In This Lesson:
Meiosis
(Lesson 2 of 3)
Today is Wednesday,
nd
December 2 , 2015
Pre-Class:
Briefly summarize the events of the six steps of
mitosis/cytokinesis.
Expect the Bag of Evil!
Have your free response questions ready but don’t discuss them.
Today’s Agenda
• Meiosis.
– Yep. Same thing you learned in your previous bio
class.
• That means I’m gonna go quickly since it’s REVIEW.
• Where is this in my book?
– Chapter 13.
By the end of this lesson…
• You should be able to argue for the existence
of meiosis, both in why it evolved and why it
bears some similarity to mitosis.
• You should be able to differentiate between
oogenesis and spermatogenesis.
The Transition
• CrashCourse – Mitosis – Splitting Up is
Complicated
Kinds of Reproduction
• Sexual
– Calm down – it only means you need two
individuals to “do it.”
• Asexual
– Only one individual.
– Remember binary fission?
Asexual Reproduction
• All DNA copied to offspring.
• Offspring is (are) clone(s).
– Used by hydra (multicellular),
paramecia, and yeast, among others.
• Kinds of asexual reproduction:
– Binary Fission
– Budding
– Fragmentation
• The big disadvantage:
– Little genetic diversity.
• Offspring are almost exactly like parents.
• Problems are usually not “taken care of.”
Budding
Budding
Sexual Reproduction
• Increases genetic diversity.
– DNA from Mom and Dad.
• Instead of just one of them.
• Requires the use of gametes. Why?
– In animals: sperm and ova (egg cells).
– Different for other living things.
46
+
46
=

92
Meiosis
• Meiosis is another process of cell division.
• Sexual reproduction only.
– Why?
• Like Mitosis, except:
– There are two cell divisions.
– # of chromosomes is halved.
Meiosis: Specific Names
• Meiosis produces gametes.
• There are specific terms for how meiosis works:
– ♀: producing ova (eggs) from oocytes is called
oogenesis.
• Oocytes are cells that produce eggs.
– ♂ : producing sperm from spermatocytes is called
spermatogenesis.
• Spermatocytes are cells that produce sperm.
Meiosis
• Meiosis is NOT a cycle:
http://upload.wikimedia.org/wikipedia/commons/5/54/Meiosis_diagram.jpg
Stages of Meiosis
• First stages – Meiosis I:
–
–
–
–
–
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
• Second stages – Meiosis II
–
–
–
–
–
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
• Important: Steps I and II are not the same!
Meiosis
• Meiosis I divides the starting diploid cell into two
haploid daughter cells.
– This is the reductive step.
• 46 chromosomes to 23 chromosomes.
• Diploid to haploid.
• 2n to n.
• Meiosis II divides the cells but keeps the chromosome
number the same.
– Process is just like mitosis but without the duplication
beforehand.
• 23 chromosomes to 23 chromosomes.
• Haploid to haploid.
• Fertilization restores the diploid cell (46 again).
Sex Determination
• This is a good time for me to remind you of
something from our last lesson: sex determination.
• Since each gamete is haploid and has half the
number of chromosomes needed for a somatic cell,
they only contain one of the two sex chromosomes.
– Each egg has an X chromosome.
– Each sperm has an X or a Y chromosome.
• Thus, it is the male that “determines” the sex of the
offspring.
– Except when he’s not. More in two slides.
Henry the VIII
• Six wives.
• Really wanted a son.
• Whose “fault” was it?
I even
look like
I’m an
idiot.
Another Sex Determination System
• If you’re using the XY sex determination system, Dad
“determines” (it’s not conscious) the sex of the
offspring by supplying either an X or Y chromosome in
the winning sperm cell.
– Remember, females can only donate an X chromosome, so
they don’t influence gender.
• If you’re using the ZW sex determination system, Mom
is in charge.
– Birds, some reptiles, some insects, and some fish use this
method.
– ZZ = Male
– ZW = Female
• Cases of Z0, ZZW, and ZZWW have been recorded.
Yet Another Sex Determination System
• Separately, there’s even parthenogenesis, where
a (usually) female gamete develops into an
offspring by itself.
– Read that again: A female gamete develops into an
offspring by itself.
