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Biology 109 Week 3 Virtual Laboratory Activity Determining time spent in different phases of the cell cycle (adapted from ‘The Biology Project - http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html) Name: Click here to enter text. right807300As we’ve learned this week, growth in multicellular organisms is carefully controlled by regulating the cell cycle. In plants, such as the onion, roots continue to grow as they search for water and nutrients deep within the soil. Because these regions are constantly growing and at any given time a significant percentage of cells will be undergoing mitosis, the tip of the root is a very good system for studying the cell cycle. In order to examine cells in the tip of an onion root, a very thin slice of the root is ‘fixed’ (a procedure which ‘freezes’ the cell division process) and placed onto a microscope slide and stained so the chromosomes will be visible. Although slicing the onion root tip and ‘fixing’ it will capture many cells in different phases of the cell cycle, keep in mind that the cell cycle is a continuous process. The key research question we will be addressing today is to estimate the amount of time cells in the onion root tip spend in each of the stages of the cell cycle. Recall from this week’s learning material that the cell cycle is divided into the following stages: 4354-3175Interphase. The cell is engaged in metabolic activity and performing its duty as part of a tissue. The DNA duplicates during interphase to prepare for mitosis (the process that results in nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. 4354-3084Prophase. Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nuclear membrane dissolves and microtubules attach at the kinetochores and the chromosomes begin moving. 4354-726Metaphase. Spindle microtubules align the chromosomes along the middle of the cell. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome. 43542268Anaphase. The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of spindle fibers. 38103810Telophase. New membranes begin to re-form around the two new daughter nuclei while the chromosomes decondense and are no longer visible under the light microscope. Cytokinesis or the separation of the two daughter cells may also begin during this stage. Determining time spent in different phases of the cell cycle The assignment In this activity, you will be presented with images consisting of cells from the tip of an onion root. Your task is to categorize each cell based on the phase it is in and count up all of the cells found in each phase. This analysis will enable us to estimate the amount of time the onion root tip cells spend in each phase. Activity 1 – Categorizing and Counting Cells Using the image above (a higher-resolution image is also available for you to download to use during this activity), count the number of cells that you can identify in each of the various stages of the cell cycle. It may be helpful to print the high resolution image out and mark off the cells as you count them. If you are unsure of the stage of a cell, count it as being in ‘Interphase’. You should count approximately 280-290 total cells in the image. Record your results in the top row in the table provided below. Interphase Prophase Metaphase Anaphase Telophase Total Number of Cells Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Percent of Total Cells (# Cells in Stage/Total # of Cells) * 100 Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. 100% After counting all of the cells and categorizing them, add up the number of cells in Interphase, Prophase, Metaphase, Anaphase and Telophase and enter this value in the ‘Total’ column in the top row (this should be around 280-290 cells). We will now use this total to calculate the percentage of cells in each stage. To do so, for each stage, divide the number of cells you counted in that stage by the total number of cells and multiply by 100. For example, if I counted 25 cells in Prophase out of a total of 285, then I would calculate the Percent of cells in Prophase as (25/285)x100 = 8.7%. After calculating all the percentages, they should add up to a total of 100%; if they do not, you have calculated incorrectly. Activity 2 – Estimating the Amount of Time Required to Complete each stage of Mitosis The dividing cells of the onion root tip are able to complete the entire cell cycle in approximately 24 hours. Since we know this, we can use the percentages calculated above (enter these values in the top row in the table below) to estimate the amount of time cells spend in each stage since the relative ratio of cells found in our ‘fixed’ time sample would represent the ratio of time out of the 24 hour period required to complete the entire cell cycle. We will estimate the amount of time spent in each stage in minutes. Since 24 hours = 24 x 60 minutes/hour = 1,440 minutes, we will use this as the amount of time required to complete the entire cell cycle. Complete the table below by multiplying the Percent of Total Cells in each stage by 1,440 minutes to estimate the amount of time needed to complete each stage and enter these values in the bottom row. All of the values in this bottom row should add up to 1,440 minutes, if they do not, you have not calculated correctly). Interphase Prophase Metaphase Anaphase Telophase Total Percent of Cells in Stage Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. 100% Amount of time Required to complete stage (% Cells x 1,440 minutes) Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. Click here to enter text. 1,440 minutes Lab Recap Questions Which stage had the greatest number of cells? What is happening inside the cell during this stage? Click here to enter text. Which stage had the least number of cells? What is happening inside the cell during this stage? Click here to enter text. How does the data you collected support what you have learned about the cell cycle? Which stage is predominant and why? Click here to enter text. The cells in the root tip of the onion are actively dividing cells (in one of the stages of mitosis). How would you predict the numbers to differ if you were to examine cells from a different part of the plant? Justify your statement with scientific concepts you are learning. Click here to enter text. You have been approached by a chemical company to test a new product that it believes will increase the growth rate of onions. How would you design an experiment to test this new product? How would your results differ from those calculated above if the product did, in fact, cause the onion to grow faster? Explain your answer. Provide a summary table if that helps you to answer the question. Click here to enter text.