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CHAPTER 12
THE CELL CYCLE
Introduction
Reproduction =continuity of life
In mammals, heat loss through the
skin occurs naturally. Which of the
following mammals would lose heat
faster………... ….….Why?
Calculate the Surface Area:Volume
Ratios for the cubes below. (SA:V)
SA= l x w x 6
V= l x w x h
#1
#2
1cm
2cm
SA= 6 cm2
SA:V = 3:1
V= 1 cm3
SA:V = 6:1
#3
3cm
SA:V = 2:1
Now, assume each CUBE is one
CELL… As the “cell” gets
larger…………
•Does its surface area get greater?
•Does its volume get greater?
•Does it’s surface area:volume ratio
get greater?
•Does increased size become an
advantage to the cell? Why or why
not?
Think back to the Elephant and the
Mouse……do you want to change
your answer? Which animal will
lose heat the fastest?
If you said the mouse, you are right!!!
The mouse would lose heat faster
due to it’s Surface Area:Volume
Ratio
When do cells divide?
1) During Growth - adult/somatic ‘stem
cells’ can form new cells/tissue - birth to
death
 2)
Embryonic development of a multicellular
organism following sexual reproduction (from
embryonic stem cells)
 3) Repair and renew cells that die from normal
wear and tear or accidents.
Note:Repair can be Regeneration
also
4) Reproduction by Mitosis - ASEXUAL
REPRODUCTION - a way to produce
offsprings in simple unicellular organisms like
bacteria (binary fission), amoeba, and in
multicellular PLANTS (cuttings)!
 Human
somatic cells (body cells) have 46
chromosomes
 Human gametes (sperm or eggs) have 23
chromosomes
Reproduction =continuity of life
CELL
CYCLE
Chromosome contains genes,
which are long sequences of DNA
 Cell division
distributes
identical sets of
chromosomes to
daughter cells
3 m of DNA to be divided and
GENOME = all of a cell’s
DNA
separated
in each round of cell
CHROMATIN = DNAdivision!!!!!!
+ packaging (proteins)
CHROMOSOME = DNA strands + protein;
appear during mitosis as threads
GENE = segment
3.2 billion base pairs
of DNA
Chromatin - thin, active
structure of DNA
First level of DNA folding
Every 200 nucleotides of DNA wrap
around a core of Histone proteins giving
a “beads on a string” look
Chromosome
The highly folded DNA
structure - an inactive form
of DNA
Chromatin lengths of 50,000100,000 nucleotides are
looped together by
nonhistone proteins
Chromosomes pack DNA into
final structure measuring
5µm long x ~1µm wide
What is the result of a successful cell
division process?
FIDELITY OF DNA REPLICATION!!
 Each
duplicated chromosome consists of two
sister chromatids which contain identical
copies of the chromosome’s DNA.
 As they condense, the
region where the strands
connect shrinks to a
narrow area, is the
centromere. The kinetochore
is where the spindle attaches.
 Why do chromosomes
duplicate to make sister
pairs?
So that they can divide
and distribute during Fig. 12.3
Mitosis
Vocab review: Sister chromatids = identical ‘post-DNA replication structures’
joined by the centromere
Chromatids - arms of a chromosome
 How
many chromosomes are there in
each of the three diagrams below? How
many chromatids?
1 Centromere = 1 chromosome
4 chromosomes……. 4 chromatids
4 chromosomes……. 8 chromatids
(after DNA has replicated)
8 chromosomes……. 8 chromatids
 The
process of the formation of the two
daughter nuclei, mitosis, is usually followed by
division of the cytoplasm, cytokinesis. They are
both part of the MITOSIS CELL CYCLE
PHASE
The mitotic phase alternates with
interphase in the cell cycle
 The
mitotic (M) phase of the cell cycle
alternates with the much longer interphase.
 The
M phase includes mitosis and cytokinesis.
