AS CIE Cell And Nuclear Division (Mitosis And Meiosis)

E: Cell and Nuclear Division
Plan
1 Introduction:
1.1 Why the need for cell division ?
1.2 Why the need for nuclear division ?
1.3 The Cell Cycle
2 Mitotic phase:
2.1 Prophase
2.2 Metaphase
2.3 Anaphase
2.4 Telophase
2.5 Cytokinesis
2
1
3
4
5
3 Uncontrolled Cell division:
3.1 Mechanism
3.2 Factors causing cancer
4 Meiosis:
4.1 Why the need for meiosis ?
4.2 Mitosis vs. Meiosis
5 Conclusions and Overviews:
Xavier DANIEL, Ph.D.
AS

 All cells come from pre-existing cells
 It is necessary to replace worn out cells in
multicellular organisms
 It is required for growth in multicellular organisms
An increase in size will require an increase in surface
area to volume ration
Cell division subdivides the cytoplasm into small units
(cells) surrounded by plasma membranes
 It is necessary for asexual reproduction in
unicellular or multicellular organisms
Why the need for cell division ?
1
AS

1
Different cells divide at different rates
Most mammalian cells
= 12-24 hours
Some bacterial cells
= 20-30 minutes
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1
All cells are only allowed to complete a certain number of
divisions
Then they die (apoptosis = programmed cell death)
How does cell division change over a lifetime?
 Childhood = cell division > cell death
 Adulthood = cell division = cell death
 The Later Years = cell division < cell death
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One parent cell that divides will produce
Two Identical Daughter Cells
Parent Cell
Two
identical
daughter
cells
Why the need for nuclear division ?
1
The two daughter cells are also identical to their parent cell
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Keeping Cells Identical
The instructions for
making cell parts are
encoded in the DNA,
So each new cell must get
a complete set of the
DNA molecules
1
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DNA Replication
DNA must be copied =
replicated before cell
division
Each new cell will then
have an identical copy
of the DNA
Original
DNA strand
Two identical
DNA strands
1
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 A diploid cell has two sets of each of its chromosomes
 A human has 46 chromosomes (2n = 46)
 In a cell in which DNA replication has occurred all the
chromosomes are duplicated and thus each consists of two
identical sister chromatids
 Each daughter cell will have one of the two sister chromatids
Maternal set of
chromosomes (n = 3)
Paternal set of
chromosomes (n = 3)
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere
1
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AS and A Biology
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Prokaryotes
 Prokaryotes have no nucleus
 They have a single circular
chromosome
 First, replicate this chromosome
 Then, simply divide their cells in
two by binary fission
1
Eukaryotes
 Eukaryotes have a nucleus
 Nucleus contain chromosomes
 First, replicate these chromosomes
 Then, divide the nucleus into two
 Each new nucleus contains all
chromosomes
 Each new cell will get one nucleus
Same need for DNA-containing organelles
Mitochondria
Chloroplasts
AS

The Cell Cycle
1
All cells have a cycle
The Cell Cycle
Two phases
Interphase Mitotic phase
Growth of the cell
Replication of DNA
Replication of centrioles
Replication of Mitochondria (and Chloroplasts)
One parent cell
becomes two daughter cells
90% of the cell cycle time 10% of the cell cycle time
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The Cell Cycle
1
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The Cell Cycle
1
Interphase
Three phases
G1 phase = Gap 1 = Growth 1
Small cell absorbs nutrients: Growth
Produces proteins, RNA, etc
S phase = Synthesis (of DNA)
DNA Replication
Cell keeps growing
G2 phase = Gap 2 = Growth 2
Cell keeps growing
Gets ready for mitosis
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The Cell Cycle
1
Pause in the cell cycle
M
Mitosis
G1
Gap 1
G0
Resting
G2
Gap 2
S
Synthesis
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M
Mitosis
G1
Gap 1
G0
Resting
G2
Gap 2
S
Synthesis
cell is formed from
a mitotic division
cell grows & matures
to divide again
cell grows & matures
to never divide again
G1, S, G2, M G1G0
epithelial cells,
blood cells,
stem cells
liver cells
brain / nerve cells
muscle cells
The Cell Cycle
1
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Prometheus
1
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The Cell Cycle
1
Review of the cell cycle
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Nucleus is well-defined
DNA loosely packed in
long chromatin fibers
Prepares for mitosis
replicates chromosomes
DNA & proteins
produces proteins &
organelles
How is DNA during Interphase ?
1
AS

