Mitosis and Meiosis
Chromosomes
☺ Chromatin – long strands of DNA wrapped around proteins
☺ Chromosome – a rod-shaped structure that forms when a single DNA molecule coils
tightly before cell division
Single Chromosome
Duplicated Chromosome
☺ Chromatid – one of two copies of each chromosome
☺ Sister Chromatid – two identical copies of the chromosome
☺ Centromere – a protein disk that attaches the two chromatids
☺ Human cells have 23 different chromosomes
☺ Contain 2 copies of each chromosomes for a total of 46
☺ Homologous Chromosomes – two copies of each chromosome that are similar in
shape, size, and have similar genetic information
Cell Division
☺ Cell Division – process by which new cells are made from an existing cell
☺ Two types of cell division
☺ Mitosis – makes somatic cells (body cells)
☺ Meiosis – makes gametes (eggs and sperm)
The Cell Cycle
☺ Cell Cycle – sequence of growth and division of a cell
☺ 5 Phases of the Cell Cycle
1. G1
☺ Growth of cell
☺ Makes new proteins and organelles
2. S
☺ DNA replicates
1
3. G2
☺ Produces proteins needed for mitosis or meiosis
4. M
☺ Mitosis or Meiosis
5. Cytokinesis – cytoplasm divides separating into 2 daughter cells
☺ G1, S, G2 combined are called Interphase
☺ Where the cell spends most of its life
Mitosis
☺ Mitosis – process by which the nucleus of a cell is divided into 2 identical nuclei with
the same number of chromosomes
☺ Mitosis is divided into 4 Stages
1. Prophase
☺ Chromatin coils into chromosomes
☺ Nuclear envelope breaks down
☺ Centrioles form and move to opposite poles
☺ As centrioles separate, spindle fibers form which are microtubules (protein
cables)
☺ Longest phase of mitosis
2. Metaphase
☺ Spindle fibers attach to the chromosomes at the centromere
☺ Chromosomes line up in the middle
3. Anaphase
☺ Centromeres split and the sister chromatids are pulled apart toward opposite
poles
☺ Each chromatid may now be called a chromosome
4. Telophase
☺ Chromosomes reach opposite poles
☺ Spindle fibers break
☺ Chromosomes uncoil
☺ Nuclear envelope forms
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☺ Cytokinesis
☺ Two daughter cells are formed
☺ Cytokinesis differs between plants and animals
☺ Animals – the cell membrane pinches in along the center of the cell
☺ Plants – have a cell wall so the cell membrane can’t pinch in
 Instead a cell plate is laid down across the center
Regulating the Cell Cycle
Normal Cell Growth
☺ Scientists have worked long and hard to discover the factors that initiate and control
cell division.
☺ A clear understanding of these control factors can benefit medical research.
☺ Scientists have found that a specific protein called cyclin regulates the cell cycle.
Uncontrolled Cell Growth
☺ Occasionally cells lose control of the cell cycle
☺ This loss of control may be caused by…
☺ Environmental factors
☺ Changes in enzyme production
☺ Cancer is a result of uncontrolled cell division
☺ Cancer cells divide uncontrollably and form masses of cells called tumors
☺ Cancer cells may break loose from tumors and spread throughout the body,
disrupting normal activities and causing serious medical problems or even death
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Meiosis
Chromosome Number
 Chromosomes occur in pairs (23 pairs)
 One from mom and one from dad
 Diploid (2n) – 2 of each kind of chromosome (46)
 Haploid (n) – one of each kind of chromosome (23)
 All species contain a specific number of chromosomes
 Pea plant – 14
 Apple – 34
 Human – 46
 Dog – 78
Why Meiosis?
 When cells divide by mitosis the new cells have exactly the same number and kind
of chromosomes as the original cells.
 If fertilization occurs by mitosis then gametes would have twice the number of
chromosomes 46 + 46 = 92
 Meiosis – a kind of cell division that halves the number of chromosomes
 Necessary to ensure correct number of chromosomes in gametes
