Download Mitosis Lab

Mitosis Lab
Name:_______________________
Background:
Mitosis and meiosis are two processes by which the DNA in a parent cell nucleus is divided up to form
new nuclei in daughter cells. Mitosis results in the formation of somatic (body) cells, while meiosis forms
gametes (reproductive cells; eggs and sperm), which have half the amount of DNA as somatic cells.
Mitosis allows organisms to produce new body cells for growth, development from a fertilized egg, and
repair of damaged tissues. If you want to observe mitosis, the best organisms to study are those that are
actively growing. In animals, these tend to be embryos and young organisms. In plants, the tips of the
roots and shoots are the areas where most cells are actively dividing to form new growth.
Plants and animals go through a few different stages in the process of dividing their nucleus for mitosis.
These stages ensure that the two nuclei of the daughter cells have the correct amount of DNA, an exact
copy of the DNA their parent cell had. Before mitosis, a cell is in interphase, during which it grows and
copies its DNA, but does not reproduce. After completing interphase, a cell begins the process of
mitosis, the division of its nucleus. The following are the stages of mitosis.
Prophase: During prophase, the chromosomes, or DNA molecules, in a cell coil tightly around
themselves, forming a compact package. This makes each chromosome less likely to become entangled
with the chromosomes around it, which could cause breakage during cell division. Chromosomes look
thicker and more distinct in prophase than they do in interphase. The two copies of each chromosome
are called sister chromatids, and they stick together at the centromere, or narrow waist, of each
chromosome. During this phase, the nuclear envelope breaks up so that the chromosomes are no longer
surrounded by a nucleus.
Metaphase: At metaphase, all the chromosomes move to the center of the cell, where they line up
along an imaginary line we call the metaphase plate. Microtubules growing from both sides of the cell
attach to proteins at the centromere, tugging them back and forth to line them up perfectly in the
center of the cell.
Anaphase: During anaphase, the two sister chromatids in each pair separate. The microtubules that are
attached to each chromatid shorten by hydrolysis, pulling each chromatid to one side of the cell or the
other.
Telophase and Cytokinesis: In telophase, two new daughter nuclei form around the DNA at either end
of the cell. While telophase is happening, the cell begins cytokinesis (the division of the cytoplasm). In
animal cells, this involves a cleavage furrow, a row of microtubules that tighten like a drawstring bag in
the middle of the cell to split the two daughter cells apart. In plant cells, cytokinesis involves a group of
vesicles full of cell wall material that migrate to the center of the cell. These vesicles fuse together to
form a cell plate, which then attaches to the existing cell wall, forming a new wall between the daughter
cells.
Reference photos:
Interphase
Late Anaphase
Prophase
Telophase
Metaphase
Late Telophase
Anaphase
Interphase
Photo source: http://www.sciencephoto.com/images/download_lo_res.html?id=670074113
Procedure:
1. Before looking at a microscope, do the online activity found at
http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/ 01.html using the computers at
your desk. Make sure that everyone in your group gets some practice identifying cells in each
phase. Identify all the pictures, but don’t fill in the table or graph at the end.
2. Obtain a light microscope for your lab group, and carefully carry it back to your group’s table
using both hands. Obtain a prepared slide of onion root tips.
3. Using the scanning objective lens, center one of the root tips from the slide and focus so that
you can clearly see the onion cells on your microscope. The root tips were sliced into thin
sections while the onions were alive, and then preserved on a slide.
4. Look at the bottom of the root tip. This region is called the root cap. Just above the root cap you
should see a region that contains many small cells. The cells in this region were actively dividing
when the root tips were preserved on the slide. The cells in this region of the slide are the cells
you should concentrate on observing for this lab. Center the image on this region, focus, and
then switch the lens up to the next highest magnification.
5. As you look at the cells on your slide, you may notice that there are no visible dark spots or
chromosomes in some of them. This slide was made by cutting a thin slice of the root, and that
slice may not contain the nucleus of every cell. For instance, if you took a hard-boiled egg and
cut it into very thin slices, not every slice would contain a piece of the yolk. Ignore the cells that
have no visible DNA, as you will not be able to tell which phase they are in.
6. Find cells in each of the following phases and zoom in on them, one by one. Draw a cell from
your onion root tip slide for each of the following phases of the cell cycle (don’t just copy the
reference photos above).
Drawings :
Interphase
Prophase
Metaphase
Anaphase
Telophase
7. For one of the 3 root tips on your slide, count and record the number of cells in each stage of
the cell cycle in the data table below. It may be easiest to have one group member look through
the microscope along each row of cells, saying aloud what stage each cell appears to be in.
Another group member can make tally marks on the data table for each stage. You will need to
count ALL the cells for one whole root tip. There will be more than 200 cells to count. There
will be parts of the root tip where almost all of the cells are in interphase.
8. Calculate the percentage of cells in each stage. This can be calculated by taking the number of
cells in that phase divided by the total number of cells in all phases, times 100:
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑝ℎ𝑎𝑠𝑒
100 ∗
= % 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑝ℎ𝑎𝑠𝑒
𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑒𝑙𝑙𝑠 𝑖𝑛 𝑎𝑙𝑙 𝑝ℎ𝑎𝑠𝑒𝑠
The percentage of cells that are in each stage at a time corresponds to how long that stage takes
compared to the others. For instance, if 90% of your cells were in interphase, then you know
that interphase takes approximately 90% of the amount of time in the cell cycle.
Phase
Interphase
Prophase
Metaphase
Anaphase
Telophase
Number of cells in phase
Percent of cells in phase
Questions:
1. Why is mitosis an important process? What would happen to an organism that could not do
mitosis at all?
2. Which stage of the cell cycle was the longest? Which was the shortest?
3. Compare your results for #2 with other groups and describe the similarities and/or
differences. If your results were different, what might explain these differences?
4. If an onion cell starts out with 16 chromosomes at the beginning of interphase, then goes
through mitosis and cytokinesis, how many chromosomes would each of the daughter cells
have?
5. If an onion cell did meiosis to form reproductive cells, how many chromosomes would be in
each of its ovules (plant egg cells) or pollen grains (plant sperm cells)?
6. If a pollen grain from one onion fertilized an ovule from another onion, how many
chromosomes would be in the zygote (fertilized egg cell)?
7. After a dividing cell finishes telophase and cytokinesis and forms two daughter cells, what
phase of the cell cycle do the daughter cells go into?
8. What is the difference between mitosis and cytokinesis?
9. How is cytokinesis different in plant and animal cells?
10. If you wanted to see mitosis happening in a plant, what parts of the plant should you look at?
Why? What about if you wanted to see mitosis happening in an animal?