The Cell Cycle. Materials 2 pipe cleaners of one color and 2 of another color Drawing paper

Transcription

1 The Cell Cycle Introduction When the cell has reached its growth potential it will begin to divide. Additionally, if a cell has become damaged or worn out it can be replaced by surrounding cells through cell division. In humans, repeated cell division allows a single fertilized egg to divide repeatedly and develop into an adult with more than 37 trillion cells. This division is referred to as the cell cycle. In plant and animal cells, this cycle is very similar but is not identical. The amount of time a cell spends in each stage of the cell cycle can be estimated by observing and counting the numbers of cells in each phase of the cycle. Consider that a slide of cells represents a split second of the cells life frozen in time. If a phase lasts a long time, there should be many cells in this stage. If a phase is very short, there should be very few cells in this stage. Now consider a cell that has uncontrollable cell growth and cell division. Check points in the cell cycle are not functioning properly and the cell continues to divide even though overcrowding has occurred. The cells of a tumor have become mutated and lack the ability to stop dividing. These cancerous cells spend more time dividing than do healthy cells, potentially developing a tumor or tumors. Essential Questions How much time is spent in each phase of the cell cycle and why is the amount of time relevant to the events of the phase? What happens if the cell cycle is not regulated? Materials 2 pipe cleaners of one color and 2 of another color Drawing paper Pencil Microscope Cell Cycle cards Onion root tip slide Colored pencils Calculators Normal and Cancerous Tissue Handout Magnifying Glasses Access to Pre-Lab Building chromosomes with pipe cleaners and simulating mitosis Part Obtain 4 pipe cleaners (two of 2 different colors) and a blank sheet of paper. 2. Using a pencil, draw a circle that takes up most of the space on your paper. This circle represents the cell membrane. Draw a second circle about the size of a grapefruit in the center of the cell membrane. This represents the nuclear envelope. 3. Using the description of each stage of the cell cycle listed below, build a model of that phase and sketch your model on Part 1 of the student data sheet. Answer questions 1-5 after completing sketches. Interphase Page 1 of 8

2 Events - The cell increases in size, carries on metabolism, and the chromosomes duplicate. These events are defined as G1, S and G2. First Gap Phase, G1: The cell increases in size and prepares for replication. The G1 phase checkpoint allows the cell to die, to rest or move on to synthesis determined by the state of the DNA and resources. If DNA is damaged or there are insufficient resources the cell will not move to the S phase. The Synthesis Phase, S: DNA is replicated and the cell will have two complete sets of DNA. The S phase checkpoint verifies the DNA does not contain errors. Second Gap Phase, G2: The cell continues to grow and prepare for division. The G2 phase checkpoint verifies that the DNA is not damaged, chromosome sets are complete, and enough cell components are available to support two cells. Appearance - The cell will contain a smooth round nucleus. There is a smaller nucleolus inside the nucleus. The duplicated chromosomes (joined at the centromere) have not condensed and cannot be seen. Prophase (Mitosis Begins) the cell stops growing and division begins Events - Nuclear division begins. The nuclear membrane disintegrates. The chromosomes thicken and the mitotic apparatus (centrioles-only in animal cells-and spindle) is assembled. Appearance - The duplicated chromosomes have tightly coiled, condensed and remained joined at the centromere. The nuclear membrane becomes less visible. Metaphase (M Check point verifies all chromosomes are attached to a spindle fiber) Events - Each sister chromatid is attached to a spindle fiber; the chromosomes are pushed and pulled by these spindle fibers to line up along the central axis. Appearance - The chromosomes appear lined up in the middle of the cell between the centrioles which are located at the opposite poles of the cell. The spindle fibers attached to each sister chromatid. Anaphase Events - Separation of sister chromatids occurs as the centromeres split. The chromosomes (now monads) migrate to opposite sides of the cell. Appearance - The sister chromatids migrate to opposite ends of the cells, pulled by spindle fibers attached to the divided centromere. Telophase Events - The chromosomes reach opposite poles (sides of cells) and the chromosomes unwind to direct metabolic activities, spindle fiber breakdown, nucleolus reappears and the nuclear envelope forms around the chromosomes. Appearance - The chromosomes are organized in bundles on opposite sides of a cell. Cytokinesis Events - In animal cells, the cellular membrane pinches in along the equator and the cell separates creating two identical daughter cells. Plant cells have a rigid cell wall and the cytoplasm is divided by the construction of a cell plate across the equatorial plane. Appearance - There will be two dark spots of nuclear material at either end of the cell. The cell membrane pinches inward to form a cleavage furrow which will appear in the middle of the cell. Part 2: Procedure Drawing and quantifying the phases of the cell cycle 1. Obtain a prepared slide of an onion root tip or a cell cycle card. 2. Start with the scanning objective (4X) and work up to the high power objective (40X) to locate then sketch an example of each of the five phases on the Student Data Sheet. Or view the cells on the cell cycle card. Page 2 of 8