Gamete Formation
 Meiosis occurs only to make gametes
 Gametes – sex cells (sperm and egg)
 Sperm – male gametes
 Egg – female gametes
 Zygote – union of an egg and sperm
 Fertilization – the uniting of male and female gametes (sperm and egg)
 Pollination – the transfer of the male pollen grains to pistil (female part) in plants
 Sexual Reproduction – involves the production and fusion of haploid sex cells
Sperm (n = 23)
Egg (n = 23)
Zygote (2n = 46)
Phases of Meiosis
Meiosis I
 Interphase I
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 DNA is copied forming duplicate chromosomes
 Prophase I
 Chromosomes coil up
 Pairs of homologous chromosomes come together and form a tetrad
 Crossing Over – homologous chromosomes exchange portions of their
chromatids
 There is an average of two to three crossovers for each pair of homologous
chromosomes
 Crossing over results in new combinations of genetic material
 Metaphase I
 Centromere attaches to a spindle fiber
 Pairs of chromosomes line up in center
 Anaphase I
 Pairs of chromosomes separate and move to opposite sides
 Telophase I
 Spindle fibers break down
 Chromosomes uncoil
 Cytokinesis
 Cytoplasm divides into two new cells
 Each cell has half the genetic info of the original cell
 Even though we now have 23 chromosomes in each cell, the chromosomes are still
in the duplicated state and it needs to divide again
Meiosis II
 Prophase II
 Centrioles form and move to opposite poles
 Chromosomes coil
 Nuclear envelope breaks down
 Metaphase II
 Spindle fibers attach to centromere
 Chromosomes line up in center
 Anaphase II
 Centromere splits and sister chromatids separate and move to opposite poles
 Telophase II
 Spindles fibers break down
 Chromosomes uncoil
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 Cytokinesis
 Cytoplasm divides into two new cells for a total of four new cells
 Each cell has 23 single chromosomes, they are haploid
 In Mitosis 1 diploid cell (2n) produces 2 diploid cells (2n)
 In Meiosis 1 diploid cell (2n) produces 4 haploid cells (n)
Mitosis
2n
2n
Meiosis
2n
2n
n
n
n
Male (sperm)
n
n
Female (egg)
2n
n
n
2n
n
n
n
polar bodies
egg
sperm
Meiosis Provides for Genetic Variation
 Genetic Recombination – the reassortment of chromosomes and the genetic
information they carry by crossing over
 3 Places for Variation during Meiosis
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1. There are 2 possibilities when homologous chromosomes line up in metaphase I
 223 = 8 million
2. 8 million different kinds of sperm or egg
 Because any egg can be fertilized by any sperm the number of different
possible offspring is 8 million x 8 million = @70 trillion
3. Crossing over leads to more variation
 Crossing over can occur anywhere at random on a chromosome
 2 or 3 crossovers per chromosome occur during meiosis
Comparing Mitosis and Meiosis
MITOSIS
MEIOSIS
Two new cells
New cells that are genetically identical to
the parent cell and each other
New cells are diploid
For growth and replace old somatic cells
Asexual reproduction
Four new cells
New cells that are genetically different
from the parent cell and each other
New cells are haploid
To make sperm and egg
Sexual reproduction
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Mitosis and Meiosis
Chromosomes
☺ Chromatin –
☺ Chromosome –
Single Chromosome
Duplicated Chromosome
☺ Chromatid –
☺ Sister Chromatid –
☺ Centromere –
☺ Human cells have ______ different chromosomes
☺ Contain 2 copies of each chromosomes for a total of ______
☺ Homologous Chromosomes –
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Cell Division
☺ Cell Division –
☺ Two types of cell division
☺ __________ – makes ____________ cells (___________ cells)
☺ __________ – makes _____________ (eggs and sperm)
The Cell Cycle
☺ Cell Cycle –
☺ 5 Phases of the Cell Cycle
1. G1


