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Meiosis
Sexual Reproduction
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Where do you come from?
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Sex:
Gender:
Woman, XX
Female
Egg, Ovum
Man, XY
Male
Sperm
+Fertilization – union of sperm an
egg to produce zygote
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Fertilization
Sperm and egg
cells – gametes
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Fetus Development
Fertilization
Mitosis to Create More Cells
Fetus
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
23 pairs of
chromosomes
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1 from mom,1 from
dad
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46 individual
chromosomes
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Body cells have all
these chromosomes
– diploid (2n)

Gametes have ½ the
amount because they
will combine to
eventually form body
cells– haploid (n)

(hap  half)
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23
n
46
2n
23
n
Homo sapiens – 46 chromosomes, 23 pairs
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Diploid and Haploid Practice
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Got it? Questions
1.
In humans, the cardiac muscle cells, skin cells, and neurons,
of humans all have ____________
chromosomes
46
2.
gametes
The sperm and egg cells are called ______________
3.
In humans an ovum (egg cell) would have _____ 23
chromosomes
4.
When a cell like a gamete has ½ the amount of
chromosomes it is called ___________and
symbolized as n
haploid
5.
When a body cell like a skin cell has the full amount of
chromosomes it is called __________, and symbolized as 2n
diploid
6.
If a sperm cell has 8 chromosomes it came from a male with
_____
chromosomes.
16
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Meiosis (My-oh-sis)
Definition: Process by which 1 diploid eukaryotic cell divides
twice to generate 4 haploid gametes

Gametes – sperm or cells

Produces 4 cells each with 1 copy of each chromosome

Purpose:


When the egg is fertilized by a sperm the new zygote has the same
number of chromosomes as the parents

Zygote has 2 of each chromosomes (1 from mom, 1 from dad)

Allows multicellular organisms to reproduce sexually
Animation: http://www.cellsalive.com/meiosis_js.htm
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
Special cells
that divide in
meiosis to
become
sperm and
egg
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Start off
diploid (2n)
Meiosis Phases – watch the chromosomes move
●
●
●
Centromeres divide the chromosome short arm and long arm.
Chromatids or sister chromatids (two identical copies of DNA) are connected by the
centromere
Telomeres protect DNA, during DNA replication.
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https://www.youtube.com/watch?v
=rB_8dTuh73c
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Purpose Continued

Provides great genetic diversity
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Offspring have the same # of chromosomes as parents
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Some of those chromosomes came from each parent
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Offspring typically have characteristics from both parents
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Prophase I
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Nuclear envelope breaks down
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DNA condenses into chromosomes
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Homologous Chromosomes Pair
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Each chromosome has 2 sister chromatids
Different from Mitosis
Crossing over occurs, which creates
Genetic diversity!!!
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

Crossing Over
Sections of DNA from the chromosome of one parent are switched with
the same section from the other parents DNA

Allows one chromosome to contain alleles from both parents

Occurs homologous chromosomes line up at equator
Increases genetic diversity
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Metaphase I

Homologous Chromosomes line up along equator of the cell


2 Homologous Chromosomes = 4 Sister Chromatids
Which side each homolog lines up on is random

50-50 chance a daughter cell will get a chromosome from mother
or father
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Homologous Chromosomes are separated
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Anaphase I
Each with 2 Sister Chromatids
1 of each homologous chromosome moves to opposite ends of the cell
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Results in 1 of each chromosome (2 sister chromatids) at each end
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Telophase I
(Final Phase)
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Nuclear envelope may reform
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2 haploid nuclei are produced
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Each has 1 of each chromosome
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Cytokinesis
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Not part of Meiosis I
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Occurs as Meiosis I is ending
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Cell divides
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Result of Meiosis I and Cytokinesis
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2 genetically different cells (Remember Crossing Over)
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Each cell contains 1 copy of each chromosomes
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2 Sister Chromatids each
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Prophase II

Nuclear envelope breaks down

DNA condenses into chromosomes

Each chromosome has 2 sister chromatids
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Metaphase II
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Sister Chromatids line up along equator of the cell
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1 of each Chromosome

2 Sister Chromatids
Different than Mitosis and Meiosis I
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Anaphase II
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Sister Chromatids are separated


1 of each chromosome moves to opposite ends of the cell
Results in 1 of each chromosome (no sister chromatids) at
each end
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Telophase II (Final Phase)

Chromosomes unwind

Nuclear envelope reforms in each new cell

4 haploid nuclei are produced

Each has 1 of each chromosome
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Cytokinesis

Not part of Meiosis II

Occurs at the end of Meiosis II
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Cells divide
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Result of Meiosis II and
Cytokinesis
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4 genetically different haploid cells
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All contain some chromosomes from mother and some from
father
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Each contains 1 of each chromosomes
No Sister Chromatids
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Comparing mitosis vs. meiosis
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Animation:
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http://highered.mheducation.com/sites/0072495855/student
_view0/chapter2/animation__comparison_of_meiosis_and_
mitosis__quiz_1_.html