AP Biology Mitosis Observations
The AP lab manual describes an investigation of whether a lectin can increase the
rate of mitosis in onion root tip cells. You will be doing something a bit different.
You will investigate the assertion that only certain regions of plant structures are
able to undergo mitosis. Specifically, you will compare the rate of mitosis in the
distal tip (far end) of an onion root with the rate in a region a bit closer to the base.
You will also be able to calculate roughly how long each phase of the cycle takes
relative to the time to complete an entire cycle.
You have available two boxes of onion root tip slides, labeled region A and region B.
Each slide contains three longitudinal sections of root tip with the region you are to
examine available. The other region is blocked by tape.
Each group should examine the cells visible in a field of view at high power in each
root tip section for each of at least two slides. For each cell within the field of view,
determine whether that cell is in interphase or in mitosis, and if in mitosis what
phase. This means you should set up a data table in which you can tally your counts.
As a group, you must organize to make sure that each slide is counted at least once.
Compile your results on the board and consider how to determine whether the
incidence of cells undergoing mitosis is significantly different in regions A and B. To
do this you will use Chi square analysis
What is Chi Square (2)? A statistical test to determine the validity of a hypothesis.
Specifically, this test will tell you if the observed variance in your data is greater
than would be expected from purely chance fluctuations.
Is there more than one hypothesis? Yes, there are two.
1) Null Hypothesis - there is no statistical difference between expected data and
observed data. In other words, the variation occurs just by chance.
2) Alternative Hypothesis - another hypothesis explains your observation.
In Chi square, you test whether to accept or reject the null hypothesis, according to
whether your calculated chi square value exceeds a critical value or not.
The formula for chi square is  2  (o  e) 2 / e
o=observed e=expected sum of the values of o-e squared divided by e
So, this is a statistical test that evaluates the how much an observed sample varies
from the expected data. (See the AP lab manual for a description of how to calculate
chi square for your sample and determine its significance.) This means you must
have some expectation of how many cells should be in interphase and how many in
mitosis for each region. How will you come up with those numbers? Do you need to
consider the phases of mitosis separately, or is it enough to just say interphase or
mitosis?
How long to divide?
In the region that is reliably undergoing mitosis, use your class tally of how many
cells are in interphase vs each segment of mitosis to create a pie chart (one of the
few acceptable uses for a pie chart!) to compare the relative duration of each phase.
What do you need to calculate for this?