The Cell Cycle & Mitosis
Chapter 12
Limits to Size

Why can’t organisms just be one giant
cell?
–
Diffusion cannot occur quickly and efficiently if
the distances involved become too large.
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–
wastes would collect inside the cell and poison it
nutrients could not reach organelles in time, so cells
would die
Information overload would occur.
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DNA does not make copies as a cell grows – what it
starts with is all that it has
Must be enough DNA blueprint to allow for protein
production
Ratio of Surface Area to Volume in Cells
Volume is a three-dimensional unit – length X width X height
Surface Area is a two-dimensional unit – length X width
SO VOLUME INCREASES AT A FASTER RATE THAN DOES SURFACE AREA…Thus,
SURFACE AREA IS THE LIMITING FACTOR IN THE SIZE OF A CELL
Functions of Cell Division
1.
Reproduction of cells
–
–
2.
Growth and Development of
Organisms
–
3.
all cells come from pre-existing cells
results in two identical cells except for size
tadpoles become frogs, ivy vines get longer
Tissue renewal
–
skin cells are being replaced, cuts and bruises heal
Cell Division: Distributing Identical Sets of
Chromosomes to Daughter Cells
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Key to cell division is the copying and equal
separation of chromosomes:
–
Chromosomes are carriers of the genetic material
that is copied and passed from generation to
generation:
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made up of DNA and protein
cells of every organism have a specific number of
chromosomes
not visible in cells except during cell division
Cell Division

Before it becomes too large, a growing somatic cell
divides forming 2 “daughter” cells by a process known
as cell division
–
Each daughter cell gets 1 complete set of genetic information
during cell division and therefore will be IDENTICAL TO THE
MOTHER CELL!
A somatic cell is a non sex cell!
Chromosomes

Spend most of their time as Chromatin – long
strands of DNA that are wrapped around proteins
and appear hazy and unorganized through the
microscope – this loose arrangement is
necessary for copying to occur.
–

When a cell gets ready to divide, the chromatin coils
and condenses into what we call Chromosomes.
When visible, chromosomes consist of two
identical Sister Chromatids that are joined in the
center at the Centromere
sister chromatids
Chromosomes

In eukaryotic cells, the genetic
information that is passed on from
one generation of cells to the next is
carried by chromosomes
–
–

Chromosomes are made of DNA
The cells of every organism have a
specific number of chromosomes
(human cells have 46
chromosomes)
Before cell division, each
chromosome is replicated and
consists of 2 identical “sister”
chromatids
–
Each pair of chromatids is attached
at an area called the centromeres
centromeres
Figure 12.3 Chromosome duplication and distribution during mitosis
A duplicating chromosome
consists of 2 sister
chromatids, which narrow at
their centromeres. The DNA
molecules of sister
chromatids are identical.
Chromosomes normally exist
in the highly condensed state
shown here only during the
process of mitosis.
The Cell Cycle


During the cell cycle, a cell grows, prepares for
division, and divides to form 2 daughter cells,
where each one of which begins a new cycle.
The 5 phases of the cell cycle are:
1.
Interphase – period of rest between cell division
2.
5.
Prophase
Metaphase
Anaphase
Telophase
6.
Cytokinesis (division of the cytoplasm)
3.
4.
phases of nuclear
division (MITOSIS)
Events of the Cell Cycle
nuclear
division
M Phase
cell divides
Events of the Cell Cycle
Interphase is divided into 3 phases:
1.
2.
3.
G1 – cell growth
S – DNA replication
G2 – preparation for Mitosis
•
During Interphase, chromosomes are in their “uncondensed” form and are
called chromatin
Mitosis (nuclear division) is the division of the nucleus and it occurs
in 4 phases:
1.
2.
3.
4.
P = prophase – chromatin condenses into chromosomes, the
centrioles separate & nuclear membrane breaks down
M = metaphase – chromosomes line up across center of cell and
each chromosome is connected to a spindle fiber at its centromere
A = anaphase – sister chromatids separate into individual
chromosomes and are pulled apart
T = telophase – chromosomes gather at opposite ends of the cell
and 2 new nuclear membranes form around them
Cytokinesis – division of cytoplasm
Concept Map of all events of Cell Cycle
Cell Cycle
includes
G1 phase
Go to
Section:
Interphase
M phase
(Mitosis)
is divided into
is divided into
S phase
G2 phase
Prophase
Metaphase
Anaphase
Telophase
Interphase: G1, S, and G2

