246 ❯ STEP 4. Review the Knowledge You Need to Score High
slides of these root cells, and you will then count the number of cells either in interphase
or in mitosis. Since you need a point of comparison, you will do similar counts with root
cells that have not been treated with this chemical (the control).Results for Part 1
So, how are you supposed to estimate how much time cells on a slide in front of you spend
in either mitosis or interphase? Say, for example, that you record your findings and get the
following breakdown: For your control, of 300 cells examined, 268 are in interphase and
32 cells are in one of the stages of mitosis (prophase, metaphase, anaphase, or telophase).
This would mean that the cell spent 89.3 percent of its time in interphase. At any moment
in time, 89.3 percent of the cells are in interphase. Here’s how to get that number. Take
the number of cells in interphase, 268, and divide that by the number of cells examined,- The result is 0.893. Move the decimal point two places to the right to get the percent-
age, 89.3 percent. By the same logic, these data also show that 10.7 percent are in mitosis.
For comparison, let’s say the chemically treated slide had 210 cells in interphase and 40 in
mitosis (for a total of 250 cells examined).
Now, once you count your mitotic versus interphase cells for both treated and
untreated roots, you need to use chi-square analysis to check if the difference is significant:
Do not panic! Even though that equation may seem intimidating, it’s really not that bad.
First, determine how many of your treated cells would be in mitosis if the chemical didn’t
have any effect.In other words, if 10.7 percent of your control cells were stuck in mitosis,
you’d expect that same percentage in your treated group: (250 treated cells) ×(0.107)=
~27 cells in mitosis. That leaves the remaining 223 cells in interphase (if you didn’t
expect that mitosis-inducing chemical to do its job). So now you use chi-square analysis
to compare what you actually saw in your chemically treated cells (40 in mitosis and 210
in interphase) to see if there are, in fact, significantly more cells stuck in mitosis. In other
words, your null hypothesis is that the treatment did not make a difference. If you find
that the chi-square value is greater than the critical value, you reject this null hypothesis
in favor of the experimental hypothesis (the chemical likely didmake a difference). See
Table 19.2.
As you can see, your chi-square value is 0.758 +6.26=7.02. To determine your crit-
ical value, you must choose a pvalue (usually 0.05) and the degrees of freedom. The degrees
of freedom (df ) equals the number of groups minus one. In this lab, there are two groups,
interphase and mitosis; therefore, df =2 – 1, or 1. Based on the chi-square table (which will
be provided for you on the AP exam), your degrees of freedom equals 3.38. Since your cal-
culated chi-square value (7.02) was bigger than 3.38, you can reject the null hypothesis that
said the treatment made nodifference. The chemical did increase the number of cells in
mitosis.x
oe
e2
2
=−
∑
()
Table 19.2 Chi-Square Table for Investigation 7.
# OBSERVED (o) # EXPECTED (e) (o-e) (o-e)^2 (o-e)^2 /e
Interphase cells 210 223 –13 169 0.758
Mitosis cells 40 27 13 169 6.26