AP Biology: Notes and Practice
The Cell Cycle
All organisms are composed of cells, and all organisms reproduce themselves. Single-celled organisms
reproduce by duplicating cellular components, including the hereditary material (DNA) in the chromosome, and
then splitting themselves in half. The process with multi-celled organisms is more complex, but involves
division of the cells within the organism. Thus, cell division is an integral component of all biological
reproduction.
Model 1 – The Eukaryotic Cell Cycle
Each original chromosome is
attached to its sister
chromatid copy at the
centromere
Mass of DNA in a human cell is about 6.6 picograms
1. The end of G1 and S phases in Model 1 indicates that DNA can be organized in two different
arrangements. Identify the names of these two arrangements and describe the differences between them.
2. Observing the cell images at the start and end of the G1 phase, describe what happens to cell in this
phase of the cycle.
3. Three major events of the cell cycle are the condensing of chromosomes, the division of the nucleus, and
the division of a cell into two daughter cells. In which phase of the cell cycle does each of these events
occur? Explain your reasoning.
AP Biology: Notes and Practice
4. The S phase stands for synthesis, which means to make or build something more complex out of simpler
parts. Scientists know that during the S phase DNA is being made in the nucleus of the cell.
Considering the result of the cell cycle, why is the S phase necessary?
5. Replicated chromosomes are often illustrated as an X shape to match how they look in real life just
before cell division. According to Model 2, which of the following diagrams correctly show an original
set of homologous chromosomes (grey) and their sister chromatids (black)—the replicated portion?
Explain your reasoning.
6. If 4 cells entered into the G1 phase of the cell cycle, how many cells would be present at the end of the
M phase? Explain your answer.
7. If a cell entered into the G1 phase with 3 picograms of DNA, how much DNA would be present in a
single cell at the end of each of the following phases. Explain your reasoning for each.
a) G1?
b) S?
c) G2?
d) M?
8. Four cells containing 5 chromosomes each enter into G1 of the cell cycle. They proceed through 1
rotation of the cycle. Construct a table that displays the total number of cells and the total number of
chromosomes per cell at the end of each major phase of the cell cycle.
AP Biology: Notes and Practice
Model 2 – Mitosis
As shown in Model 1, the cell cycle consists of 4 phases: G1, S, G2 and M. G1, S, and G2 together make up
interphase, the preparation for cell division. “M” is the abbreviation for mitosis, the phase in which a cell
undergoes nuclear division. Mitosis consists of its own sub-phases, which are displayed in the model below.
9. Referring to Model 2, describe the relationship between a chromatid and a chromosome.
10. Refer to Model 2 to answer the following questions:
a) Describe the apparent relationship between centrioles and spindle fibers.
b) Identify the phases in which the spindle fibers are present and the action they perform in each of
these phases.
c) Predict the purpose of the spindle fibers during mitosis.
AP Biology: Notes and Practice
11. Refer to Model 2 to answer the following questions:
a) Describe the changes that the nuclear membrane undergoes throughout the phases of mitosis.
b) Predict how each phase of mitosis would be different if the nuclear membrane did not break
down.
c) Explain why the changes to the nuclear membrane described in part a occur.
12. Cellular division has two parts – mitosis and cytokinesis. Describe the differences between these two
parts of cellular division and explain why mitosis has to come before cytokinesis in the cell cycle.
13. During cytokinesis the chromosomes unwind and become a pile of very long, thin, thread-like DNA.
DNA in this structure is referred to as chromatin; it requires far less energy for a cell to structure its
DNA as chromatin than as chromosomes. DNA remains in its chromatin structure until mitosis occurs
again, at which point the DNA coils back into chromosomes. Explain why DNA recoils into
chromosomes for mitosis, despite the energy cost to the cell.
AP Biology: Notes and Practice
14. Fill in the table below where each of the phases of nuclear and cell division is listed in the left hand
column and write the events that occur during that phase in the right hand column.
15. The point of attachment between two sister chromatids is called the centromere region, shown in the
figure below. The centromere region contains proteins called kinetochores that connect with the spindle
fibers during mitosis. Colchicine is a poison that competitively binds kinetochore proteins. Describe
the effects on mitosis in a cell that has been treated with colchicine.
AP Biology: Notes and Practice
16. Imagine that a cell is undergoing mitosis as shown in model 2. All event occur exactly as pictured,
except that the chromosomes line up in the following pattern during metaphase:
Explain how the resulting daughter cells would be affected by this chromosome arrangement.