In eukaryotes, heritable
information is passed to the
next generation via processes
that include the cell cycle and
mitosis or meiosis plus
fertilization
Cell Cycle and Mitosis 8.1 to 8.11
All the DNA in a cell constitutes the cell’s
genome
 A genome can consist of a single DNA
molecule (common in prokaryotic cells) or
a number of DNA molecules (common in
eukaryotic cells)
 DNA molecules in a cell are packaged into
chromosomes

Genetic information is stored and
transmitted through DNA
Fig. 12-3
20 µm




Every eukaryotic species has a characteristic
number of chromosomes in each cell nucleus
Somatic cells (nonreproductive cells) have
two sets of chromosomes - DIPLOID
Gametes (reproductive cells: sperm and
eggs) have half as many chromosomes as
somatic cells - HAPLOID
Eukaryotic chromosomes consist of
chromatin, a complex of DNA and protein
that condenses during cell division
Genetic Information is stored and
transmitted through DNA
Fig. 12-4
0.5 µm
Chromosomes
Chromosome arm
Centromere
DNA molecules
Chromosome
duplication
(including DNA
synthesis)
Sister
chromatids
Separation of
sister chromatids
Centromere
Sister chromatids
The cell cycle is a complex set of stages
that is highly regulated with
checkpoints, which determine the
ultimate fate of the cell

Majority of the cell cycle

Time when a cell’s metabolic activity is
very high and the cell performs various
functions

3 stages
◦ G1 – cell growth
◦ S – DNA replication (Synthesis of DNA)
◦ G2 – prepare for mitosis
Interphase

About 10% of the cell cycle

2 stages
◦ Mitosis – nuclear division
◦ Cytokinesis – cytoplasm division

Results in two genetically identical cells
M Phase (mitotic phase)
The cell cycle is directed by
internal controls or checkpoints.
•
For many cells, the G1 checkpoint seems
to be the most important one
•
If a cell receives a go-ahead signal at the
G1 checkpoint, it will usually complete the
S, G2, and M phases and divide
•
If the cell does not receive the go-ahead
signal, it will exit the cycle, switching into
a nondividing state called the G0 phase
Cell Cycle Checkpoints

Two types of regulatory proteins are involved
in cell cycle control: cyclins and cyclindependent kinases (Cdks)

The activity of cyclins and Cdks fluctuates
during the cell cycle

MPF (maturation-promoting factor) is a
cyclin-Cdk complex that triggers a cell’s
passage past the G2 checkpoint into the M
phase
Cell Cycle Checkpoints
Fig. 12-17
M
G1
S
G2
M
G1
S
G2
M
G1
MPF activity
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin concentration during
the cell cycle
Degraded
cyclin
G2
Cdk
checkpoint
Cyclin is
degraded
MPF
Cyclin accumulation
Cdk
Cyclin
(b) Molecular mechanisms that help regulate the cell cycle

An example of an internal signal is that
kinetochores not attached to spindle
microtubules send a molecular signal that
delays anaphase

Some external signals are growth factors,
proteins released by certain cells that
stimulate other cells to divide

For example, platelet-derived growth factor
(PDGF) stimulates the division of human
fibroblast cells in culture
Internal and External signals provide
stop-and-go signs at the checkpoints
Fig. 12-18
Scalpels
Petri
plate
Without PDGF
cells fail to divide
With PDGF
cells proliferate
Cultured fibroblasts
10 µm
Fig. 12-19
Anchorage dependence
Density-dependent inhibition
Density-dependent inhibition
(a) Normal mammalian cells
25 µm
25 µm
(b) Cancer cells

Mitosis is conventionally divided into five
phases:
◦ Prophase
◦ Prometaphase
◦ Metaphase
◦ Anaphase
◦ Telophase

Cytokinesis is well underway by late
telophase
Mitosis passes a complete genome from
the parent cell to the daughter cell
Fig. 12-6b
G2 of Interphase
Chromatin
Centrosomes
(with centriole(duplicated)
pairs)
Prophase
Early mitotic Aster Centromere
spindle
Nucleolus Nuclear Plasma
envelope membrane
Chromosome, consisting
of two sister chromatids
Prometaphase
Fragments Nonkinetochore
of nuclear
microtubules
envelope
Kinetochore
Kinetochore
microtubule
Fig. 12-7
Aster
Centrosome
Microtubules
Chromosomes
Sister
chromatids
Metaphase
plate
Kinetochores
Centrosome
1 µm
Overlapping
nonkinetochore
microtubules
Kinetochore
microtubules
0.5 µm
Fig. 12-6d
Metaphase
Anaphase
Metaphase
plate
Spindle
Centrosome at
one spindle pole
Telophase and Cytokinesis
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Fig. 12-8b
Chromosome
movement
Kinetochore
Microtubule
Motor
protein
Chromosome
Tubulin
Subunits
Fig. 12-9
Cleavage furrow
Contractile ring of
microfilaments
100 µm
Daughter cells
(a) Cleavage of an animal cell (SEM)
Vesicles
forming
cell plate
Wall of
parent cell
Cell plate
1 µm
New cell wall
Daughter cells
(b) Cell plate formation in a plant cell (TEM)

What must occur before mitosis?
◦ DNA replication
◦ Pass G2 checkpoint

What is “checked” at the ‘M’ checkpoint?
◦ Chromosome attachment to microtubule

What follows mitosis?
◦ Cytokinesis

What are the products of the mitosis?
◦ 2 genetically identical cells

Why do we need mitosis?
◦ Growth, repair, asexual reproduction
Summary of Mitosis