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Meiosis
A reduction division
Meiosis
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Multi-celled organisms
produce gametes
(cells) through the process
of meiosis.
In humans and many other organisms, these cells are
called ova(eggs) and sperm.
When an ovum and a sperm combine, a new organism
is formed with a full set of chromosomes, half of
which came from the father, and half of which came
from the mother.
So what is a
chromosome
anyway?
Meiosis is a type of cell division
where one cell divides to form
new cells, each of which have half
the number of chromosomes as
the original (ex – Humans have ?
normally?).
 This condition is called haploid.
 Cells with the original number of
chromosomes are called diploid.
 Another way to say this is that
each new daughter cell gets half
of the parent cell’s DNA.
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Humans have 23 pairs of
chromosomes, for a total of
46.
In each pair, one
chromosome comes from
mom and one comes from
dad.
Both members of the
chromosome pair are the
same size, shape, and carry
the same genetic
information.
 There
are two main stages of meiosis:
Meiosis I and Meiosis II.
 Each of these is divided into four
stages: Prophase, Metaphase, Anaphase
and Telophase
 Before the cell starts dividing, it has to
complete Interphase – What happens
during this very important time?
Meiosis
We refer to the 23 pairs as
homologous pairs. Why?
 During the earliest stage of
meiosis, the members of
each pair will temporarily
attach together to form a
structure called a tetrad.
Why are there four instead
of two?
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homologous pair
tetrad
Prophase I
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Very similar to Prophase of
mitosis.
What happens during Prophase?
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Chromatin does what?
N guys do what?
Spindle fibers?
Chromosomes (tetrads) do what?
Centrioles do what?
There are a few extras to add!
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The tetrads actually exchange some
of their DNA in a process called
crossing-over (Swapping spit –
Disgusting!!)
Metaphase I
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_______,
Tetrads not
individual
chromosomes, line up
on the equator of the
cell.
Spindle fibers attach
_______
to the centromeres of
chromosomes.
Anaphase I
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Spindle fibers contract
tetrads apart.
and pull _____
Entire double
stranded chromosome
pulled to opposite
ends of the cell.
Chromosome number
is now half of original
(haploid).
Telophase I
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Cell plate (plant) or
cleavage furrow (animal)
divides the _________.
cytoplasm
Cell usually goes directly
into Prophase II, but if
not, chromosomes relax,
nuclei reform, and
spindle fibers break
down.
half
Each daughter cell has _____
of the number of
chromosomes as the original.
Chromosomes not to scale
Summary of Meiosis II
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The mechanics of Meiosis II are identical
_______ to
Mitosis.
Each haploid daughter cell produced by Meiosis
I divides again, so that four haploid cells are
produced altogether.
In _____________,
spermatogenesis all four cells survive to
produce viable sperm.
In _________,
oogenesis only one cell survives to produce
a viable ovum.
Overview
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Tetrads form (Pro I)
Crossing over can occur
(Pro I)
Tetrads line up and are
separated (Meta I)
Daughter cells are haploid
Produces gametes (ova and
sperm)
Compare
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Critical differences
between Mitosis and
Meiosis:
Tetrads form (Pro I)
Crossing over can
occur (Pro I)
Tetrads line up and are
separated (Meta I)
Daughter cells are
haploid
Only occurs in germ
cells
Produces gametes (ova
and sperm)
Mitosis vs. Meiosis I
Left: Metaphase
of MITOSIS: no
pairing!!!!
Right: Metaphase
I of Meiosis:
homologous
chromosomes
Pair UP!