In This Lesson:
Meiosis
(Lesson 3 of 3)
Today is Friday,
November 13th, 2015
Pre-Class:
Briefly summarize the events of the five steps of
mitosis/cytokinesis:
-Prophase
-Metaphase
-Anaphase
-Telophase
-Cytokinesis
Expect the Bag of Evil!
Today’s Agenda
•
•
•
•
•
•
•
Mitosis Gizmo
Labeling Mitosis [Quia]
Guided Notes?
Reproduction review
Meiosis
Meiosis Web Lesson
And maybe…I don’t know…something interesting and
seemingly off-topic.
• Where is this in my book?
– Academic: P. 275 and following…
– Honors: P. 137 and following…
By the end of this lesson…
• You should be able to describe the process
most living cells go through in order to
prepare gametes for reproduction.
Cell Division Gizmo
• Now, we’re going to put our knowledge to the
test with a gizmo from ExploreLearning.
– Called Cell Division Gizmo.
– Directions are on the accompanying Quia quiz
called Cell Division Gizmo.
– [Log-in Instructions]
• When you are done, attempt the Quia quiz
called Labeling Mitosis.
– This one’s important!
Labeling Mitosis
• Log-in to Quia and take the quiz entitled,
“Labeling Mitosis.”
• This is an excellent review tool and will be
online for the remainder of the year.
• Note: “Labeling Meiosis” is different, but will
also be available all year long.
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).
• Kinds of asexual reproduction:
– Binary Fission
– Budding
– Fragmentation
Asexual Reproduction
• The big disadvantage:
– Little genetic diversity.
• Offspring are almost exactly like parents.
• Problems are usually not “taken care of.”
Sexual Reproduction
• Increases genetic diversity.
– DNA from Mom and Dad.
• Instead of just one of them.
• Requires the use of gametes.
– In animals: sperm and ova (egg cells).
– Different for other living things.
Meiosis
• Meiosis is another process of cell division.
• Sexual reproduction only.
– Why?
• Like Mitosis, except:
– # of chromosomes is halved.
– Two cell divisions.
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.
– From 46 to 23, diploid to haploid – the reductive
step.
• Meiosis II divides the cells but keeps the
chromosome number the same.
– Process is just like mitosis but without the
duplication beforehand.
– From 23 to 23, haploid to haploid.
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.
• Increases genetic variability.
• Side Notes:
– The spot at which the chromatids
cross is called the chiasma.
– Crossing over occurs in a process
called synapsis.
http://library.thinkquest.org/20465/meiosis.html
Crossing Over: Another View
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.
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
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
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
Labeling Meiosis
• Visit Quia and try the quiz entitled Labeling
Meiosis.
• This is very similar to the Labeling Mitosis quiz
and will also be on our Unit 4 Test.
• We will do it as a class in a few moments…
Aside: 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.
Time for an Activity
• The Meiosis Web Lesson!
– Find those web lesson sheets.
• Website is linked on my page:
– http://www.sumanasinc.com/webcontent/anisam
ples/majorsbiology/meiosis.html
Exit Ticket
1. Which step of meiosis (I or II) reduces the
number of chromosomes?
2. What is the purpose of “crossing over?”
• You must turn this in or show me on your way
out.
• P.S. Yay the cell cycle is done!