• Sometimes this leads to a haploid adult
(ants/bees, for example), but often the egg and
another haploid cell (the polar body – more later)
fuse to create a diploid cell.
• For more on all this:
– TED: Aaron Reedy – Sex Determination More
Complicated Than You Thought
Parthenogenesis vs.
Normal Fertilization
Back to Meiosis
• One last thing before we get started:
– DNA is copied before meiosis begins, but never
again in the rest of the process.
• Key: Watch for the difference in meiosis that
reduces chromosome number where mitosis
didn’t.
Prophase I
46 Chromosomes
92 Chromatids
• Chromatin condenses to X-shaped chromosomes.
• Maternal/paternal chromosomes pair up to form
tetrads (pair of X-shaped chromosomes, four
chromatids).
• Crossing over occurs.
http://www.regentsprep.org/regents/biology/units/reproduction/crossingover.gif
http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect16.htm
About crossing over…
• Biology’s way of “shaking
things up.”
• Sections of chromosomes are
exchanged with one another.
– Note: The sections are traded
between chromatids but NOT
sister chromatids.
• Increases genetic variability.
– Crossing over occurs in a
process called synapsis.
– The spot at which the
chromatids cross is called the
chiasma.
– Resulting chromosomes are
called recombinant
chromosomes.
Crossing Over: Another View
http://library.thinkquest.org/20465/meiosis.html
About tetrads…
• A tetrad is a set of two
X-shaped chromosomes
next to one another.
• Tetrads exist starting in
Prophase I and are split
apart in Anaphase I.
Tetrad
http://home.comcast.net/~mjmayhew42/Biology%20notes/meiosis%20notes_files/image005.gif
Metaphase I
46 Chromosomes
92 Chromatids
• Tetrads line up in the middle of the cell.
– Remember, these are pairs of X-shaped
chromosomes.
– Half the tetrad is from Mom, half is from Dad.
http://www.uic.edu/classes/bios/bios100/lecturesf04am/metaphase1m.jpg
http://www.sinauer.com/cooper/4e/micro/16/16-02_Meiosis-Metaphase1(NL-Large).jpg
Compare Metaphases
• Metaphase – Mitosis • Metaphase I - Meiosis
Anaphase I
23 Chromosomes
46 Chromatids
on each side!
• Tetrads pulled apart (stay as X-shaped
chromosomes).
– Important: The sister chromatids remain joined to
one another.
http://biog-101-104.bio.cornell.edu/bioG101_104/tutorials/cell_division/lily_review_fs.html
Compare Anaphases
• Anaphase – Mitosis
• Anaphase I - Meiosis
Telophase I and Cytokinesis
• Chromosomes gather at cell poles.
• Cell divides.
http://biog-101-104.bio.cornell.edu/bioG101_104/tutorials/cell_division/lily_review_fs.html
23 Chromosomes
46 Chromatids
in each cell!
Summary of Meiosis I in Diagrams
Prophase I
Metaphase I
Anaphase I
Telophase I
End Results of Meiosis I
• After meiosis I, we end up with two haploid cells.
• Still not ready to be gametes.
– Need one more division.
• Time for Meiosis II.
– Booyah!
• Not really.
Meiosis II
• Meiosis II is like Mitosis, except this time, we’re
gonna end up getting haploid cells from
haploid cells.
– Remember, Meiosis is NOT a cycle.
• The good news? Meiosis II is the same as
Mitosis!
– Samesies!
Prophase II
•
•
•
•
23 Chromosomes
46 Chromatids
in each cell!
[SAME AS MITOSIS]
Chromosomes start in the X-shape.
Nuclear envelope dissolves, spindle appears.
No crossing over this time.
http://www.sinauer.com/cooper/4e/micro/16/16-05_Meiosis-Prophase2(NL-Large).jpg
Metaphase II
23 Chromosomes
46 Chromatids
in each cell!
• [SAME AS MITOSIS]
• Chromosomes line up in the middle of the
cell.
http://www.sinauer.com/cooper/4e/micro/16/16-06_Meiosis-Metaphase2(NL-Large).jpg
Anaphase II
23 Chromosomes
in each cell!
• [SAME AS MITOSIS]
• Chromosomes pulled apart at centromeres,
move toward poles.
