercise 3: Estimating the Time Spent in the Phases of Mitosis Cells in your body reproduce at different rates. Skin cells reproduce frequently (about once per day); liver cells reproduce rarely (about once per year); after initially formed nerve and muscle cells almost never reproduce. The whole process of mitosis, prophase to telophase, takes ~ 90 min. The following exercise will allow you to estimate how long each phase lasts. Procedure: 1. Each team of two will need one of the provided onion root tip micrographs. This area of the root is undergoing rapid cell reproduction. 2. Identify the phases of the cell cycle for 25 randomly chosen cells. Record this information in table 1 below. It is important that you choose cells randomly so as not to bias your data. 3. Once you have identified 25 cells, trade jobs with your lab partner. Record total data in Table 1. Table 1: Number of cells in each phase of the cell cycle counted by self and lab partner. Phase Interphase Prophase Metaphase Anaphase Telophase Total Your 25 Partners 25 TOTALS 4. After identifying a total of 50 cells, trade data with 3 other groups so that you have a total of 200 cells identified. Record this data in Table 2. 7
Table 2: Number of cells in each phase of the cell cycle counted by four lab pairs. Phase Interphase Prophase Metaphase Anaphase Telophase Total Your Total Other #1 Other #2 Other #3 TOTALS 5. Now you are ready to estimate the amount of time these cells spend in each different phase. In an onion root tip, the entire cell cycle takes about 12 hours (depending on the plant s health and where you are looking in the root). We will use 12 hours (720 minutes) as a rough estimate. 6. Calculate the percentage of time spent in each phase by counting the total number of cells in each phase (total in interphase, in prophase and so on) and dividing each by the total number of cells you counted (this should be 200 if you have compared with 3 other groups). Record this data in Table 3. 7. Multiply the percentage of time in each phase by the total time of the cell cycle (720 minutes) and this gives you an estimate of the time spent in each phase. Record this data in Table 3. Table 3: Estimate of time spent in each phase of the cell cycle. Phase Interphase Prophase Metaphase Anaphase Telophase Total % of cells in each phase Time estimate 100% 720 minutes 8. Most of the cells in your body are not dividing as quickly as onion root tips; however, the amount of time spent in the various phases of mitosis is about the same as you have calculated. 8