Observing the Cell Cycle in Onion Root Tips
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(Mitosis)
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Introduction:
In this lab, you will examine the dividing root-tip cells of an onion. You will examine the
specimen to identify the various phases of the cell cycle, to see which phases cells spend
more time in, and you will answer related analysis questions to further your understanding of
the eukaryotic cell cycle.
Pre-lab:
1. What are the three general phases of the cell cycle? List them and describe what
happens in each general phase.
Interphase – normal cell function; cell prepares for division; DNA is replicated
Mitosis – actual division of DNA and organelles; cell membrane pinches in
Cytokinesis – division of the cytoplasm; two daughter-cells formed
2. What would happen if our cells didn’t go through interphase correctly?
DNA replication happens in interphase, so a problem is most likely to happen
then. DNA replication problems often result in different kinds of mutations which
can be very dangerous for the cell. Mutations can lead to cancer and other genetic
diseases.
3. What could happen if our cells didn’t go through mitosis correctly?
The formation of healthy daughter cells depends upon the equal sharing of DNA
and organelles. If the DNA is not shared equally, then many chromosomal
disorders can occur (such as Down syndrome)
4. When you look into the microscope, which phase of the cell cycle do you think most
cells will be in –interphase, mitosis, or cytokinesis? Why?
Most cells will be in interphase. Cells only divide when they are supposed to
(growth/development, repair, replace) and spend the rest of their time in
interphase performing their normal cell functions or preparing to divide.
5. Why do you think it is easiest to see the nucleus dividing, rather than cell organelles
dividing?
In order for the nucleus to divide, the DNA must be copied. Having two copies of
DNA makes the nucleus more visible. Organelles are too small to be seen with the
microscopes we use.
6. Just because cells are dividing does not mean that the cell is cancerous – where in our
bodies are cells dividing fast? Why do those cell parts need to divide fast?
Cells in our stomach need to divide quickly because they are in a really harsh
environment. These cells cannot survive very long due to the stomach acid that is
trying to digest everything it contacts.
Lab:
Look at the onion root tip slides. Choose a sample of about 50 cells. Look for a group of cells that
appear to have been actively dividing at the time that the slide was made. The cells will appear in
rows, so it should be easy to keep track of them. The dark stained bodies are the nuclei.
Stage
Sketch
Description of what’s
happening in each stage
Estimated number or %
of cells in that stage
Interphase
Normal cell functions;
cell prepares to divide;
the DNA is replicated
(copied)
62%
Prophase
Nuclear membrane
disappears; DNA condenses
and forms chromosomes;
centrioles begin to separate
and spindle fibers form
12%
Metaphase
Chromosomes line up in
the middle; centrioles
have moved to the poles;
spindle fibers are fully
formed
4%
Anaphase
Centrioles pull the
chromosomes apart so
each side receives one
copy
6%
Telophase
Nuclear membrane forms
around both sets of
chromosome; spindle fibers
disappear as the cell
membrane pinches in
8%
Cytokinesis
Cytoplasm divides in half
as the cell membrane
finishes developing; two
identical daughter cells
are created
8%
Post lab:
1. Why is the onion root a good specimen for studying mitosis?
Root cells need to divide in order to grow deep to provide support and get nutrients.
2. The majority of the cells in your specimen were in which stage of the cell cycle?
Most of our cells were in interphase.
3. Why does it make sense that most cells in your specimen were in that stage?
Most cells do not constantly divide, and only do so when needed. Most cells are
either doing their normal cell functions or preparing to divide.
4. Cancerous tissue is composed of cells undergoing uncontrolled, rapid cell division. How could you
develop a procedure to identify cancerous tissue by counting the number of cells undergoing mitosis?
Since most normal cells are in interphase, you would want to make a procedure that
can count the number of cells in interphase and in mitosis. If more are in interphase,
the cell is normal; if more are in mitosis, then the cells may be cancerous.
Use this image to determine your percentages and to help answer the remaining
questions on page 2 of your lab.