• Almost all human cells contain 46
chromosomes, and are diploid (2n).
• Q: If a sperm cell has 46 chromosomes (2n) &
an egg cell has 46 chromosomes (2n), when
they combine during fertilization, how many
chromosomes will the “baby” have?
The Problem!
No Good!
If egg and sperm had same number of chromosomes as other
body cells . . . baby would have too many chromosomes!
Meiosis – The solution
• Meiosis produces cells that have only 23
chromosomes and are 1N – haploid (half)
You tell me!
• Drosophila (fruit flies)
– Diploid: 2N = 8
– Haploid: 1N = ?
• Lettuce
– Diploid: 2N = 8
– Haploid: 1N = ?
• Goldfish:
– Diploid: 2N = ?
– Haploid: 1N = 48
The Meiosis Solution!
Much Better!
Egg and sperm with half the number of chromosomes than other
body cells . . . baby would have just the right amount!
Chromosomes
• Most human cells have 46 (23 pairs)
chromosomes
– One set of 23 from mom, and one of 23 from dad
• Of the 46 (23 pairs) chromosomes:
– 44 chromosomes (22pairs): autosomes
– 2 chromosomes (1 pair): sex chromosomes (XX or
XY)
• All human cells have 46 chromosomes except for
the egg & sperm cells
• Egg = 23
• Sperm = 23
(1 set of chromosomes, 1N)
• All other cells = 46 (2 sets of chromosomes, 2N)
– (liver, cardiac, blood cells)
Mitosis vs. Meiosis
• Mitosis
Asexual Reproduction
– Produces cells that are
genetically identical of
parent cell
• Meiosis
Sexual Reproduction
– Combines genes from
sperm & egg so offspring
are genetically different
from parents
Meiosis
• Makes 4 cells genetically different from parent
cell & from each other
• halves the number of chromosomes (46 23)
• Used for sexual reproduction
Gametes (egg or
sperm cells with
23 chromosomes,
haploid)
46 chromosomes,
diploid
• Fertilization restores the correct number of
chromosomes for the offspring (baby)
Where does meiosis happen?
• In malestestes
• In femalesovaries
VOCABULARY
Gametogenesis - production of gametes
Sperm
or
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egg
VOCABULARY
Spermatogenesis – production of four
equal sized gametes called sperm (1N)
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VOCABULARY
Oogenesis: only one egg (1N), other three are
called polar bodies and are not involved in
reproduction.
Meiosis
• Occurs in 2 stages:
– Meiosis I
– Meiosis II
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
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Anaphase I
Telophase I
and Cytokinesis
Interphase I – prior to Meiosis
Cells undergo a round of DNA replication,
duplicating their DNA (and other organelles).
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MEIOSIS I
Prophase I
Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
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Homologous Chromosomes
• Pair of chromosomes - maternal
& paternal - similar in shape &
size.
1 tetrad
• carry genes controlling the same
inherited traits.
4 chromatids
Homologous Chromosomes
Homologous chromosomes
sister chromatids
paternal
Tetrad
sister chromatids
maternal
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Crossing Over - Segments of non-sister chromatids break
and reattach to the other chromatid – causes VARIATION!
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
Crossing-over: entirely new
chromosomes are formed as
new combinations of alleles are
produced
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Check out the tetrads!
MEIOSIS I
Metaphase I
Spindle fibers attach to the centromeres on tetrads.
Tetrads line up in the middle: metaphase plate.
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INDEPENDENT ASSORTMENT
• Tetrads line up randomly at along metaphase plate
• Leads to variation
Dad’s chromosome - blue
Mom’s chromosome - red
MEIOSIS I
Anaphase I
Spindle fibers pull the homologous chromosomes
toward opposite ends of the cell.
Tetrads split into single chromosomes (notice that
chromosomes are different than parent due to crossing
over).
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MEIOSIS I
Telophase I
Nuclear membranes form.
Cell separates into two cells.
The two cells produced have chromosomes and alleles that
are different from each other and from the diploid cell that
entered meiosis I.
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Telophase I
• Each pole now has
haploid (1n) set of
chromosomes.
• Cytokinesis occurs and
two haploid daughter
cells are formed.
Chytokinesis
Meiosis I results in 2 haploid (1N) daughter cells,
each with half the number of chromosomes as the
original cell (23 for humans).
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Overview of Meiosis 1
4 duplicated chromosomes (2n)
2 duplicated chromosomes
(1n)
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Phases of Meiosis
Meiosis II
Cells enter second phase of cell division
Unlike meiosis I, neither cell goes through
chromosome replication.
Each of the cell’s chromosomes has 2 chromatids
(DNA is still duplicated in each cell)
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Phases of Meiosis
Meiosis II – similar to mitosis
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
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Anaphase II Telophase II
and
Cytokinesis
Meiosis II
Prophase II
• Centrioles separate and move to opposite
poles of cell
• new spindle fibers appear
Meiosis II
Metaphase II
• Sister chromatids (chromosomes) line up at
metaphase plate
• Spindle fibers attach at centromere
Meiosis II
Anaphase II
• sister chromatids separate & move
toward opposite ends of the cell
Meiosis II
Telophase II and Cytokinesis
• Nuclei reform.
• Cytokinesis occurs.
• Four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Fertilization
• Restores original chromosome number
– 2346
– N+N2N
SIGNIFICANCE OF MEIOSIS
 Halving the chromosome number ensures
that when gametes with the haploid number
fuse to form a zygote the normal diploid
number is restored.
 Meiosis leads to increased variation as it
occurs to form both egg and sperm, when
these combine during fertilization, even more
variation of offspring results
Each gamete (egg/sperm) is
not identical to its parent
N
When the two gametes
combine, they form unique
offspring – not identical to
either parent (zygote)
Offspring may express some
traits of the parent and/or
entirely different traits from
the parents depending on
how genes interact with
each other
2N
N
• Meiosis Animation
• meiosis square dance
Meiosis vs. Mitosis
• Comparison of Meiosis and Mitosis