Chapter 9: Meiosis
Pages 207-213
Sexual Reproduction
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the union of 2 specialized sex cells (gametes)
to form a single cell called a zygote
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Gametes usually come from 2 different parents
Results in genetic variation among offspring
How is Chromosome Number Kept the
Same with Sexual Reproduction?
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Homologous Chromosomes - each chromosome
in a somatic (body) cell has a partner
chromosome
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Similar in size, shape, and position of centromeres
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The sex chromosomes are called X and Y
Human females have a homologous pair of X
chromosomes (XX)
Human males have one X and one Y
chromosome
The 22 pairs of chromosomes that do not
determine sex are called autosomes
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Each pair of homologous chromosomes
includes one chromosome from each parent
The 46 chromosomes in a human somatic
cell are two sets of 23: one from the mother
and one from the father
A diploid cell (2n) has two sets of
chromosomes
For humans, the diploid number is 46 (2n =
46)
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A gamete (sperm or egg) contains a single
set of chromosomes, and is haploid (n)
For humans, the haploid number is 23 (n =
23)
Each set of 23 consists of 22 autosomes and
a single sex chromosome
In an unfertilized egg (ovum), the sex
chromosome is X
In a sperm cell, the sex chromosome may be
either X or Y
Gametes are Haploid

When egg and sperm join at fertilization,
each gamete is haploid
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The diploid number is restored in the fertilized egg
(zygote).
When the zygote divides by mitosis to form the
first 2 cells of the embryo, each daughter cell
receives the diploid number of chromosomes
Polyploid
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3 or more sets of chromosomes
Rare in animals
Common among plants
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Important factor in plant evolution
Larger and hardier than diploid
Wheat, cotton
Meiosis
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Diploid cells -> Haploid Cells
Meiosis: 1 cell undergoes 2 divisions –
Meiosis I and II
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Produces 4 haploid cells
Similar to mitosis, except:
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2 divisions
Only 1 DNA duplication (first division)
Genetic information is shuffled
http://www.johnkyrk.com/meiosis.html
Meiosis
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In a cell in which
DNA synthesis has
occurred, each
chromosome is
replicated
Each replicated
chromosome
consists of two
identical sister
chromatids
Prophase I
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Remember – the chromosomes are
duplicated during the S phase (before
meiosis)
Homologous chromosomes synapse – line
up side by side
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Form tetrads
Crossing over occurs – genetic material is
exchanged between homologous chromatids
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Produces a new combination of genes
Increases the genetic variation among offspring
Metaphase I
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Kinetochores are attached by spindle fibers
to the poles
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Both sister chromosomes are attached to the
same pole, the other sister chromosomes to the
other pole
Tetrads (4 chromatids) line up on the
equatorial plane of the cell
Tetrads are held together at chiasmata –
sites of prior crossing-over
Anaphase I
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Paired homologous chromosomes separate
and move toward opposite poles
Each pole receives a random mixture of
maternal and paternal chromosomes
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Only one member of each homologous pair is
present at each pole.
Sister chromatids are united at their
centromere regions.
Telophase I
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One of each pair of homologous
chromosomes is at each pole.
Each new nucleus contains the haploid
number of chromosomes, but each
chromosomes is a duplicated chromosome.
Cytokinesis occurs:
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Often called interkinesis
Video
Meiosis II
Prophase II
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Chromosomes
condense again
NO crossing over
occurs
Metaphase II
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Chromosomes line up
on equatorial planes of
their cells
Chromatids are in
groups of two
Anaphase II
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Sister chromatids
separate
chromosomes move to
opposite poles
Telophase II
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Nuclei form at opposite
poles of each cell
Cytokinesis occurs
Results of Meiosis
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Four n cells with unduplicated chromosomes
Animals – production of gametes
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“gametogenesis”
Spermatogenesis or oogenesis
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Plants – production of spores
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Tutorial
Oogenesis
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Human females have about 700,000 primary
oocytes at birth
Suspended in Prophase I until puberty
Usually 1 egg is ovulated per cycle ~ 28 days
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Oocyte fully matures by finishing meiosis
Human female ovulates ~ 400 times/lifetime
Meiosis II only finishes if fertilization occurs