In This Lesson:
Meiosis
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.
• Hank again! Talks Sex
By the end of this lesson…
• You should be able to explain meiosis, both in
why it evolved and why it bears some
similarity to mitosis.
• You should be able to differentiate between
oogenesis and spermatogenesis.
Kinds of Reproduction
• Sexual
– 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).
46
+
46
=
92

We don’t have 92 chromosomes!!!
Meiosis
• Meiosis is another process of cell division.
• Sexual reproduction only.
– Why?
• Like Mitosis, except:
– There are two cell divisions.
– # of chromosomes is halved.
– Genetic material is varied.
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.
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 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.
Henry the VIII
• Six wives.
• Really wanted a son.
• Whose “fault” was it?
Soooooo many wives, so few lived.
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
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 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.
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!
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
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
• All four haploid cells produced by meiosis can
become viable sperm.
• The process is continuous starting at puberty.
– Each ejaculation is a release of 100-600 million
sperm.
– 1 tsp of ejaculate contains 250, 000, 000 million
sperm
- males make 60,000 sperm / minute
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 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.
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Three Ways that Genetic Diversity
Occurs During Meiosis
Crossing Over
During prophase of meiosis I, the double-chromatid homologous pairs of
chromosomes cross over with each other and often exchange chromosome segments.
This recombination creates genetic diversity by allowing genes from each
parent to intermix, resulting in chromosomes with a different genetic complement.
The exchange occurs between non-sister chromatids.
Because genes often interact with each other, the new combination of genes
on a chromosome can lead to new traits in offspring.
Random Chromatid Assortment (Independent Assortment)
A second source of genetic diversity occurs during metaphase 1, when chromosomes
line up at the equator of the cell.
Crossing over in meiosis I leads to non-identical chromatids.
The outcome of which chromosome will go to which gamete is random,
so that each gamete has a potentially unique combination of genetic material.
223 combinations exist = 8,388,608
Fertilization
Fertilization creates genetic diversity by allowing each parent to randomly
contribute a unique set of genes to a zygote.
While fertilization is not part of meiosis, it depends on meiosis creating
haploid gametes. The fertilized cell restores the diploid number. Without meiosis,
the number of chromosomes per cell would double in each generation of offspring,
leading to unstable conditions that could threaten the viability of a species.
So………. Which little swimmer does the job!