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Meiosis
Objectives:
1. To Compare Mitosis with
Meiosis
2. To recognize the
mechanisms of Meiosis
that increase genetic
diversity.
3. To understand the effects
of abnormal Meiosis
Mitosis Review
 IPMAT
Big Events of Mitosis
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Interphase: DNA replicates
Prophase: Chromatid pairs form
Nuclear envelope
disappears.
Spindle forms
Metaphase: Chromatid pairs align at
the equator of the
spindle.
Anaphase: Chromatid pairs separate and
Chromosomes are pulled to
opposite sides of the cell.
Telophase: “Opposite” of Prophase
Cytokinesis
Mitosis Review
Mitosis Rap.....You Have to See This
Reasons for Mitosis?
 Production
of 2 daughter cells that
are genetically identical
 Provides a) a means of asexual
reproduction for unicellular
organisms
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b) replacement of worn out
and damaged cells
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c) growth of an organism
Chromosomes, Chromatids
and Centromeres
A packaged
chromosome
Chromatid
Identical
chromatid
Chromosome
arm
Centromere
Chromosome
arm
Two identical
chromosomes
Replication
Anaphase
From Zygote to Embryo
Zygote
Zygote
2n
2n
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Cleavage
From Zygote to Embryo
Blastula
Meiosis
 Special
cell division that produces
“haploid” sex cells or gametes.
 Consists of 2 stages, Meiosis I and
Meiosis II
 One replication and two cell divisions
 For every special reproductive cell
(germ cell) that undergoes Meiosis,
four haploid sex cells are produced
Meiosis I
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Interphase I
- DNA Replicates(This cell: 2n = 4)
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Prophase I
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Nuclear envelope disappears
Replicated Chromatin coils into chromatid pairs
Spindle forms
Synapsis forms Tetrads
Synapsis
 Animation
 Chromatid
pairs join to form Tetrads
 After tetrad formation, arms of the
chromatid pairs exchange segments
in a process called Crossing Over
Crossing over
Where the chromatid pairs cross over is
called the Chiasma.
 Increases genetic diversity in sex cells
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Crossing Over
Prophase I:
Tetrad formation/
crossing over
Because of crossing over, every
gamete receives a unique set of
genetic information.
Telophase II
Metaphase I
Anaphase I
Telophase I
Metaphase I
 Tetrads
spindle.
align at the equator of the
Anaphase I
 Tetrads
separate in an event called
Disjunction.
 Chromatid pairs are pulled to
opposite sides of the cell
Telophase I
 Cytokinesis
splits the cell in two and
ends Meiosis I
Meiosis II
 Cells
skip Interphase II and move
right into Prophase II
 Meiosis
Mitosis
II follows the stages of
Prophase II
 Nuclear
envelope disappears
 Spindle forms
 Chromatids are floating throughout
the cytoplasm
Metaphase II
 Chromatids
the spindle
align at the equator of
Anaphase II
 Chromatid
pairs split and
chromosomes are pulled to opposite
sides of the cells
Telophase II
 Cytokinesis
splits the 2 cells into 4
Haploid sex cells. (n=2)
Mother cell
Stages Of Meiosis:
Meiosis I
Interphase
Prophase I:
Condensing
Prophase I:
Chromosomes
Tetrad formation/
Metaphase I
crossing over
Meiosis II
Telophase I
Anaphase I
Stages Of Meiosis:
Meiosis II
TelophaseIII
Prophase
The products of meiosis are 4
haploid cells each with a
unique set of chromosomes.
Metaphase II The products of
mitosis are 2 diploid
cells with identical
chromosomes.
Anaphase II
Telophase II
Meiosis Review

Meiosis
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Meiosis 2
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Meiosis in a 2n = 2 Cell
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Meiosis and Crossing Over
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Here it is!
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Another look!
(The quiz)
Meiosis Dry Lab
Using a special germ cell with a 2n = 6,
draw the stages of Meiosis I and Meiosis II
as a review of this special cell division.
 Draw your diagrams neatly. Make sure any
important structures or events are clearly
labelled.
 Your finished review will go in your lab
book.
 I will provide paper and “cell simulators”
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Spermatogenesis
vs
Oogenesis
Spermatogenesis
 - produces sperm
cells
 - ratio of germ
cells to sex cells is
1:4
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Oogenesis
 - produces egg
cells
 - ratio of germ
cells to sex cells is
1:1 (3 “polar
bodies die each
time)
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Spermatogenesis
vs
Oogenesis
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Spermatogenesis

Oogenesis
Terms to Remember:
Synapsis
 Tetrads
 Crossing Over
 Disjunction
 Spermatogenesis
 Oogenesis
 Germ Cells
 Haploid
 Diploid
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Nondisjunction
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Occasionally, during Anaphase I, the
tetrads do not separate in an event called
Nondisjunction.
The result is sex cells with either 1 extra
chromosome or 1 less chromosome
Nondisjunction
 Nondisjunction
 An
extra chromosome creates a
Trisomy (3 chromosomes where
there should be 2)
 One less chromosome creates a
Monosomy (1 chromosome where
there should be 2)
Chromosome Number Disorders
 Trisomy
21 – Down Syndrome
Chromosome Number Disorders
 Trisomy
18 – Edward’s Syndrome
Chromosome Number Disorders
 Trisomy
xxy – Kleinfelters Syndrome
Chromosome Number Disorders
 Monosomy
xo – Turners Syndrome
 (single x female)
Down Syndrome
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Welcome To Holland
by Emily Perl Kingsley
©1987 by Emily Perl Kingsley. All rights reserved. Article printed with
permission of the author.
I am often asked to describe the experience of raising a child with disability
- to try to help people who have not shared that unique experience to
understand it, to imagine how it would feel. It's like this......
When you're going to have a baby, it's like planning a fabulous vacation trip
- to Italy. You buy a bunch of guide books and make your wonderful plans.
The Coliseum. The Michelangelo David. The gondolas in Venice. You may
learn some handy phrases in Italian. It's all very exciting.
After months of eager anticipation, the day finally arrives. You pack your
bags and off you go. Several hours later, the plane lands. The stewardess
comes in and says, "Welcome to Holland."
"Holland?!?" you say. "What do you mean Holland?? I signed up for
Italy! I'm supposed to be in Italy. All my life I've dreamed of going to
Italy."
But there's been a change in the flight plan. They've landed in Holland and
there you must stay.
The important thing is that they haven't taken you to a horrible, disgusting,
filthy place, full of pestilence, famine and disease. It's just a different place.
Down Syndrome
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So you must go out and buy new guide books. And you must learn
a whole new language. And you will meet a whole new group of
people you would never have met.
It’s just a different place. It's slower-paced than Italy, less flashy
than Italy. But after you've been there for a while and you catch
your breath, you look around.... and you begin to notice that
Holland has windmills....and Holland has tulips. Holland even has
Rembrandts.
But everyone you know is busy coming and going from Italy... and
they're all bragging about what a wonderful time they had
there. And for the rest of your life, you will say "Yes, that's where
I was supposed to go. That's what I had planned."
And the pain of that will never, ever, ever, ever go away...because
the loss of that dream is a very very significant loss.
But... if you spend your life mourning the fact that you didn't get
to Italy, you may never be free to enjoy the very special, the very
lovely things ... about Holland.
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