Chapter 12
The Cell Cycle and
Mitosis
The Key Roles of Cell Division
Cell division functions in reproduction,
growth, and repair
Unicellular organisms (ex. Amoeba) will
divide to reproduce entire organisms
Cell division also will allows a multicellular
organism to develop from a single cell
The Key Roles of Cell Division
DNA is passed from one generation of
cells to the next without dilution.
-cell duplicates it DNA
- moves the 2 copies to opposite ends
of the cell
- and then splits into 2 daughter cells
The Key Roles of Cell Division
Concept 12.1
Cell Division distributes identical sets of
chromosomes to daughter cells
A cell’s genetic material is called its
genome
- prokaryote = single long DNA strand
- eukaryote = number of DNA
molecules
Concept 12.1
The replication and distribution of DNA is
manageable because it is packaged into
chromosomes
- the nuclei in human somatic cells
contain 46 chromosomes
- the nuclei in human gametes
contains 23 chromosomes
Concept 12.1
Concept 12.1
The DNA-protein complex is called the
chromatin and is a long thin fiber.
After the chromatin is duplicated, it will
prepare for division. It will condense
and coil up to form chromosomes.
Concept 12.1
Each duplicated chromosome has 2 sister
chromatids.
- each contains identical copies of the
chromosome’s DNA molecule
- they are connected together at the
centromere
Concept 12.1
0.5 µm
Chromosomes
Chromosome arm
Centromere
DNA molecules
Chromosome
duplication
(including DNA
synthesis)
Sister
chromatids
Separation of
sister chromatids
Centromere
Sister chromatids
Concept 12.2
The mitotic phase alternates with
interphase in the cell cycle
The mitotic (M) phase (mitosis and
cytokenesis) is the shortest part of the
cell cycle.
Interphase accounts for about 90% of
the cell cycle.
Concept 12.2
Interphase can be divided into subphases
- G1 (first gap), S (synthesis), and G2
(second gap)
- during subphases, cell grows by
producing proteins and organelles
- chromosomes are only duplicated
during the S phase
Concept 12.2
G1
S
(DNA synthesis)
G2
Concept 12.2
Mitosis is broken down into 4 subphases
- prophase, metaphase, anaphase, and
telophase
Prophase
- chromatin coil into chromosomes
- nucleoli disappears
- spindles begin to appear as centrosomes
move to the poles of the cell
Concept 12.2
The cell moves into prometaphase
- nuclear envelope fragments
- kinetochore attaches to forming
spindles
- cell prepares for metaphase
Concept 12.2
Concept 12.2
Metaphase
- centrosomes area at opposite poles
- chromosomes are on equator of cell,
the metaphase plate
Anaphase
- begins when the centromeres of the
chromosomes separate
Concept 12.2
- sister chromatids begin moving
toward opposite poles
- by the end, the poles have equal sets
of chromosomes
Telophase
- daughter nuclei form at the poles
- nuclear envelope begins to reform
Concept 12.2
- chromosomes become less tightly
coiled
-Cytokenesis, the division of the
cytoplasm, follows immediately
Concept 12.2
Concept 12.2
The mitotic spindle distributes
chromosomes to the daughter cells
-during interphase, the single
centrosome replicates to form 2
centrosomes; during the early stage of
mitosis, they separate and move toward
opposite poles helping the spindle fibers
Concept 12.2
Concept 12.2
Cytokenesis divides the cytoplasm
- in animals, cytokenesis occurs by the
formation of a cleavage furrow
- in plants the cleavage cannot occur
because of the cell wall; vesicles will
move to the center of the cell to form
the cell plate.
Concept 12.2
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
(a) Cleavage of an animal cell (SEM)
Daughter cells
(b) Cell plate formation in a plant cell (TEM)
Concept 12.2
The origins of mitosis are believed to be from
bacterial organisms of cell reproduction
- prokaryotes reproduce by binary fission
(“dividing in half”)
- prokaryotes do not have mitotic spindles;
instead, once the DNA replicates, the copies of
the region move apart rapidly
Concept 12.2
Origin of
replication
E. coli cell
Two copies
of origin
Origin
Cell wall
Plasma
membrane
Bacterial
chromosome
Origin
Concept 12.3
The cell cycle is driven by specific chemical
signals present in the cytoplasm
Sequential events of the cell cycle are directed
by a distinct cell cycle control system
- driven by a built in clock
- the cell cycle is regulated at certain
checkpoints by internal and external controls
Concept 12.3
Concept 12.3
The checkpoint is a control point where
stop and go-ahead signals can regulate
the cycle
- kinases, a type of regulatory protein
that activate or inactivate other
proteins, give the signals for G1 and G2
checkpoints
Concept 12.3
To be active the kinase must be attached
to a cyclin (kinases become cyclindependent kinases or Cdks)
- the activity of Cdks rises and falls with
changes in the [cyclin]
- first called MPF = “maturation
promoting factor” or “M-phasepromoting factor”
Concept 12.3
- when cyclins accumulate during G2,
MPF initiates mitosis
Internal and external cues help regulate
the cell cycle
- for cells to divide a growth factor, a
specific protein, is released to stimulate
cell division
Concept 12.3
Concept 12.3
The discovery of growth factors has led
us to understand density-dependent
inhibition of cell division
- when a cell population reaches a
certain density, the amount of growth
factors and nutrients needed for
division becomes insufficient for
increased growth
Concept 12.3
Most animal cells also exhibit anchorage
dependence
- to divide, the cell must be attached to a
substratum (ex. inside of a culture jar or
extracellular matrix of a tissue)
Concept 12.3
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
25 µm
25 µm
(a) Normal mammalian cells
(b) Cancer cells
Concept 12.3
Concept 12.3
Cancer cells have escaped from cell cycle
controls
- they do not respond to the control
mechanisms
- they do not stop dividing when growth
factors are depleted; don’t respond to
density dependant inhibition
Concept 12.3
- if cancer cells stop dividing, it is at
random points and not at the
checkpoints
Caner begins when a single cell tissue
undergoes a transformation
- if the cell evades destruction by the
immune system it may form a tumor
Concept 12.3
- if the abnormal cell remain at the
original site, it is called a benign
tumor
- a malignant tumor becomes
invasive enough to impair the functions
of organs
- the spread of cancer cells from the
original site is called metastasis
Concept 12.3
Concept 12.3