Chapter 11: The Cell Cycle and Cell Division
Introduction
11.1 How Do Prokaryotic and Eukaryotic Cells Divide?
The life cycle of an organism is closely linked to cell division.
Cell division is important in growth and repair of tissues in multicellular organisms, and
in the reproduction of all organisms.
Four events must occur for cell division:
• A reproductive signal initiates cell division
• Replication of DNA
• Segregation: distribution of DNA into two new cells
• Cytokinesis: separation of cellular material into the two new cells
In prokaryotes, binary fission results in two new single-celled organisms.
External factors such as nutrient concentration and environmental conditions are the
reproductive signals.
For many bacteria, abundant food supplies speed up the division cycle.
Most prokaryotes have one chromosome, a single molecule of DNA. Often forms a
circle, but is compacted and folded. Two important regions:
• ori—where replication starts (origin)
• ter—where replication ends (terminus)
Replication occurs as the DNA is threaded through a “replication complex” of proteins.
The ori regions move toward opposite ends of the cell, aided by special proteins.
When replication is complete, the daughter DNA molecules are segregated at opposite
ends.
In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell
divisions.
Cytokinesis begins by a pinching in of the plasma membrane; protein fibers form a ring.
New cell wall materials are synthesized, resulting in separation of the two cells.
In eukaryotes, signals for cell division are related to the needs of the entire organism.
Many cells in multicellular organisms become specialized and seldom divide.
Eukaryotes usually have many chromosomes; replication and segregation are more
intricate.
© 2014 Sinauer Associates, Inc.
, Newly replicated chromosomes are closely associated (sister chromatids).
Mitosis separates them into two new nuclei.
Cytokinesis proceeds differently in animal and plant cells (plants have cell walls).
Cells resulting from mitosis are genetically identical to the parent cell.
Meiosis is nuclear division in cells involved in sexual reproduction.
• The cells resulting from meiosis are not identical to the parent cells. It results in
new gene combinations.
11.2 How Is Eukaryotic Cell Division Controlled?
Cell cycle: period from one cell division to the next; divided into mitosis/cytokinesis and
interphase.
Interphase: nucleus is visible and cell functions, including DNA replication, occur;
begins after cytokinesis, ends when mitosis starts (M phase). Duration of interphase
is highly variable.
Interphase has three subphases: G1, S, and G2
• G1: between cytokinesis and S phase; chromosomes are single, unreplicated
structures.
• Duration of G1 is variable, from a few minutes to years. Some cells enter a resting
phase (G0).
• At the G1-to-S transition the commitment is made to DNA replication and
subsequent cell division. Now called the restriction (R) point.
• S phase: DNA replicates; sister chromatids remain together.
• G2: cell prepares for mitosis (e.g., by synthesizing structures to move the
chromatids).
Specific signals trigger the transition from one phase to another.
Identification of these signals came from cell fusion experiments.
For example, cells in the S phase produce a substance that activates DNA replication.
The signals act through cyclin-dependent kinases (Cdk’s).
Protein kinases catalyze transfer of a phosphate group from ATP to a protein
(phosphorylation). The shape and function of the protein changes. Cdk’s
play important roles in the cell cycle.
Cdk is activated by binding to cyclin (allosteric regulation); this alters its shape and
exposes the active site.
There are many different cyclin–cdk complexes acting at different stages of the cell cycle.
At the G1-S transition:
© 2014 Sinauer Associates, Inc.
Introduction
11.1 How Do Prokaryotic and Eukaryotic Cells Divide?
The life cycle of an organism is closely linked to cell division.
Cell division is important in growth and repair of tissues in multicellular organisms, and
in the reproduction of all organisms.
Four events must occur for cell division:
• A reproductive signal initiates cell division
• Replication of DNA
• Segregation: distribution of DNA into two new cells
• Cytokinesis: separation of cellular material into the two new cells
In prokaryotes, binary fission results in two new single-celled organisms.
External factors such as nutrient concentration and environmental conditions are the
reproductive signals.
For many bacteria, abundant food supplies speed up the division cycle.
Most prokaryotes have one chromosome, a single molecule of DNA. Often forms a
circle, but is compacted and folded. Two important regions:
• ori—where replication starts (origin)
• ter—where replication ends (terminus)
Replication occurs as the DNA is threaded through a “replication complex” of proteins.
The ori regions move toward opposite ends of the cell, aided by special proteins.
When replication is complete, the daughter DNA molecules are segregated at opposite
ends.
In rapidly dividing prokaryotes, DNA replication occupies the entire time between cell
divisions.
Cytokinesis begins by a pinching in of the plasma membrane; protein fibers form a ring.
New cell wall materials are synthesized, resulting in separation of the two cells.
In eukaryotes, signals for cell division are related to the needs of the entire organism.
Many cells in multicellular organisms become specialized and seldom divide.
Eukaryotes usually have many chromosomes; replication and segregation are more
intricate.
© 2014 Sinauer Associates, Inc.
, Newly replicated chromosomes are closely associated (sister chromatids).
Mitosis separates them into two new nuclei.
Cytokinesis proceeds differently in animal and plant cells (plants have cell walls).
Cells resulting from mitosis are genetically identical to the parent cell.
Meiosis is nuclear division in cells involved in sexual reproduction.
• The cells resulting from meiosis are not identical to the parent cells. It results in
new gene combinations.
11.2 How Is Eukaryotic Cell Division Controlled?
Cell cycle: period from one cell division to the next; divided into mitosis/cytokinesis and
interphase.
Interphase: nucleus is visible and cell functions, including DNA replication, occur;
begins after cytokinesis, ends when mitosis starts (M phase). Duration of interphase
is highly variable.
Interphase has three subphases: G1, S, and G2
• G1: between cytokinesis and S phase; chromosomes are single, unreplicated
structures.
• Duration of G1 is variable, from a few minutes to years. Some cells enter a resting
phase (G0).
• At the G1-to-S transition the commitment is made to DNA replication and
subsequent cell division. Now called the restriction (R) point.
• S phase: DNA replicates; sister chromatids remain together.
• G2: cell prepares for mitosis (e.g., by synthesizing structures to move the
chromatids).
Specific signals trigger the transition from one phase to another.
Identification of these signals came from cell fusion experiments.
For example, cells in the S phase produce a substance that activates DNA replication.
The signals act through cyclin-dependent kinases (Cdk’s).
Protein kinases catalyze transfer of a phosphate group from ATP to a protein
(phosphorylation). The shape and function of the protein changes. Cdk’s
play important roles in the cell cycle.
Cdk is activated by binding to cyclin (allosteric regulation); this alters its shape and
exposes the active site.
There are many different cyclin–cdk complexes acting at different stages of the cell cycle.
At the G1-S transition:
© 2014 Sinauer Associates, Inc.
