Mitosis / Meiosis Lab

3.A.1: Observing Mitosis in Plant & Animal Cells Using Onion Root Tip & Whitefish Blastula

Introduction / Purpose:
In this lab we learned about the processes of Mitosis in plant and animal cells. By using a microscope we were able to get a clear view of the cells in an onion root tip and a whitefish blastula. In these cells we observed the different stages of mitosis and were able to get a visual understanding of how cells divide. By seeing all the stages of mitosis, we learned what happens in each stage.

Procedure:
Examine prepared slides of either onion root tip or whitefish blastula. Locate the meristematic region of the onion or locate the blastula, with the 10X objective and then use the 40X objective to study individual cells. For convenience in discussion, biologists have described certain stages of the continuous mitotic cell cycle, as outlined on this page and the next. Identify one cell that clearly represents each phase. Sketch and label the cell in the boxes provided.

Image Credit goes to our fellow Early Bird Biology classmates

Analysis:

The purpose of this section of the experiment was to observe and gain experience in how an onion root tip cell looks when it is undergoing each phase of the cellular cycle. The observation of these cells helped us see how each cell looks like during each phase of the cycle, and how plant and animals cells may be different when dividing since a plant cell must accommodate for the presence of a cell wall.

3.A.2: Time For Cell Replication

Introduction / Purpose:
For this experiment we used a microscope to further observe the cells of an onion root tip. Once again we were able to observe the process of mitosis and examine the differences between each stage. We were instructed to take a tally of how many cells were seen in each stage. Through our results were were able to get an understanding of which stages take the longest amount of time. 

Procedure:
1. Observe every cell in one high-power field of view and determine which phase of the cell cycle it is in. This is best done in pairs. The partner observing the slide calls out the phase of the each cell while the other partner records. Then switch so the recorder becomes the observer and vice versa. Count at least two full fields of view. If you have not counted at least 200 cells, then count a third field of view.

2. Record your data on Table 3.1

3. Calculate the percentage of cells in each phase, and record it in Table 3.1  

Analysis:

The purpose of the experiment was to try to estimate the amount of time a cell spends in each stage of the cell cycle by observing onion root tip cells under a microscope and counting the number of cells in each phase. The phase in which most cells were observed to be in would be the longest stage. The stage in which the least amount of cells were observed to be in would be the shortest stage. From the data we gathered, it was determined that the highest number of cells were currently in Interphase, thus making it the stage in which cells spend the majority of their time. From the rest of the data and the times recorded, it is plausible to believe that the cells spend the least amount of time in the Telophase stage of the cycle. The cycles from longest to shortest, as demonstrated by the data, were Interphase, Prophase, Metaphase, Anaphase, and Telophase. A way to improve the data and gather perhaps more accurate results would be to do a double count of the cells, or have a technique to more accurately count the number of cells in each phase.

3.B.1: Simulation of Meiosis

Introduction / Purpose:
In this experiment we created visual representations of each stage in meiosis. This allowed us to gain a better understanding of what occurs in each stage, and in what order the processes are completed. After this experiment we should be able to identify each stage and understand what occurs in that stage. 


Procedure:

  

 

Analysis:

Through this exercise, it was possible to observe the results of crossing over. It was helpful to observe a difference between Meiosis 1 and 2 during the exercise. Meiosis 2 separates the chromatins of the chromosome into two daughter cells all formed by the division of Meiosis 1. Several difference between Mitosis and Meiosis were also noted. Meiosis produces haploid cells, only occurs with gamete, and occurs in two staged. Mitosis produces diploid cells, occurs with somatic cells, and does not include a Mitosis 2.

3.B.2: Crossing Over During Meiosis

Introduction / Purpose:
In this experiment we viewed photos of hybrid Sordaria asci. We observed these photos and identified all the points of crossing over. It was difficult to see all the points at which crossing over occurred, but by studying the photos we were able to get a better understanding of the process. The purpose of this experiment was to help us understand a visual representation of crossing over.

Procedure:
1. Obtain two images of hybrid Sordaria asci containing both tan and black ascospores.

2. This is best done in pairs. Have one partner count off the amount of non-crossing over 4:4 asci while the other partner keeps a tally. Write down your results in Table 3.3

3. Next have one partner count off the number of crossing over asci while the other partner keeps a tally. Asci patterns can appear in a few different varieties (image in column 2 in Table 3.3). Write down your results in Table 3.3

4. Add number of non-crossing over 4:4 asci with the number of crossing over asci to get the total amount of asci. Write down your results in Table 3.3. To calculate the map unit (an arbitrary unit of measurement used to describe relative distances between linked genes) use the number of asci showing crossing over and divide it by the total amount of asci. Take this number and multiply it by 100. Write down your results in Table 3.3 and create a simple sketch in the box provided.

Analysis:

One of the main observations made from the experiment was that genes further away from the centromere are more likely to cross over to the other chromosome. This was seen as the sordaria sample  greater amount of crossing over occurring also had a larger distance separated the genes and centromere than the sordaria with less crossing over occurring. It may also be more likely that genes further from the centromeres are more likely to cross over because it is the ends of the chromatids that crossover and exchange genes.