CHAPTER 9 THE CELL CYCLE
Scientific Skills Exercise
Teaching objective: Students will practice reading a histogram in this exercise. The students
then relate the data to phases of the cell cycle based on relative amounts of DNA.
Teaching tips: A version of this Scientific Skills Exercise can be assigned in MasteringBiology.
The method used to measure DNA content in this experiment is flow cytometry, also sometimes
called fluorescence-activated cell sorting (FACS). The fluorescent dye (propidium iodide in this
example) binds to
amount of DNA the cell contains. The flow cytometer instrument passes individual cells through
vel. The analysis
software calculates how many cells in each sample had a given range of fluorescence and graphs
the relationship as cell frequency (or cell count) versus fluorescence interval. In the experiment
producing the results shown here, the researchers ran 10,000 cells through the flow cytometer for
each treatment.
Students may need some help with the idea of frequency data in the form of cell counts. Students
may be used to seeing DNA content of cells over time, and may tend to fall into thinking of the
x-axis as representing time and the height of the peak as representing DNA content. A clearer
way for students to think of it is that in the case of the cell cycle data seen here, there are
basically two states that most cells will be in: G1 (less DNA) and G2 (more DNA). The spread of
cells in between these two states represents cells in S phase, transitioning from the low to high
amount of DNA. After a cell enters mitosis, it will have the doubled DNA content until it goes
through cytokinesis, at which point it will be counted as two cells, and those cells would
immediately show up in the G1 peak to the left along the x-axis. An analogy to use: If you were
to count the number of students in two sequential classrooms along a hallway at any one time,
you get relatively high head counts for each classroom location. But there will be some students
walking from classroom G1 to classroom G2; they are in transition between locations and will
represent a low head count spread out along the hallway between the two rooms (S phase). The
M to G1 transition would be like teleporting back to the first classroom and never showing up in
the hallway at all!
Answers:
1. (a) The relative amount of DNA is shown on the x-axis. Because the fluorescent dye is binding
to DNA in each cell, the amount of fluorescence corresponds to the amount of DNA (the more
3. (a) The G1 phase of the cell cycle has the greatest number of cells because the highest peak in
the treated sample corresponds to the lower fluorescence level, comparable to the G1 cells in the
Suggested Answers for End-of-Chapter Essay Questions
See the general information on grading short-answer essays and the suggested rubric at the
beginning of this document.
8. Scientific Inquiry
(a) The motor proteins at the kinetochore ends of kinetochore microtubules walk the
9. Focus on Evolution
Natural selection favors organisms with attributes that lead to most offspring, and cell division is
a crucial function for both unicellular and multicellular eukaryotes. Although the number of
chromosomes would be correct in each daughter cell, the two cells would not contain identical
10. Focus on Information
Sample key points:
The continuity of life depends on cells dividing and faithfully passing heritable information
to daughter cells.
The chromosomes of a eukaryotic cell duplicate in the S phase.
Sample top-scoring answer:
The continuity of life reproduction of unicellular organisms and the development, growth, and
repair of multicellular organisms depends on the transmission of identical copies of heritable
11. Synthesize Your Knowledge
Cancer cells divide without being subject to the usual cell cycle controls. They can divide in the
absence of growth factors or other molecules that are necessary for normal cells to pass
particular checkpoints. Furthermore, cancer cells are not inhibited by density and do not require