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Cell Division
 Why do cells divide?
• For survival
 To prevent DNA overload
◦ The larger a cell becomes the more demands the
cell places on its DNA
 To make sure that enough nutrients and
waste can get across the cell membrane.
• For repair – damaged tissue
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 Why a cell divides?
• For growth & development
• For reproduction
 Asexual reproduction in some organisms, or
 Special cell cycle called meiosis, to make
egg & sperm cells.
• For renewal -In order to replace old worn out cells.
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Cell Cycle
 Before a cell ever divides it must go
through a series of steps known as
the cell cycle to get ready for cell
division.
 Cell Cycle:
• Series of events that takes place in a cell
leading to its duplication (replication)of DNA
and cell division.
 Time required to complete the cell
cycle varies greatly but often ranges
from 10-24 hours. (embryonic cells divide
every 30 minutes!)
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Cell Cycle
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Cell Cycle
 The cell cycle consist of 3 main
stages:
• Interphase: The cell grows, carries on
normal metabolic processes, and
prepares for division by duplicating
organelles.
• Mitosis: Division of the nuclear material.
• Cytokinesis: Division of the cytoplasm.
 Cell division includes both mitosis &
cytokinesis.
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 Before the cell can divide, it must
replicate its DNA
 DNA Replication
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Chromosome Structure
 During interphase the DNA is unwound and
being used – genes are being translated.
 During cell division the DNA is packaged up
into chromosomes.
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Chromosome Structure
A. Chromatid
B. Chromosome
Each chromosome
contains 2 sister
chromatids.
 In humans we have 46
chromosomes that
make 23 pairs.
 Each pair contains
similar information.
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Chromosomes
 Diploid (2n): having two copies of
each chromosome.
 Haploid/Monoploid (n): cell with half
the normal number of chromosomes
as a diploid cell.
 Homologous Pairs: matching pairs of
chromosomes in a diploid cell.
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Cell Cycle: Interphase
 Interphase consists of 3 main phases:
1. G1: Growth Phase
 Cell grows rapidly and carries out routine functions
 Phase takes most of the cell’s life
 Muscle and nerve cells never divide, so they remain in G1
2. S: Synthesis Phase
 Cell’s DNA is copied
 At the end of the stage, each chromosome consists
of 2 chromatids attached at the centromere.
3. G2: Second Growth Phase
 Hollow microtubules are assembled
 Microtubules are used to move chromosomes during
mitosis
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Cell Cycle: Interphase
 Cells that are not actively dividing
(certain muscle, liver, lung, kidney,
nerve cells) are considered to be in
G0 phase (perhaps an extended G1)
 The G0 phase is considered a resting
phase because the cell is not:
dividing, getting ready to divide, or
getting larger
• BUT, the cell IS metabolically active.
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Cell Cycle: Interphase
 Most cells at least occasionally enter
G0 phase from G1 - some never leave G0
and essentially die with the organism
 The transition from G0 back to G1 is
thought to commit the cell to
completing the cell cycle and
dividing
• it is at this time in which the cell begins
preparing for DNA Replication
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Cell Cycle: Mitosis
 During mitosis the nucleus divides
into 2 nuclei.
 Each of the daughter nuclei ends up
with the SAME number of
chromosomes as the parent nucleus.
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Cell Cycle: Cytokinesis
 During cytokinesis the cytoplasm
splits into two dividing it equally
among the two new nuclei.
 The cell membrane then pinches into
2 creating two new cells.
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Cell Cycle
 REMEMBER!!!
• During interphase
the cell is getting
ready to divide.
• Cell division occurs
in 2 parts: Mitosis &
Cytokinesis
• During mitosis the
NUCLEUS is dividing.
• During cytokinesis
the CYTOPLASM is
dividing.
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Cell Cycle: Prokaryotes
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Cell Cycle Overview
 Animation
Ms. Coyle
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CELLULAR DIVISION
Mitosis
Growth
Zygote
2n
♀ Egg
♂ Sperm
Gametes
n
Meiosis
-reproduction
 Cells divide in eukaryotic organisms
via TWO processes:
• Mitosis
• Meiosis
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Meiosis
 Before any cell division can occur,
what MUST happen to the DNA???
