Meiosis
Homework
Objectives
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Learn the purpose, products, and stages of
meiosis

Learn why meiosis produces genetic diversity

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“Why aren’t brothers identical to each other?”
Be able to compare & contrast mitosis and
meiosis
Meiosis

Think silently about the answer: “When
a cell goes through mitosis, it produces
2 daughter cells that have (choose one:
twice as many, the same number, half
as many) chromosomes as the parent
cell.”
Reproduction


Think silently about the answer: “When a cell
goes through mitosis, it produces 2 daughter
cells that have the same number of
chromosomes as the parent cell.”
But one cell produces a daughter cell with the
same number of chromosomes as itself, and
you are the product of two cells (sperm &
egg), why don’t you have twice as many
chromosomes as your parents?
Meiosis

In order for sexual reproduction to occur, meiosis
must happen. Meiosis is the division of a diploid
cell to make haploid gametes.

Gamete = is a haploid cell (sperm or egg) made
for sexual reproduction.
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The word gamete will be used a LOT in the next few
weeks of class, be sure you know it.
When two haploid gametes fertilize each other,
they create a new diploid cell called a zygote.
Overviews of Meiosis
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http://www.youtube.com/watch?v=D1_mQS_FZ0
Candy Meiosis!

Just like before, do not open your bag or take
things out unless I tell you to, and do not eat your
candy.

Meiosis is much more complicated than mitosis.
Pay careful attention that you are color-coding
your diagrams correctly.

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Red for one kind of gummi, orange for the other.
Dark blue for one kind of sour, light blue for the other.
Meiosis: Interphase

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Interphase: The cell is going about its
normal activities, and at the end,
prepares for meiosis.
Make a cell…


Take out ONE long piece of string and
ONE short piece of string and make a cell
membrane and nuclear membrane.
Take out TWO dots, for centrioles.
Meiosis: Interphase

Take out ONE gummi worm and sour
worm of each color and put them in the
nucleus. Each of these is one
chromosome. The two gummis, and
two sours, are homologous
chromosomes.
QuickTime™ and a
decompressor
are needed to see this picture.
Interphase
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Now, the cell will prepare for meiosis.

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Replicate your chromosomes to make sister
chromatids.
Replicate your centrioles.
The main features of interphase were the
same as in mitosis: growth, normal life, and
DNA replication to make sister chromatids.
Meiosis I

Prophase I

(Don’t draw a picture yet!)
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Chromosomes condense
and become visible.
Homologous chromosomes
pair up.
Crossing-over occurs.
Spindles form between
centrioles. (use only TWO licorice)
Nuclear membrane
dissolves.
Meiosis I

Prophase I: Crossingover

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Crossing-over happens
randomly.
It increases genetic
diversity by scrambling
the combinations of
different versions of
genes.
Only homologous
chromosomes cross-over.
Meiosis I

Prophase I: Crossingover


It doesn’t work to crossover candy, but you’ll
draw it.
From now on, color-code
your chromosomes to be
crossed-over, like this:
Meiosis I

Metaphase I

QuickTime™ and a
decompressor
are needed to see this picture.

Homologous
chromosomes line up in
pairs in the middle of the
cell.
Each spindle fiber
attaches to both in a pair
of homologous
chromosomes.
Meiosis I

Anaphase I

QuickTime™ and a
decompressor
are needed to see this picture.
Spindles pull the
homologous
chromosomes towards
opposite ends of the cell.

This is an important
difference from mitosis!
Meiosis I

Telophase I

QuickTime™ and a
decompressor
are needed to see this picture.

Each in the pair of homologous
chromosomes is now at the
opposite side of the cell.
Nuclear membranes reform.


Take out another short string.
Spindles dissolve.
Meiosis I

Cytokinesis

The cell membrane
pinches off to form two
cells.

Take out another long
string.
Checkpoint

Meiosis is half done.