Mitosis
2
Five phases in mitotic phase
Prophase Metaphase Anaphase Telophase Cytokinesis
Pro – Meta – Ana - Telo
First phase Last phase Cell movement
Middle phase
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After DNA duplication, chromatin condenses
coiling & folding to make a smaller package
DNA
chromatin
mitotic chromosome
How is DNA during Prophase ?
2
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Duplicated chromosomes
are called
sister chromatids
They are held together
by the centromere
2
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28
 Karyotype
 A picture of the chromosomes
from a (here, human) cell
arranged in homologous pairs by
size
 First 22 pairs are called
autosomes
 Last pair are the sex
chromosomes
 XX female or XY male
2
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 Prophase
 Chromatin condenses
visible chromosomes
 Long sister chromatids attached at the centromere
 Centrioles move to opposite poles of cell
 animal cell
 Protein fibers cross cell to form mitotic spindle
 microtubules
 will coordinate movement of chromosomes
 Nucleolus disappears
 Nuclear envelope breaks down
What happens during Prophase ?
2
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 Prometaphase
spindle fibers attach to centromeres
creating kinetochores
microtubules attach at kinetochores
connect centromeres to centrioles
chromosomes begin to move
An additional phase ?
2
After Prophase, not yet in Metaphase...
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Kinetochore Fiber
Chromosome
Kinetochores…
2
After Prophase, not yet in Metaphase...
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Mitosis
2
Five phases in mitosis
Prophase Metaphase Anaphase Telophase Cytokinesis
Pro – Meta – Ana - Telo
First phase Last phase Cell movement
Middle phase
Prometaphase
Before metaphase
AS

 Metaphase
 Chromosomes are aligned along middle
of cell
metaphase plate
spindle fibers coordinate movement
helps to ensure chromatids separate
properly
so each new nucleus receives only 1
copy of each chromosome
What happens during Metaphase ?
2
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What happens during Metaphase ?
2
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What happens during Anaphase ?
2
 Anaphase
 Sister chromatids separate
move to opposite poles
pulled at centromeres
pulled by motor proteins
 Poles move farther apart
polar microtubules lengthen
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Telophase
 Chromatids arrive at opposite poles
2 nuclei are formed (envelope)
chromatin disperses
 individual chromosomes no longer
visible
2 nucleoli are formed
 Spindle fibers disperse
What happens during Telophase?
2
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Comparison of Anaphase and Telophase
2
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Origin of
replication
One chromosome
Circular
double-stranded DNA
replication
of DNA
elongation of cell
cell pinches
in two
ring of
proteins
Binary Fission in Prokayotes
1
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Cytokinesis in Animals
constriction belt of actin
microfilaments around equator
of cell
cleavage furrow forms
splits cell in two
What happens during Cytokinesis ?
2
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 Cytokinesis in Plants
cell plate forms
vesicles line up at equator
derived from Golgi
vesicles fuse to form 2 cell
membranes
new cell wall laid down between
membranes
new cell wall fuses with
existing cell wall
2
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2
Cell plate
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2
Plant
Animal
Cell plate
Cleavage
furrow
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interphase prophase (pro-metaphase)
metaphase anaphase telophase
cytokinesis
Summary of Mitosis
2
Animal Cells
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Summary of Mitosis
2
Animal Cells
Interphase
Prophase Metaphase
Anaphase
Telophase
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Summary of Mitosis
2
Animal Cells
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Mitosis in whitefish blastula
Summary of Mitosis
2
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Summary of Mitosis
2
Plant Cells
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onion root tip
Summary of Mitosis
2
Plant Cells
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Summary of Mitosis
2
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What happens after Mitosis ?
2
The cell returns to
interphase
1) G1, S, G2, mitosis again
Or
2) G1, G0
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 If mitosis is not controlled,
unlimited cell division
occurs causing cancerous
tumors
 Oncogenes are special
proteins that increase the
chance that a normal cell
develops into a tumor cell
Cancer cells
Uncontrolled Cell Division
3
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3
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 The cell cycle is controlled at three checkpoints:
 1. G1/S checkpoint
 -the cell “decides” to divide
 2. G2/M checkpoint
 -the cell makes a commitment to mitosis
 3. late metaphase (spindle) checkpoint
 -the cell ensures that all chromosomes are
attached to the spindle
3
Control of the Cell Cycle
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3
The checkpoints of the Cell Cycle
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 Growth factors:
 -can influence the cell cycle
 -trigger intracellular signaling systems
 -can override cellular controls that otherwise inhibit
cell division
 platelet-derived growth factor (PDGF) triggers cells
to divide during wound healing
3
Control of the Cell Cycle
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Factors causing cancer
3
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Cancer is a failure of cell cycle control
Two kinds of genes can disturb the cell cycle
when they are mutated:
1. tumor-suppressor genes
2. proto-oncogenes
3
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Tumor-suppressor genes
-prevent the development of many cells containing mutations
-for example, p53 halts cell division if damaged DNA is
detected
-p53 is absent or damaged in many cancerous cells
3
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XD
3
Tumor-suppressor genes
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Proto-oncogenes
-some encode receptors for growth factors
-some encode signal transduction proteins
-become oncogenes when mutated
-oncogenes can cause cancer when they are
introduced into a cell
3
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Proto-oncogenes
3
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E: Cell and Nuclear Division
Plan
1 Introduction:
1.1 Why the need for cell division ?
1.2 Why the need for nuclear division ?
1.3 The Cell Cycle
2 Mitotic phase:
2.1 Prophase
2.2 Metaphase
2.3 Anaphase
2.4 Telophase
2.5 Cytokinesis
2
3
4
1
5
3 Uncontrolled Cell division:
3.1 Mechanism
3.2 Factors causing cancer
4 Meiosis:
4.1 Why the need for meiosis ?
4.2 Mitosis vs. Meiosis
5 Conclusions and Overviews:
AS