3 3. Observe the number of cells in each stage and predict which stage will be the longest. Record your prediction on the analysis sheet (Part 2, #6). 4. Select a row of cells. Scan down the row and identify the stage of the cell cycle for each cell. Identify the stage, report the stage to your partner. Your partner will keep a record of the stage of each cell by placing a tally mark in the appropriate column on the data sheet. Count all the cells that are in the row. Do not skip any cell that can be identified. Continue counting until 50 cells have been identified. 5. Switch places with your partner and repeat step 4. Each partner is responsible for identifying 50 cells, totaling Total the tally marks for each stage. Since 100 cells were counted, this number represents the percent of cells in each stage. 7. Use the formula on the data sheet to calculate the number of hours the cell spends in each phase of the cycle. 8. Answer the analysis questions 7-10 on the student data sheet. 9. Follow safe handling procedures and clean the microscope and slide. Elaborate. Part 3. Uncontrollable cell division, Mitosis Gone Wrong Now that cells of normal cell growth and division have been observed, it is time to consider how the tally may differ if viewing a snap shot of cancer cells frozen in time. These cells would reveal Mitosis Gone Wrong or uncontrolled cell division. Based on your tally in part B, predict how the percentages for each phase may change if Mitosis Gone Wrong occurs. Write down predictions in the Extend section C on the Student Data Sheet. Research may be required to answer questions Refer to the normal tissue vs. cancerous tissue handout to complete questions Page 3 of 8

4 STUDENT DATA SHEET THE CELL CYCLE Name: Date: Part 1/ Pre-lab: Model Sketch each phase of the animal cell cycle here (based on the model developed in part A) INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE ANALYSIS 1. How are chromosomes different in prophase and metaphase than in anaphase and telophase? 2. What is the difference between chromatids and chromosomes? 3. Why does mitosis occur? 4. What is the significance of the process of mitosis? 5. What is the final stage of mitotic division? Part 2: Sketch each phase of the onion cell cycle here. INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE 6. Predict which phase of Cell Cycle is the longest? Shortest? Page 4 of 8

5 DATA: Place tally marks in the appropriate columns for each cell counted. INTERPHASE PROPHASE METAPHASE ANAPHASE TELOPHASE Total Total Total Total Total % % % % % Interphase Prophase Metaphase Anaphase Telophase Number of hours in each phase: Number of hours = 24 hr x Number of Cells Counted 100 Interphase Prophase Metaphase Anaphase Telophase 7. Use tally data to answer the following questions: a. What is the longest phase of the cell cycle? b. What is the longest phase of mitosis? c. What is the shortest phase of the cell cycle? d. What is the shortest phase of mitosis? 8. Was your prediction correct? 9. Describe the cell plate that was present during telophase. How would telophase differ in an animal cell? 10. If the pie chart below represents 24 hours in the life of an onion cell, color in the amount of time spent in each phase based on your calculations. Provide a key to accommodate coloration of pie chart. Page 5 of 8

6 Cell Cycle Part 3. Elaborate. Predictions of Mitosis Gone Wrong % % % % % Interphase Prophase Metaphase Anaphase Telophase 11. Normal cells spend most of their time in which phase of the cell cycle? Why? 12. Would you expect the percentages of cells in each phase of the cell cycle to change if uncontrolled cell division occurred? Explain your answer. 13. Cancer is when is uncontrolled. 14. How are cancer cells different from normal cells? 15. List and give examples of three different types of carcinogens. a. b. c. 16. What is the purpose of Checkpoints in the cell Cycle? Page 6 of 8

7 Refer to the normal tissue vs. cancerous tissue handout to complete questions In the chart below, make a tally of the number of cell in each phase of mitosis for the normal tissue. PROPHASE METAPHASE ANAPHASE TELOPHASE 18. In the chart below, make a tally of the number of cell in each phase of mitosis for the cancerous tissue. PROPHASE METAPHASE ANAPHASE TELOPHASE 19. Explain difference seen in the phases of mitosis when comparing the normal tissue with cancerous tissue cards. 20. Consider the following statement: The majority of cancers are genetic in nature, with only about 5% resulting from inheritance. Explain this statement. Page 7 of 8

8 Normal Tissue vs. Cancerous Tissue A. Normal Tissue B. Cancerous Tissue Disclaimer: The Normal Tissue and Cancerous Tissue diagrams are for comparison purposes only. The Cancerous Tissue diagram has been altered and should only be used for the purposes of this lab. Page 8 of 8