1. What is the cell cycle?
The cell cycle is a regular series of events that cells go through as they grow and divide.
The cell cycle for prokaryotic cells is a quick succession of growth, DNA replication, and cell division. Cell division in prokaryotes is a one-step process called binary fission (shown right).
Eukaryotic cells have a more complex cell cycle than prokaryotic cells.
1. Review What is the difference between a prokaryotic cell and a eukaryotic cell?

2. How is the eukaryotic cell cycle divided?
The time between cell divisions is called interphase. The length of interphase varies depending on cell type.
Eukaryotic interphase is divided into three steps, or phases: G1, S, and G2.
Eukaryotic cells divide during the M phase of the cell cycle. The M phase consists of two steps: mitosis and cytokinesis.
2. List What are the three steps of interphase in eukaryotic cells?

3. What happens during each phase of eukaryotic interphase?
G1: Cells do most of their growing during this phase. It begins when mitosis is complete and ends when DNA replication begins.
S: DNA is synthesized as chromosomes are replicated.
G2: Many of the molecules and cell structures required for cell division are produced; usually the shortest phase of the cell cycle.
During interphase, the cell grows and replicates its DNA.
3. Describe What happens during the S-phase of interphase?

4. What happens during the M phase of the eukaryotic cell cycle?
The M phase is usually much shorter than interphase and results in two daughter cells.
The first step of the M phase is mitosis. The cell’s nucleus divides during mitosis.
The second step of the M phase is cytokinesis, during which the cell’s cytoplasm is divided.
4. Describe What happens during mitosis?

5. What are the steps of mitosis?
Mitosis consists of four steps: prophase, metaphase, anaphase, and telophase.
Prophase: nuclear envelope breaks down, DNA condenses, spindle begins to form.
Metaphase: replicated chromosomes, which appear as paired sister chromatids, line up across the center of the cell and attach to spindle.
(contd.)

6. What are the steps of mitosis?
Anaphase: sister chromatids separate and move toward ends of the cell.
Telophase: chromosomes disperse, nuclear envelope reforms.
5. Review During which phase of mitosis do sister chromatids line up along the center of the cell?

7. What completes the M phase of the cell cycle?
Cytokinesis completes the M phase of the cell cycle. It may begin while telophase is still taking place.
During cytokinesis, the cytoplasm (which includes all of the contents of a eukaryotic cell outside the nucleus) draws inward, eventually pinching off into two nearly equal parts. Each part contains a nucleus.
(contd.)

8. What completes the M phase of the cell cycle?
In plant cells and other eukaryotic cells that have a cell wall, a cell plate forms halfway between the divided nuclei. It gradually develops into cell membranes and forms a complete cell wall surrounding each daughter cell.
Upon the completion of cytokinesis and the M phase, a cell enters interphase.

9. When does DNA replicate?
Before a cell divides in the M phase, its DNA is duplicated by a process called DNA replication.
DNA replication occurs at the beginning of the S phase of interphase.
It ensures that each daughter cell that results from cell division will have a complete set of DNA molecules.
6. Review Why does DNA replicate before cell division?
DNA replication

10. How does DNA replicate? (contd.)
During replication, proteins separate the two strands of the DNA double helix creating replication forks.
The enzyme DNA polymerase then produces two new complementary strands of DNA, following the rules of base pairing.
(contd.)

11. How does DNA replicate?
Replication “bubbles” move in both directions along each chromosome until the entire DNA molecule has been replicated.
At the end of the S phase, each DNA molecule consists of one “old” strand and one “new” strand.

12. What are the rules of base pairing?
Hydrogen bonds create a nearly perfect fit between nitrogenous bases along the center of the DNA molecule.
These bonds form only between certain base pairs.
Adenine bonds with thymine; guanine bonds with cytosine.
7. Review What kind of bond holds the nitrogenous bases together along the DNA molecule?

13. How is the cell cycle controlled?
Movement through the cell cycle is subject to control by internal and external regulators.
Internal regulators ensure that a cell does not move from one phase to another until certain events have taken place.
External regulators respond to events outside the cell and direct cells to either speed up or slow down the cell cycle.
8. Infer A set of regulatory proteins prevents a cell from entering anaphase until all of its chromosomes are attached to the mitotic spindle. Are these regulatory proteins internal regulators or external regulators?

14. What role does the cell cycle have in the growth of organisms?
As an organism grows, its cells cannot simply get larger and larger. The reason is that as cells get bigger, their surface area to volume ratio decreases.
If a cell gets too big, its surface area is not large enough to get adequate oxygen and nutrients in and waste out through the cell membrane.
Cells must divide to accommodate the needs of a growing organism.

15. Why does the rate of the cell cycle vary among cell types?
Cells types lost or damaged to wear and tear move quickly through the cell cycle to provide replacements.
In adults, cells in skin and the digestive tract move through the cell cycle very quickly; cells in the nervous system divide very rarely.
During embryonic development, cells progress through the cell cycle very quickly, often dividing as frequently as every hour or two.
(contd.)

16. Why does the rate of the cell cycle vary among cell types?
Upon injury, cells often speed up their passage through the cell cycle to aid in healing. The timing of the cell cycle returns to normal once the injury heals.
9. Infer Which would be more damaging, an injury to the lining of the digestive tract or an injury to the spinal cord? Explain.
When a person breaks a bone, cells at the edges of the injury are stimulated to speed up the cell cycle so new cells can heal the break.