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What is the difference between each of
these cells, and the cell you started with?
Are these cells diploid or haploid?
http://www.phschool.com/science/biology_
place/biocoach/meiosis/mei1ani.html
Meiosis II
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(There is no interphase between meiosis I and meiosis II.)

Prophase II:


Chromosomes visible. Nuclear membranes
dissolve.
Centrioles replicate.
QuickTime™ and a
decompressor
are needed to see this picture.
Meiosis II

Metaphase II



Chromosomes line up in the middle of the cell.
Spindles form. (Use TWO licorice per cell.)
Each spindle attaches to one chromosome’s
centromere.
QuickTime™ and a
decompressor
are needed to see this picture.
Meiosis II

Anaphase II

Sister chromatids are pulled apart.

Like mitosis, but different from Anaphase I!
QuickTime™ and a
decompressor
are needed to see this picture.
Meiosis II

Telophase II




Sister chromatids wind up at opposite ends of the
cell.
Nuclear membranes form.
Spindles dissolve
(The image shows some cytokinesis too, sorry)
QuickTime™ and a
decompressor
are needed to see this picture.
Meiosis II

Cytokinesis


The cell membrane pinches off.
There are now four gametes created from the
single beginning cell. Note: because of crossingover, each has a unique genetic code!
QuickTime™ and a
decompressor
are needed to see this picture.
Checkpoint

Meiosis is done!


What is the difference between each of
these cells, and the cells you started with?
Are these cells diploid or haploid?
Practice!

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Go over meiosis with your team. Be sure that you
know what happens in each stage, what each stage is
called, and what each part of the cell is called.
I will check two tables at a time. When you call me
over, you must:
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Have gone through the whole process with your group at
least once.
Have your cell in Interphase I and be ready to begin.
Have your neighboring table also meeting these
requirements.
You must narrate this to me WITHOUT referencing your
study guides. That means you must practice!
Meiosis Review

Many forgot to turn in your Mitosis
worksheet yesterday!

Bring your Meiosis worksheet up to be
stamped.
Sexual Reproduction

Why reproduce sexually? Why not
asexually, it seems simpler!

Discuss with partner, what would be the
advantage/s to the organism?
Sexual Reproduction

The main advantage of sexual reproduction is
diversity.

Sexual reproduction makes a new generation that
shuffles the genes of the old generation around.
More diversity is very adaptive!

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If you have a diverse population and something bad
happens, you’ll probably have at least a few individuals
who can survive.
Every time you make a new combination of genes, you
have a chance of it producing an awesome new trait.
And many more benefits, even though it’s energetically
costly.
Compare and Contrast

Make a Venn Diagram comparing and
contrasting mitosis and meiosis. Think
of everything that you can.
Mitosis
Meiosis
Essential Differences


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Purpose
 Mitosis - Asexual reproduction, or to replace damaged cells, or because
it got too big, Meiosis - Sexual reproduction only.
Process
 Mitosis = 1 division, Meiosis = 2 divisions
 Crossing-over only in Meiosis
 Mitosis = Single-file in Metaphase, Meiosis = Homologous chromosomes
paired in Metaphase I, single-file in Metaphase II
 Mitosis = Anaphase pulls sister chromatids apart, Meiosis = Anaphase I
pulls homologous chromosomes & Anaphase II pulls sister chromatids
Products
 Mitosis = 2 cells, Meiosis = 4 cells
 Mitosis = Diploid -> Diploid, Meiosis = Diploid -> Haploid
 Mitosis = Identical cells, Meiosis = Each one w/ unique DNA
Compare and Contrast
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
http://highered.mcgrawhill.com/olc/dl/120074/bio17.swf
http://www.johnkyrk.com/meiosis.html
Essays
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The exam on the Cell Cycle, Mitosis, and
Meiosis is on Friday.
In your remaining time, pick a study method
that’s effective for you:
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Make Flashcards (if you brought cards)
Make Essay Outlines
Read from textbook
Make quizzes for each other
Make diagram of meiosis
Check concepts with me