19 Lab Protocol: Mitosis and Meiosis
Observe Prepared Slides Showing Mitosis in Onion Root Tips
It is not possible to fully understand mitosis by just looking at textbook diagrams that represent the different phases of mitosis. In this lab, you will have the opportunity to observe and analyze prepared slides of onion root tips on which chromosomes have been stained. As you observe the specimens, you will photograph and label various cells. On every photograph, you should document the total magnification and provide a scale bar to help viewers comprehend the sizes of structures. Always use the highest magnification that allows you to photograph the entire structure of interest. Note: you will not use the 100X objective lens since that requires immersion oil to increase resolution.
Cross section of onion root tip:
- Photograph 1: Full view of the cross section of an onion root tip to show the diameter of the root tip and the sizes and shapes of cells. Use arrows to identify cells undergoing mitosis and label each with the name of the phase.
Longitudinal section of onion root tip:
- Photograph 2: View of the longitudinal section of an onion root tip at lowest magnification to show the shape of the root tip and the sizes and shapes of cells in the various locations.
- Photograph 3: View of cells near root tip where abundant mitotic cell divisions are occurring. This photograph should show both internal and epidermal cells. Indicate the three-dimensional measurements of epidermal and internal cells using photograph 1 and photograph 3.
- Photograph 4: View of cells at end of specimen opposite of the root tip. This photograph should show both internal and epidermal cells. How do these cells differ in size and shape compared to the cells in photograph 3?
- Photograph 5: A cell in prophase using an arrow to mark the cell of interest.
- Photograph 6: A cell in metaphase using an arrow to mark the cell of interest.
- Photograph 7: A cell in anaphase using an arrow to mark the cell of interest.
- Photograph 8: A cell in telophase using an arrow to mark the cell of interest.
What proportion of cells are undergoing mitosis near the root tip?
What proportion of cells are undergoing mitosis at the opposite end of the specimen?
Which phase(s) of mitosis is/are represented more often than others?
Speculate on what this might mean.
Use Chromosome Models to Examine the Different Phases of Meiosis
To understand genetics and inheritance, it is essential to understand how meiosis leads to the production of genetically diverse gamete types. Meiosis is an important step in the process of gametogenesis and is the mechanism by which cells become haploid. We will use chromosome models to help us visualize the different phases of interphase, meiosis I and meiosis II. Before beginning to model meiosis, it is important to understand what each part of the model represents (Figure 1).
Figure 1: Chromosome models to diagram the events of meiosis.
While representing the different phases, it is important to apply the conservation of matter and recognize the events that increase/decrease the amount of DNA or chromosomes in a cell. The only way to increase the amount of DNA is DNA replication. The only way the amount of DNA and the number of chromosomes will decrease within a cell is by cell division. Each daughter cell will then have half of the DNA and half of the chromosome number. The number of chromosomes in a cell increases when the two sister chromatids of a duplicated chromosome separate from one another during anaphase II.
You will do two things as you model the phase of meiosis:
- Photograph and name each phase (and place the photographs in order).
- Complete the table below counting the number of cells, number of chromosomes per cell, and the number of chromatids per chromosome at the end of each phase.
|
phase |
Number of cells |
Number of chromosomes per cell |
Number of chromatids per chromosome |
|
G1 of interphase |
1 |
4 |
1 |
|
G2 of interphase |
|
|
|
|
Prophase I |
|
|
|
|
Metaphase I |
|
|
|
|
Anaphase I |
|
|
|
|
Telophase I /cytokinesis |
|
|
|
|
Prophase II |
|
|
|
|
Metaphase II |
|
|
|
|
Anaphase II |
|
|
|
|
Telophase II/cytokinesis |
4 |
2 |
1 |
Observe Prepared Slides of Graafian Follicles and the Epididymis
Only specialized cells called germ line cells have the possibility of undergoing meiosis leading to gamete production. These specialized cells, found in the ovaries of females and testes of males, are surrounded by cells that will not undergo meiosis but do support gamete production. In mammals, eggs are some of the biggest cells, and sperm cells are some of the smallest.
- Photograph the Graafian follicle with the largest developing egg cell. Make sure that the entire follicle is within view along with surrounding support cells. Record the magnification used and include a scale bar. The egg cell is expected to be spherical in shape. What is the diameter of the egg cell? If a support cell is also spherical in shape, what is its diameter?
- Photograph a portion of the epididymis that includes sperm cells and surrounding support cells at 400X or 630X magnification. Indicate the magnification used and include a scale bar with which to estimate cell size. What is the shape of the sperm cell? How long is the sperm cell minus the flagellum?
- Use the internet and state your source when answering the following questions: How many mitochondria are typically found in a human somatic cell? a human egg cell? a human sperm cell? Why is it important that a sperm cell have functional mitochondria?
- Continue to use the internet and state the source for your answers. Mitochondria carry genetic information in the form of DNA. Maternal inheritance is the mechanism by which mitochondria are inherited in humans and many other species. Maternal inheritance means that functional mitochondria are exclusively inherited through the egg cell. What happens to the sperm mitochondria?