2. S

3. G2

4. M

5. Cytokinesis –
☺ G1, S, G2 combined are called _____________.
☺
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Mitosis
☺ Mitosis –
☺ Mitosis is divided into 4 Stages
1. Prophase
☺
☺
☺ _____________ form and move to ____________
___________
☺ As centrioles separate, spindle fibers form which are microtubules (protein
cables)
☺ _______________ phase of mitosis
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2. Metaphase
☺ Spindle fibers attach to the chromosomes at the __________________
☺ Chromosomes line up in the _______________
3. Anaphase
☺
☺ Each chromatid may now be called a __________________
4. Telophase
☺ Chromosomes reach _____________ ____________________
☺
☺ Chromosomes _______________
☺ Nuclear envelope _________________
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☺ Cytokinesis
☺ _______ daughter cells are formed
☺ Cytokinesis differs between plants and animals
☺ Animals – the cell membrane ____________ in along the center of the cell
☺ Plants – have a cell wall so the cell membrane does not pinch in
☺ Instead a ___________ ___________ is laid down across the center.
Regulating the Cell Cycle
Normal Cell Growth
☺ Scientists have worked long and hard to discover the factors that initiate and control
cell division.
☺ A clear understanding of these control factors can benefit medical research.
☺ Scientists have found that a specific protein called ____________ regulates the cell
cycle.
Uncontrolled Cell Growth
☺ Occasionally cells lose control of the cell cycle
☺ This loss of control may be caused by…
☺
☺
☺ _____________is a result of uncontrolled cell division
☺ Cancer cells divide uncontrollably and form masses of cells called ______________.
☺ Cancer cells may break loose from tumors and spread throughout the body,
disrupting normal activities and causing serious medical problems or even death.
12
Meiosis
Chromosome Number
 Chromosomes occur in pairs (_____ pairs)
 One from ________ and one from __________
 Diploid (_____) –
 Haploid (____) –
 All species contain a specific number of chromosomes
 Pea plant –
 Apple –
 Human –
 Dog –
Why Meiosis?
 When cells divide by mitosis the new cells have exactly the ________ number and
kind of chromosomes as the original cells.
 If fertilization occurs by mitosis then gametes would have twice the number of
chromosomes ______ + ______ = ______
 Meiosis –
 Necessary to ensure correct number of chromosomes in gametes
Gamete Formation
 Meiosis occurs only to make gametes
 Gametes –
 Sperm – ________ gametes
 Egg – ___________gametes
 Zygote –
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 _______________ – the uniting of _________ and __________ gametes (sperm
and egg)
 _______________ – the transfer of the male pollen grains to pistil (female part) in
plants
 Sexual Reproduction – involves the production and fusion of ___________ sex cells
Sperm (n = 23)
Egg (n = 23)
Zygote (2n = 46)
Phases of Meiosis
Meiosis I
 Interphase I
 DNA is _________ forming duplicate chromosomes
 Prophase I

 ________ of homologous chromosomes come together and form a ________
 Crossing Over –
 There is an average of _______ to _______ crossovers for each pair of
homologous chromosomes
 Crossing over results in new combinations of genetic material
 Metaphase I
 _____________ attaches to a spindle fiber

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 Anaphase I
 Pairs of chromosomes ______________ and move to opposite sides
 Telophase I

 Chromosomes ____________
 Cytokinesis
 Cytoplasm divides into ________ new cells
 Each cell has ________ the genetic info of the original cell
 Even though we now have 23 chromosomes in each cell, the chromosomes are still
in the ______________________ state and it needs to divide again
Meiosis II
 Prophase II
 _______________ form and move to opposite poles
 Chromosomes ___________
 Nuclear envelope breaks down
 Metaphase II
 Spindle fibers attach to __________________
 Chromosomes line up in _____________
 Anaphase II
 Centromere splits and ______________ _____________ separate and move
to opposite poles
 Telophase II
 Spindles fibers break down
 Chromosomes __________
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 Cytokinesis
 Cytoplasm divides into two new cells for a total of __________ new cells
 Each cell has 23 __________ chromosomes, they are ________________
 In Mitosis 1 diploid cell (2n) produces 2 diploid cells (2n)
 In Meiosis 1 diploid cell (2n) produces 4 haploid cells (n)
Mitosis
2n
Meiosis
2n
Male (sperm)
Female (egg)
2n
2n
16
Meiosis Provides for Genetic Variation
 Genetic Recombination – the reassortment of chromosomes and the genetic
information they carry by crossing over
 3 Places for Variation during Meiosis
1. There are 2 possibilities when homologous chromosomes line up in metaphase I
 223 = _____ million
2. 8 million different kinds of ________ or __________
 Because any egg can be fertilized by any sperm the number of different
possible offspring is 8 million x 8 million = @70 trillion
3. ___________ _____________ leads to more variation
 Crossing over can occur anywhere at _______ on a chromosome
 ____ or ____ crossovers per chromosome occur during meiosis
Comparing Mitosis and Meiosis
MITOSIS
MEIOSIS
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