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!

In This Lesson:Meiosis

(Lesson 3 of 3)

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• 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

46 Chromosomes92 Chromatids

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.

http://home.comcast.net/~mjmayhew42/Biology%20notes/meiosis%20notes_files/image005.gif

Tetrad

Metaphase I• 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.sinauer.com/cooper/4e/micro/16/16-02_Meiosis-Metaphase1(NL-Large).jpg

46 Chromosomes92 Chromatids

http://www.uic.edu/classes/bios/bios100/lecturesf04am/metaphase1m.jpg

Compare Metaphases

• Metaphase – Mitosis • Metaphase I - Meiosis

Anaphase I

• 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

23 Chromosomes46 Chromatidson each side!

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 Chromosomes46 Chromatidsin 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• [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

23 Chromosomes46 Chromatidsin each cell!

Metaphase II

• [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

23 Chromosomes46 Chromatidsin each cell!

Anaphase II• [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

23 Chromosomesin each cell!

Telophase II and Cytokinesis

• 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

23 Chromosomesin each cell!

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

46

46 46

Start with one diploid cell that has 46 chromosomes.

Mitosis(diploid to diploid)

End with two diploid daughter cells that each have 46 chromosomes.

Summary of Meiosis (Males)

46

23 23

Start spermatogenesis with one diploid

spermatocyte that has 46 chromosomes.

Meiosis I(diploid to haploid)

23 23 23 23

Meiosis II(haploid to haploid)

End with four haploid sperm cells that each have

23 chromosomes.

Summary of Meiosis (Females)

46

23 23

Start oogenesis with one diploid oocyte that has 46

chromosomes.

Meiosis I(diploid to haploid)

23 23

Meiosis II(haploid to haploid)

End with one haploid ovum with 23

chromosomes and three polar bodies.

23 23

First polar body

Second polar body Second polar body 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/

anisamples/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!