What is the cell cycle?

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
  cells nucleus divides during mitosis.
• The second step of the M phase is cytokinesis,
  during which the cells 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?

• 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.