Mitosis
In this lab, you will be learning about the process of mitosis. Mitosis consists of different stages that allow the cell to prepare for division. Cell division occurs for the following reasons: growth, development, and repair. Each cell goes through its own cell cycle. It begins when a previous cell splits and it ends when the cell either dies or splits again. The first phase of the cell cycle is called G1. During G1, the cell primarily grows. This is then followed by the S phase. During the S phase, the cell replicates its DNA. The S phase is then followed by the G2 phase, which is when the cell further prepares for cell division by copying organelles and creating microtubules to be used later. The next phase is the M phase, which is what today’s lab is about. M stands for mitosis, and it occurs at the end of the cell cycle. Mitosis consists of 4 phases: prophase, metaphase, anaphase, and telophase, followed by a process called cytokinesis. During prophase, the nuclear envelope dissolves and the chromosomes condense and become visible. During metaphase, the chromosomes line up along the middle of the cell. During anaphase, the sister chromatids begin to separate and travel to opposite ends of the cell. During telophase, the chromosomes end up at opposite ends of the cell, the nuclear envelope reforms, and then the chromosomes begin to unravel. See the picture below to get an idea of what each phase of mitosis looks like. Between mitosis events, the cell goes through Interphase (inter= between). During interphase, the cell grows and can go into a dormant phase. If not, it will continue to prepare for mitosis.
In today’s lab, you will see these different phases of mitosis under the microscope and then you are going to determine how long each cell spends in each phase of mitosis.
Identifying the Stages of Mitosis
Obtain your slide box from your lab kit, and obtain your microscope.
Place the slide called “Onion Root Tip Mitosis” on the microscope and begin searching for cells in the different stages of mitosis.
Draw each of the phases that you saw in the space below:
Determining Lengths of Time Spent in Mitosis Phases
Looking at the onion root slide, locate 50 cells that appeared to be actively dividing when the slide was made. Notice how these cells are arranged in columns. This will allow you to easily keep track of them while you are doing this activity.
For each of the 50 cells you chose, identify which stage of mitosis the cell is in, and keep track using the “tally marks” section of the table below.
Once you have tallied 50 cells, count how many were present for each phase of mitosis and write that number in the “count” section of the table.
Think about it:
Which phase had the most cells? __________________________
Which phase had the least cells? ________________________
Calculate the percentage of cells found in each phase. This can be done by dividing the number of cells in the phase by 50, which is the total number of cells in the count, and then multiply your answer by 100. Enter this number in the “percentage” section of the table.
The percentage of cells found in each phase is a measure of how long each phase lasts. For example, if 25 percent of the cells are in prophase, then prophase takes 25 percent of the total time it takes for a cell to undergo mitosis. Mitosis in onion cells takes about 80 minutes.
Calculate the actual time for each phase using this information and the percentage you have just determined. Use the following equation to find the duration of each phase of mitosis in onion cells.
Enter the time (in minutes) of each phase under the “Time (in minutes)” column of the table.
Duration of phase (in minutes) = percentage/100 x 80 minutes
Mitosis Phase Tally Marks Count Percent Duration (minutes)
Prophase,
Metaphase,
Anaphase
Telophase