• Chromosomes are no longer X-shaped.
http://www.sinauer.com/cooper/4e/micro/16/16-07_Meiosis-Anaphase2(NL-Large).jpg
Telophase II and Cytokinesis
•
•
•
•
23 Chromosomes
in each cell!
Nuclear envelope re-forms.
Cell divides.
Chromosomes return to chromatin.
4 GENETICALLY DISTINCT haploid cells result!
http://www.sinauer.com/cooper/4e/micro/16/16-08_Meiosis-Telophase2(NL-Large).jpg
Summary of Meiosis I in Diagrams
Prophase I
Metaphase I
Anaphase I
Telophase I
Summary of Meiosis II in Diagrams
Prophase II
Metaphase II
Anaphase II
Telophase
II
Summary of Mitosis in Diagrams
Prophase
Metaphase
Anaphase
Telophase
The Finished Products
• After meiosis, here’s what’s left:
• ♂: 4 sperm cells
• ♀: 1 ovum, 3 polar bodies
– Polar bodies are shriveled “non-eggs.”
• In other words, meiosis in females results in only one viable egg.
– Why polar bodies? To provide the egg enough cytoplasm
to nourish the potential embryo.
• Side note: The egg (not the sperm or polar bodies) has all the
organelles for the potential zygote.
– Compare the size of sperm and egg:
• http://learn.genetics.utah.edu/content/begin/cells/scale/
Summary of Mitosis
Start with one diploid cell
that has 46 chromosomes.
46
End with two diploid
daughter cells that each
have 46 chromosomes.
Mitosis
(diploid to diploid)
46
46
Summary of Meiosis (Males)
Start spermatogenesis
with one diploid
spermatocyte that has 46
chromosomes.
End with four haploid
sperm cells that each have
23 chromosomes.
46
Meiosis I
23
(diploid to haploid)
23
Meiosis II
(haploid to haploid)
23
23
23
23
Spermatogenesis Details
• The precursor cell to spermatogenesis is a
spermatogonium (2n).
– The spermatogonium divides by mitosis, producing
another spermatogonium (to repeat the process) and a
primary spermatocyte.
– The primary spermatocyte undergoes meiosis, producing
secondary spermatocytes after Meiosis II and four sperm
cells after Meiosis II.
• All four haploid cells produced by meiosis can
become viable sperm.
• The process is continuous starting at puberty.
– Each ejaculation is a release of 100 million-600 million
sperm.
Spermatogenesis
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/meiosis/meiosis.htm
Summary of Meiosis (Females)
Start oogenesis with one
diploid oocyte that has 46
chromosomes.
End with one haploid
ovum with 23
chromosomes and three
polar bodies.
46
Meiosis I
23
(diploid to haploid)
23
First polar body
Meiosis II
(haploid to haploid)
23
23
23
Second polar body
Second polar body
23
Second polar body
Oogenesis Details
• Oogenesis starts with an
oogonium that differentiates to
a primary oocyte within a follicle
(egg “container”).
– No cell division – oogonia don’t
mitotically divide.
• Prior to birth, primary oocytes
are halted in Prophase I.
– “Prior to birth” meaning before a
female is born.
• Meiosis I is completed when the
ovum matures, while Meiosis II
only completes when the egg is
fertilized.
• Only one of the four haploid
products can become a viable
ovum.
Oogenesis
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/meiosis/meiosis.htm
Oogenesis Facts
• Women are born with 2 million primary oocytes.
No more are made.
• Oocyte maturation starts at puberty, but by that
time only 400,000 are left.
• Each month, 1000 primary oocytes mature and
begin proceeding through Meiosis I, but most die.
• Usually only one every 28 days matures successfully
and is released in ovulation.
• Human females ovulate about 400 times total.
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/meiosis/meiosis.htm
Reproductive Strategies
• In meiosis: Notice how males produce as much
sperm as possible (at “low cost”), whereas
females invest a lot into one cell.
• In ecology/behavior: Notice how males (typically)
attempt to pass their genes on by mating with as
many individuals as possible with little parental
“duties,” whereas females (as young bearers)
invest their time in their single brood.
• Side side note: This explains why females’
menstrual cycles synchronize if they live in close
proximity to one another.
Kitchen Sink:
Alternation of Generations
• As humans, we’re pretty much diploid.
• In a weird way, though, we can think of
ourselves as having a “haploid life stage.”