 Interphase accounts
for 90% of the cell
cycle.
A Cell’s lifetime of
growth & division can
be referred to as a
Cell Cycle
Fig. 12.4
The Cell Cycle -Phases
For a 24 hour cycle,
M Phase lasts about
1 hour.
G2
DNA
synthesis
S
Mitosis
G0
G1
Cells not
cycling
Interphase
Has 3 subphases:
G1 –first gap phase
(growth)
S phase (DNA
replication)
G2 –second gap
phase (preparation for
cell division)
Interphase – G1
The cell doubles
in size, and its
enzymes,
ribosomes,
mitochondria and
other cytoplasmic
molecules and
structures also
increase in number
Interphase – S
The DNA
replicates and
sister chromatids
(exact copies) are
formed
Interphase – G2
 Centriole division is
completed in animal
cells, cytoplasm makes
proteins in
preparation for
mitosis
Karyotype: Chromosomes
(23 pairs in humans)
The Cell Cycle -Control: 3 important
checkpoints - if they are not crossed, cell cannot
divide
Most important – cells can arrest here and
enter G0 (skeletal muscle cells, brain cells)
Before anaphase
- All chromatids
have to
connected to the
spindle fiber
Some cells arrest here - heart cell;
The Cell Cycle -Control
G1- Most important – growth factors,
Cdk (cyclin dependent kinase), Cyclins
– APC
(Anaphase
promoting
complex)
– MPF (Mitosis promoting
factor), (Kinase + Cyclins)
How do kinases work?
Can activate many proteins/enzymes needed for each phase by
phosphorylating it (phosphorylation cascade - remember this?)
 Cyclin levels
rise sharply throughout
interphase, then fall abruptly during mitosis.
 Peaks in the activity of one cyclin-Cdk
complex, MPF, correspond to peaks in cyclin
concentration.
CDK is around,
Cyclin is
Fig. but
12.14a
degraded:
Copyright ©Why?
2002 Pearson Education, Inc., publishing as Benjamin Cummings
Cyclin controls Cdk
CANCER – cells divide without
arresting in G1 or G2
Cells that do not divide
Nerve, & Heart Cells
Cells that do not normally divide
Cells that divide constantly
Liver Cells
Skin cells,
Sperm Cells,
Bone Marrow cell
Benign tumors are not cancerous. They:
•can usually be removed
•do not come back in most cases
•do not spread to other parts of the body and the
cells do not invade other tissues
Malignant tumors are cancerous. They:
•can invade and damage nearby tissues and
organs
•metastasize (cancer cells break away from a
malignant tumor and enter the bloodstream or
lymphatic system to form secondary tumors in
other parts of the body)
 Each
of us inherited 23 chromosomes from each
parent: one set in an egg and one set in sperm.
MEIOSIS =
GAMETE
FORMATION
46 chromosomes
23 chromosomes
46 chromosomes
23 chromosomes
46 chromosomes
MITOSIS =
SOMATIC CELL
DIVISION
 Mitosis
is a continuum of changes.
 Mitosis
is usually broken into five subphases:
prophase,
 prometaphase,
 metaphase,
 anaphase, and
 telophase.

Interphase: chromosomes have been
duplicated but loosely packed (euchromatin).
 The centrosomes (contain centrioles in animal
cells) have been duplicated and begin to
organize microtubules into an aster (“star”).
Interphase
Prophase - the
chromosomes are tightly
coiled, with sister
chromatids joined
together.
 The nucleoli disappear.
 The mitotic spindle begins
to form; centrosomes
(plant)/centrioles (animal)
move toward opposite
ends (poles) of the cell.
Prophase
 Prometaphase,
the nuclear envelope
fragments and microtubules from the
spindle interact with the chromosomes.
 Microtubules from one
pole attach to one of two
kinetochores, special
regions of the centromere,
while microtubules from
the other pole attach to
the other kinetochore.
