The Cell CycleMitosis and Meiosis
Interphase- G1, S, G2
Mitosis or Meiosis
The Cell Cycle
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The sequence of growth and
division of a cell
Interphase = G1, S, G2
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Interphase is when the cell grows,
and the organelles double prior to
the actual splitting of the nucleus.
93% of a cell’s life is spent in
interphase.
Interphase has three parts
Growth 1 (G1)
 Synthesis (S)
 Growth 2 (G2)
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G1, S, G2
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G1 is when organelles double.
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S when DNA is replicated.
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Remember each new cell needs a
complete set of organelles.
Each cell needs a complete and
identical set of DNA
G2 Proteins needed for Mitosis are
produced.
Mitosis
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The process by which the cell nucleus divides
into two identical cell nuclei.
In some Human cells interphases lasts 15.3
hours, while mitosis lasts only .7 hours.
Occurs in a series of steps
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Prophase
Metaphase
Anaphase
Telophase
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Cytokinesis
Chromosomes
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Must duplicate and separate during
Mitosis
Structures of the tightly packaged
DNA
DNA is tangled up into a substance
of chromatin
The chromatin is packaged on the
chromosome
Chromosomal structure
Prophase
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Chromosomes now called chromatids because
they doubled to form short thick rods which pair
up and line up in the center of the nucleus.
A centromere connects the two halves of the
doubled chromatids.
Spindle fibers begin to form.
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Spindle fiber – a fibrous structure from the
cytoplasm which forms to the centriole.
Centrioles move to opposite sides of the cell.
The nuclear membrane breaks down.
Prophase
Metaphase
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Centromeres of the chromatid pairs line up in
the middle of the cell.
Metaphase plate- location where the
centromeres line up in the center of the cell.
By the end of metaphase each chromatid has
attached to spindle fibers.
Metaphase
Anaphase
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The spindle fibers pull the
chromatids apart.
This separates each one from its
duplicate. These move to opposite
sides of the cell.
Now there are two identical sets of
chromosomes.
Anaphase
Telophase
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When the chromosomes reach
opposite sides of the cell the
spindle fibers break up.
The nuclear membrane begins to
reform.
A furrow begins to develop
between the two sets of
chromosomes.
Telophase
Cytokinesis
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The two identical cells completely
divide and the cell membrane is
completely formed.
Mitosis Movie 1
Mitosis movie 2
Meiosis
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Diploid (2n) - A cell with two of each kind of
chromosome.
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One chromosome from each parent.
If two body cells were to combine nuclei, the
number of chromosomes would double.
In order for sexual reproduction to occur, each
cell involved must reduce its chromosome
number by half.
Haploid (n)- A cell with one of each kind of
chromosome.
Haploid cells
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Haploid cells are called gametes
Gametes are either sperm or eggs
Organism
diploid
gamete
Human
 Pea
 Fruit fly
 Dog
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46
14
8
78
23
7
4
39
Homologous chromosomes
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Are paired chromosomes with genes for the
same trait arranged in the same order.
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Ex. Eye color, hair color, height, one may
code for blue, blonde, tall, its homolog may
code for brown, blonde, short
Homologous chromosomes may have
different alleles on them
Allele- gene form for each variation of a trait
of an organism.
Meiosis
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Meiosis is the process of cell division in which
gametes are formed and the number of
chromosomes is halved. So that sexual
reproduction and zygote formation can occur.
Zygote- Fertilized egg which has a diploid
number of chromosomes.
Stages of Meiosis
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InterphaseChromosomes replicate
 Each chromosome consists of 2 identical sister
chromatids
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Prophase I
Each Pair of homologous chromosomes come
together to form a tetrad.
 Tetrad- 2 homologous chromosomes come
together and the 4 chromatids overlap.
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Crossing over
Tetrads are so tight that non-sister chromatids
from the homologous pair actually exchange
genetic material.
 Crossing over- The exchange of genetic
material by non-sister chromatids during late
prophase I of meiosis.
 Results in a new combination of alleles
Metaphase I
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Homologous chromosomes line up together in
pairs.
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* In mitosis homologous chromosomes line up in
the middle independently of each other.
Anaphase I
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Spindle fibers attach to the centromeres of
each pair.
Homologous chromosomes separate and
move to opposite ends of the cell.
Centromeres DO NOT split like they do in
mitosis
Now each cell will get one chromosome from
each homologous pair.
Telophase I
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Spindle fibers break down
Chromosomes uncoil
Cytoplasm divides
Another cell division is needed because the
number of chromosomes has not been reduced
After telophase I there maybe a short
interphase, but not always. It is important to
note that if a cell does have a second
interphase, there is No replication of
chromosomes.
Meiosis I
Meiosis II
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Is basically just like mitosis, but remember the
chromosomes did not duplicate in interphase II.
Prophase II
Chromosomes begin to line up in the middle of the
cell.
 Spindle fibers begin to form
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Metaphase II
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Chromosomes line up on the metaphase plate
Meiosis II
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Anaphase II
Centromeres split
 Sister chromatids separate and move to opposite
sides of the cell
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Telophase II
Nuclei reform
 Spindle fibers disappear
 Cytoplasm divides into two.
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The number of chromosomes in each daughter
cell has now been reduced by half.
Meiosis II
Meiosis Movie 1