Interphase is very long (cells spend most of time
here)…
–
G1 phase – cell growth; cells increase in size and synthesize
new proteins and organelles
–
S phase – chromosomes are replicated and the synthesis of
DNA molecules takes place; key proteins associated with the
chromosomes are synthesized during this time
–
G2 phase – shortest of 3 phases; many of the organelles and
molecules required for cell division are produced
INTERPHASE
Nucleus well defined bounded by nuclear envelope.
Easily identifiable nucleolus.
Genetic material in uncondensed form of chromatin. – chromosomes cannot be
seen.
M phase: Mitosis - PMAT
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Prophase
Metaphase
Anaphase
Telophase
http://www.sumanasinc.com/webcontent/a
nimations/content/mitosis.html
Prophase: 1st and Longest Phase

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Chromatin coils and condenses –
becomes visible as chromosomes;
centrioles separate and take up positions
on opposite sides of the nucleus;
chromosomes become attached to spindle
fibers;
nucleolus disappears, nuclear membrane
breaks down
PROPHASE
Prometaphase
Metaphase: Shortest Phase


Chromosomes line up in center of cell
along metaphase plate
For each chromosome, the kinetochores
of the sister chromatids are attached to
microtubles coming from opposite poles of
the cell
METAPHASE
Figure 12.6 The mitotic spindle at metaphase
Anaphase
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Anaphase begins suddenly when the paired
centromeres that join the sister chromatids separate
from each other.
NOW EACH CHROMATID IS A SEPARATE
CHROMOSOME….they begin moving toward
opposite poles of the cell.
Chromosomes continue to move until they have
separated into two groups near the poles of the
spindle.
Anaphase is over when the chromosomes stop
moving!
ANAPHASE
Telophase: Final Phase of Mitosis
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Two daughter nuclei form at the two poles of the
cell.
Chromosomes begin to relax back down into
chromatin.
Nuclear envelope re-forms around each cluster
of chromatin.
Spindle begins to break apart and nucleolus
reappears in each daughter cell
MITOSIS IS NOW COMPLETE, BUT NOT
CELL DIVISION!
TELOPHASE
Figure 12.5x Mitosis
Cytokinesis

Division of the cytoplasm itself.
–
Can take place in a number of ways:
animal cells – “draw-string” effect forms
cleavage furrow (which pinches the cell into
two parts)
 in
plant cells – cell plate forms from inside
out, and cell wall begins to appear
 in
CYTOKINESIS
Figure 12.8 Cytokinesis in animal and plant cells
Figure 12.5 The stages of mitotic cell division in an animal cell:
G2 phase; Prophase; Prometaphase
Figure 12.5 The stages of mitotic cell division in an animal cell:
Metaphase; Anaphase; Telophase and Cytokinesis.
How do cells know when to divide?
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Closeness of neighboring cells.
Presence of proteins called CYCLINS:
–
2 types:
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internal regulators
external regulators
Close Contact Between Cells
WHEN CELLS COME INTO CONTACT WITH
OTHER CELLS, THEY STOP GROWING!
Cyclins

Regulate the timing of the cell cycle in
eukaryotic cells
–
TYPES:

Internal regulators: respond to events inside the cell;
–

Ex. Don’t begin mitosis until all chromosomes are copied
External regulators: respond to events outside the cell;
direct cells to speed up or slow down the cell cycle.
–
Ex. Growth factors that stimulate the growth and division of
cells are external regulators
(REFER TO YOUR BOOK AND KNOW SOME SPECIFIC EXAMPLES)
Uncontrolled Cell Growth

If growth is not controlled, then crowding and
even tissue damage may result:
–
Ex. Cancer: disorder where body’s own cells lose the
ability to control growth; can crowd and even damage
tissue in surrounding area – forms tumor.
–
Benign vs. malignant tumors – benign are localized
and not spreading; malignancies are capable of
breaking off and starting up in another location –
metastasis.
Figure 12.17 The growth and metastasis of a malignant breast tumor
Figure 12-17x1 Breast cancer cell
Figure 12-17x2 Mammogram: normal (left) and cancerous (right)
Treatments for Cancer:

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surgery
radiation therapy – expose to x-rays
–
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releases free ions/free radicals, these attach to DNA,
prevents cell division
chemotherapy – causes nausea, lowering of
immune system due to healthy cells that are affected
–
–
venchristine – attaches to mitotic spindle
methotrexate – anti-metabolite attaches to DNA, prevents
DNA from replicating
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used to treat leukemia, brain tumors, testicular tumors