 Mitosis is carried on in most cells in
the body with the exception of the
sex cells (egg & sperm).
 The sex cells divide by a different
process called meiosis.
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Meiosis
 Meiosis differs from mitosis for several
key reasons:
• Cells go through 2 divisions during
meiosis – only 1 division in Mitosis
• Daughter cells have ½ the number of
chromosomes in meiosis (haploid)
• Daughter cells are genetically different
than parent cells.
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Meiosis: Meiosis Stages in Order
 Meiosis occurs in two
stages and is used in
sexual reproduction.
 The cells produced
from Meiosis are NOT
genetically identical
to their parents due
to crossing over.
 Crossing over occurs
in Prophase 1 only.
 Meiosis produces
haploid cells with 23
chromosomes total.
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Crossing Over
 Crossing Over: the
switch of genes
between pairs of
homologous
chromosomes
during Prophase I
of Meiosis, when
the homologous
pairs are still joined
together.
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Crossing Over Animation
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Variation in Organisms
 Meiosis results in variation amongst
organisms several ways:
• Crossing over of the chromosomes
during Prophase I
• Random rearrangement of
chromosomes during Meiosis I – 4 new
cells are produced that are all different
• Random fertilization of gametes (sperm
& egg)
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Meiosis
 The process of meiosis remains the
same whether you make egg cells or
sperm cells.
 The process of making egg cells is
known as oogenesis.
 The process of making sperm cells is
spermatogenesis.
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Spermatogenesis & Oogenesis
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Spermatogenesis vs. Oogenesis
 Spermatogenesis and oogenesis are alike
in that they both produce haploid
gametes.
 These two process differ in three important
ways:
1. Only one ovum results from each diploid parent cell
that undergoes oogenesis, whereas four sperm cells
result from each parent cell that undergoes
spermatogenesis.
2. An ovary at birth contains all the primary oocytes it
will ever have, whereas the testes produce new
primary spermatocytes throughout the male’s
reproductive years.
3. Oogenesis is not completed without stimulation from
a sperm cell, whereas spermatogenesis produces
mature sperm in an uninterrupted sequence.
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Comparison
Mitosis
Meiosis
Function
Growth &
regeneration of new
cells
Creates egg or sperm
cells
Location In Body
All body cells
Ovaries or testicles
Number of Parent
Cells
1
1
Number of Daughter
Cells
2
4
Number of Cell
Divisions
1
1
Change in Number of No change
Chromosomes
(Diploid)
½ the number of
original chromosomes
(Haploid)
Difference in DNA
between Parent &
Daughter Cells
Identical to parent
cells
Daughter cells
genetically different
than parent cells and
from each other
Crossing Over Occurs
No
Yes
Homologous
Chromosomes
Ms. Coyle
Act independently of
one another
Form pairs during
Metaphase I
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Mitosis vs. Meiosis
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Karoytoype
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Human Genome Project
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Human Genome Project
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Regulating Cell Cycle
 There are three checkpoints:
1. G1 Checkpoint
 Decides when a cell can divide based on
environmental conditions, health and cell
size
 Favorable conditions begin S phase
◦ If not favorable, a resting period begins
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Regulating the Cell Cycle
2. G2 Checkpoint
 DNA repairs enzymes and checks DNA
replication
 Once this checkpoint is passed, then mitosis
begins
3. Mitosis Checkpoint
 Signals end of mitosis and G1 begins again
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Regulating the Cell Cycle
 What happens when checkpoints
fail?
1. Cancer can occur
•
Cancer is the uncontrolled growth of cells.
2. Mutation missed by checkpoint can
cause overproduction of growth
hormone
3. Damage done to a cell by
environmental factors can cause cells
to constantly repair
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Cancer
 When cells do not respond to the
normal cell cycle control
mechanisms, a condition called
cancer can result.
 Cancer is the uncontrolled growth
and division of cells ─ a failure in the
regulation of the cell cycle.
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Cancer
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