– We did start out as haploid sperm/egg cells,
remember?
• Even so, diploid is dominant for us.
Alternation of Generations
• Organisms use haploid/diploid stages in different ways.
– For others, haploid stages are dominant, while for others,
both stages are equally dominant.
– This is called alternation of generations.
• Big in plants (and some animals) – more details later.
http://upload.wikimedia.org/wikipedia/commons/8/80/Alternation_of_generations_simpler.svg
Alternation of Generations
• Follow the image:
– A diploid (2n) sporophyte generates haploid spores by meiosis.
– The spores develop through mitosis into a haploid (n)
gametophyte, which releases haploid gametes.
– The gametes fuse to form a diploid (2n) zygote, which develops
mitotically into a diploid sporophyte again.
Alternation of Generations
• What that ultimately means for mitosis and
meiosis:
– Mitosis is a way to maintain chromosome number.
– Meiosis is a way to halve chromosome number.
• Meiosis does not always produce gametes.
– Sometimes it makes spores, which are like
gametes except they don’t fuse with one another.
• They just start developin’.
Alternation of Generations in Humans?
• As we said, the diploid stage is dominant in us.
• What if we had a true “alternation of generations”
like in some plants? What would it look like?
– Sperm and egg cells would not just fuse and make a
diploid zygote. They would be considered spores.
– The spores would develop into multicellular “sperm
creatures” and “egg creatures.”
– Then, “spermasaurus” and “eggzilla” would release
gametes of their own, which would fuse and make
humans.
• Actually, I think I’m going to make this into a horror movie.
Alternation of Generations
• Another look:
http://www.funsci.com/fun3_en/guide/guide3/micro3_en.htm
Alternation of Generations
The dominant
organism is haploid,
producing a diploid
zygote that
undergoes meiosis.
The dominant
organism is diploid,
producing a diploid
zygote that
undergoes mitosis.
There is no dominant
organisms. There is
alternation between
haploid and diploid
karyotypes.
Variation
Summary Slide
• Key: Sexual reproduction introduces genetic
variation.
– Genetic recombination causes independent
assortment of chromosomes.
• Chromosomes get randomly split up during Anaphase
I/II, so they are randomly passed into gametes.
– Crossing over (synapsis) mixes up alleles further.
– Random fertilization (which sperm and which
egg) adds to the variation.
Independent Assortment
• Independent assortment of chromosomes
leads to gametes of offspring that are not the
same as the gametes of their parents.
– Independent assortment produces 223 (8,388,608)
different combinations of alleles in the gametes.
Crossing Over
• Crossing over can lead to infinite variety in gametes.
– Two parents can produce a zygote with over 70 trillion (223 x
223) possible diploid combinations.
Labeling Meiosis
• Visit Quia and try the quiz entitled Labeling
Meiosis.
• This is very similar to the Labeling Mitosis quiz.
• We will do it as a class in a few moments…
Closure:
Mitosis and Meiosis In Karyotypes
• See if you can follow the processes of mitosis
and meiosis through actual karyotype images.
• If you can keep things straight, you’re in good
shape.
– Pseudo-rhyme?
Closure:
Mitosis Through Karyotypes
• Somatic cell in G1 or G0:
Closure:
Mitosis Through Karyotypes
• Somatic cell in G2 (after S phase):
Closure:
Mitosis Through Karyotypes
• After cytokinesis:
Closure: Just like Mitosis?
• Meiosis I is different from Mitosis:
– Tetrads are pulled apart instead of X-shaped
chromosomes.
– Crossing over happens in Prophase I.
• Identical genes are not passed on.
• Meiosis II is just like Mitosis except:
– Chromosomes are not duplicated beforehand.
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Closure:
Meiosis Through Karyotypes
• Somatic cell in testes/ovaries undergoing
meiosis, starting in G1 or G0:
Closure:
Meiosis Through Karyotypes
• Somatic cell in G2 (after S phase), entering
Meiosis I:
Closure:
Meiosis Through Karyotypes
• Two haploid cells entering Meiosis II:
Closure:
Meiosis Through Karyotypes
• Four unique haploid gametes produced after
Meiosis II:
Closure
• CrashCourse – Meiosis – Where the Sex Starts
• Bizarre Aquatic Creatures Are Secretly Lesbian
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