 Sexual reproduction
 During fertilization,
sperm and ovum fuse
forming a diploid zygote
 The zygote develops into
an adult organism
 If the gametes were
diploid,
 The zygotes would be 4n
= tetraploid !!
 The gametes have to be
haploid
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS FERTILIZATION
Ovary Testis Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Why the need for meiosis ?
4
AS

4
Meiosis
=
Two consecutive mitosis
Meiosis I and Meiosis II
Without an S phase in between
Phase between Meiosis I and Meiosis II = Interkinesis
One diploid cell (2n) produces 4 haploid cells (n)
AS

4
Meiosis
=
Meiosis I
Prophase I Metaphase I Anaphase I Telophase I Cytokinesis I
+
Meiosis II
Prophase II Metaphase II Anaphase II Telophase II Cytokinesis II
AS

Meiosis: Two Part Cell Division
Homologs
separate
Sister
chromatids
separate
Haploid
Meiosis
I
Meiosis
II
Diploid
Haploid
What is happening is Meiosis ?
4
AS

Meiosis I: Reduction Division
Nucleus
Spindle
fibers Nuclear
envelope
Early Prophase I
(Chromosome number
doubled)
Late Prophase I Metaphase I
Anaphase I Telophase I (haploid)
4
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Tetrads Form in Prophase I
Homologous chromosomes
(each with sister chromatids)
Synapsis: the making of
TETRADS
4
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 Crossing-over
 Homologous
chromosomes in a
tetrad cross over
each other
 Pieces of
chromosomes or
genes are
exchanged
 Produces Genetic
recombination in
the offspring
4
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Homologous Chromosomes During
Crossing-Over
4
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Crossing-over multiplies the already huge number of
different gamete types produced by independent assortment
Crossing-Over
4
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Metaphase I
Homologous pairs of
chromosomes align
along the equator of
the cell
4
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Homologs separate and
move to opposite poles
Sister chromatids remain
attached at their centromeres
Anaphase I
4
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Nuclear envelopes reassemble
Spindle disappears
Cytokinesis divides cell into two
Telophase I and Cytokinesis I
4
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Meiosis II: Reducing Chromosome Number
Prophase II
Metaphase II
Anaphase II
Telophase II
4 Different haploid cells
4
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Prophase II
Nuclear envelope
fragments.
Centrioles duplicate
in each daughter cell
Spindles form
4
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Metaphase II
Chromosomes align
along equator of cell
4
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Anaphase II
Sister chromatids
separate and move to
opposite poles.
Equator
Pole
4
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Telophase II and Cytokinesis II
Nuclear envelope
assemble
Chromosomes
decondense.
Spindle disappears.
Cytokinesis divides cell
into two.
4
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Gametes (egg & sperm) form
Four haploid cells with one
copy of each chromosome
One allele of each gene
Different combinations of
alleles for different genes
along the chromosome
What are the results of Meiosis ?
4
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Mitosis vs. Meiosis
4
 Meiosis
 DNA duplication
followed by 2
cell divisions
 Sysnapsis
 Crossing-over
 One diploid cell
produces 4
haploid cells
 Each new cell
has a unique
combination of
genes
 Mitosis
 Homologous
chromosomes do not
pair up
 No genetic exchange
between homologous
chromosomes
 One diploid cell
produces 2 diploid
cells or one haploid
cell produces 2
haploid cells
 New cells are
genetically identical
to original cell (except
for mutation)
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Mitosis vs. Meiosis
4
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Mitosis Meiosis
# of DNA
replications
1 1
# of divisions 1 2
# of daughter cells 2 4
n # of daughter cells 2n n
Purpose Growth, asexual Sexual
Daughter cells like
parent?
Yes No
Daughter cells like
each other?
Yes No
Mitosis vs. Meiosis
4
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92
Mitosis Quizz
5
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93
Mitosis Quizz
5
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94
Interphase
Early prophase
Mid-Prophase
Late Prophase
Metaphase
Late Anaphase
Early Anaphase
Early Telophase,
Begin cytokinesis
Late telophase,
Advanced cytokinesis
Mitosis Quizz
5
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95
Early, Middle, & Late Prophase
Late Prophase
Metaphase Anaphase
Late Anaphase Telophase
Telophase & Cytokinesis
?
?
Mitosis Quizz
5
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96
Metaphase
Prophase
Anaphase
Telophase
Mitosis Quizz
5
Locate the phases of mitosis
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Can You Identify the Stages of Mitosis?
Put the
following
mitosis
stages in the
correct
sequence
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AS CIE Cell And Nuclear Division (Mitosis And Meiosis)

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