Chapter 12
The Cell Cycle
The Key Roles of Cell Division
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The ability of organisms to produce more of their own kind is the one
characteristic that best distinguishes living things from nonliving
things.
The continuity of life is based on the reproduction of cells, or cell
division.
The division of one prokaryotic cell reproduces an entire organism.
Cell division also enables multicellular eukaryotes to develop from a
single cell. (fertilized egg)
Cell division also functions in renewal and repair, replacing cells that
die from normal wear and tear or accidents.
– the cell division process is an integral part of the cell cycle
• cell division results in genetically identical daughter cells
Cellular Organization of the Genetic Material
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A cell's endowment of DNA, its genetic information is
called its genome.
Although a prokaryotic genome is often a single DNA
molecule, eukaryotic genomes usually consist of a
number of DNA molecules.
The overall length of DNA in a eukaryotic cell is
enormous.
--the DNA molecules in a cell are packaged into
chromosomes
Cells duplicate their genetic material before they
divide, ensuring that each daughter cell receives an
Eukaryotic Chromosomes
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Eukaryotic chromosomes consist of
chromatin, a complex of DNA and protein that
condenses during cell division
• In animals somatic cells have two sets of
chromosomes and gametes have one set of
chromosomes
– human somatic cells have 46 chromosomes
and gametes have 23 chromosomes
Distribution of Chromosomes
During Cell Division
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In preparation for cell division DNA is replicated and
the chromosomes condense
Each duplicated chromosome has two sister
chromatids, which separate during cell division
Each sister chromatid has a centromere, a region
containing specific DNA sequences where the
chromatid is attached most closely to its sister
chromatid.
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Eukaryotic cell division consists of two parts
– Mitosis, the division of the nucleus
– cytokinesis, the division of the cytoplasm
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Cytokinesis usually occurs immediately after Mitosis
• In meiosis sex cells are produced after a reduction in
chromosome number.
Meiosis in humans occurs only in the gonads (ovaries
and testes).
Phases of the Cell Cycle
The cell cycle consists of two phases:
– the mitotic phase
– interphase
• The mitotic phase alternates with interphase in the cell
cycle.
• Interphase can be divided into subphases:
– G1 phase
– S phase
– G2 phase
Mitosis
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The mitotic phase is made up of mitosis and cytokinesis.
Interphase: duplication of genetic material, ends with visible chromosomes
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Mitosis is conventionally broken down into five stages:
– Prophase: chromatin coils and condenses into chromosomes; mitotic spindle
forms from MTOC’s; nucleoli disappear
– Prometaphase: nuclear membrane dissolves; spindle microtubules attach to
the chromosomes at the kinetochores
– Metaphase: the spindle is completely formed; chromosomes align in single
file with centromeres on metaphase plate
– Anaphase: the chromatids separate and migrate to spindle poles
– Telophase: reverse of prophase
– Cytokinesis: occurs after Mitosis
The Mitotic Spindle: A Closer Look
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The mitotic spindle is an apparatus of
microtubules that controls chromosome
movement during mitosis
• The spindle arises from the centrosomes and
includes spindle microtubules and asters
• Some spindle microtubules attach to the
kinetochores of chromosomes and move the
chromosomes to the metaphase plate
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In anaphase, sister chromatids separate and move along
the kinetochore microtubules toward opposite ends of the
cell
– movement of the chromosomes is driven by addition or
subtraction of protein subunits to the kinetochore end of
spindle microtubules
Nonkinetechore microtubules from opposite poles overlap
and push against each other, elongating the cell
• In telophase genetically identical daughter nuclei form at
opposite ends of the cell
Cytokinesis
Cytokinesis differs in plants and animals
– in animals, a ring of microfilaments contracts around periphery of
cell
• this forms a cleavage furrow that eventually divides the
cytoplasm
– in plants, vesicles containing cell wall material collect on spindle
equator
• the vesicles fuse from the inside out forming cell plate which
gradually develops into a new cell wall between new cells
• membranes surrounding the vesicles fuse to form new parts of
the plasma membranes
Mitosis in a plant cell
Binary Fission
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Prokaryotes (bacteria) reproduce by a type of cell
division called binary fission.
Binary Fission means “division in half” refers to this
process and to the asexual reproduction of singlecelled eukaryotes.
• In binary fission the bacterial chromosome replicates
– the two daughter chromosomes actively move apart
Bacterial cell division by binary fission
Cell cycle control system
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Molecules present in the cytoplasm regulate progress
through the cell cycle
The sequential events of the cell cycle are directed by
a distinct cell cycle control system, which is similar
to a clock
• There are three major check points in cell cycle:
–
-G1 of interphase
– G2 of interphase
– M phase
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The clock has specific checkpoints where the cell
cycle stops until a go-ahead signal is received
– release of growth factor at each of these checkpoints
allows cell cycle to continue
The Cell Cycle Clock: Cyclins and CyclinDependent Kinases
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Rhythmic fluctuations in the abundance and activity of
cell cycle control molecules pace the sequential
events of the cell cycle.
Two types of regulatory proteins are involved in cell
cycle control
– cyclins and cyclin-dependent kinases (Cdks)
• The activity of cyclins and Cdks fluctuates during the
cell cycle
Stop and Go Signs:Internal and External Signals
at the Checkpoints
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Both internal and external signals control the cell cycle
checkpoints
– Growth factor, is a protein released by certain cells
that stimulates other cells to divide
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The effect of an external physical factor on cell division is
clearly seen in density-dependent inhibition.
In density-dependent inhibition crowded cells stop
dividing
• Most animal cells exhibit anchorage dependence in which
they must be attached to a substratum to divide
Loss of Cell Cycle Controls in
Cancer Cells
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Cancer cells do not respond normally to the body’s control
mechanisms
– cancer cells do not exhibit density-dependent inhibition
– some do not show anchorage dependence
• Uncontrolled cell division forms tumors
– benign tumor-no metastasis
– malignant tumor-metastasize
• named for organ or tissue of origin
A benign tumor becomes malignant when cancerous cells from
the tumor mass spread to new sites and continue to proliferate
– movement of the cancer cells is mediated by either blood or
lymph systems
COMMON TREATMENTS FOR
CANCER
Radiation
– disrupts normal processes of cell division
• cancer cells are more susceptible
• Chemotherapy
– disrupts cell division