 Pole to pole fibres are
also present
 Metaphase:
Sister chromatids are all
arranged at the metaphase plate, an
imaginary plane equidistant between the
poles
Metaphase
 Anaphase-
the centromeres divide,
separating the sister chromatids.
 Each is now pulled toward the pole to which it
is attached by spindle fibers.
 Movement of sister chromatids involves
elongation of the pole to pole fibers and
shortening of the pole to kinetochore fibers
 By the end, the two poles have same number
of chromosomes.
Anaphase
 Anaphase-
the centromeres divide,
separating the sister chromatids.
 Each is now pulled toward the pole to which it
is attached by spindle fibers.
 Movement of sister chromatids involves
elongation of the pole to pole fibers and
shortening of the pole to kinetochore fibers
 By the end, the two poles have same number
of chromosomes.
Anaphase
Telophase: cell continues to elongate as pole
to pole spindle fibers from each centriole
push off each other.
 Two nuclei begin for form, surrounded by
the fragments of the parent’s nuclear
envelope.
 Chromatin becomes
less tightly coiled.
 Cytokinesis, division
of the cytoplasm
begins.
Telophase
Cytokinesis
Prophase
Anaphase
Metaphase
Interphase
Telophase
Interphase
Metaphase
Prophase
Telophase
Anaphase
Cytokinesis
Interphase
Anaphase
Prophase
Telophase
Metaphase
Cytokinesis
Microscope observations goals:
a) Draw pencil pictures and Label all
parts
b) Describe what you see in the phase.
How many cells did you see in that phase?
Make a % estimate of how long the cell
spends in each phase based on how many
cells you see at that phase.
c) Compare plant (onion root tip) and
animal cell (fish blastula) mitosis
Plant cell Mitosis
Animal cell Mitosis
Nucleolus
Chromatin
Nuclear
membrane intact
Has Cell wall,
Has No Cell wall,
No centrioles, only
Centrioles present
centrosome
Interphase
Prophase
Metaphase
Anaphase
Telophase and
Cytokinesis
Cytokinesis-2 Daughter Cells
Fig. 12.5 right
Mitotic Spindle Movement
 The
mitotic spindle has fibers composed of
microtubules and associated proteins
 The tubulin comes from partial disassembly
of the cytoskeleton.
 The spindle fibers elongate by incorporating
more subunits of the protein tubulin.
 Assembly
of the spindle microtubules starts
in the centrosome.
 The
centrosome (microtubule-organizing
center) of animals has a pair of centrioles at
the center.
.
Microtubules disassemble at the kinetochores
causing spindle fibres to shorten in length and draw
sister chromatids to the poles.
Fig. 12.7a
 Experiments
support the
hypothesis that
spindle fibers
shorten during
anaphase from
the end attached
to the
chromosome
(kinetochore),
not the centriole.
 Nonkinetichore
pole to pole microtubules
are responsible for lengthening the cell
along the axis defined by the poles.
Cytokinesis divides the
cytoplasm:
 Cytokinesis,
division of
the cytoplasm,
typically follows
mitosis.
 In animals, the first
sign of cytokinesis
(cleavage)
is the appearance of a
cleavage furrow in
the cell surface near
the old metaphase
plate.
Fig. 12.8a
 On
the cytoplasmic
side of the cleavage
furrow (a contractile
ring of actin
microfilaments and
the motor protein
myosin form).
 Contraction of the
ring pinches the cell
in two.
Fig. 12.8a
 Cytokinesis
in plants, which have
cell walls, involves a completely
different mechanism.
 During telophase, vesicles
from the Golgi coalesce at
the metaphase plate,
forming a cell plate.
 The
plate enlarges until its
membranes fuse with the
plasma membrane.
Fig. 12.8b
Fig. 12.9
Mitosis in eukaryotes may have evolved
from binary fission in bacteria
Fig. 12.10
Fig. 12.11
Identify the following phases