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Today is Thursday, December 4 th, 2014 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. In This Lesson: Meiosis (Lesson 2 of 3)
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
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 = 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:
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. 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 Chromatin condenses to X-shaped chromosomes. Maternal/paternal chromosomes pair up to form tetrads (pair of X-shaped chromosomes, four chromatids). Crossing over occurs. 46 Chromosomes 92 Chromatids
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
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
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. 46 Chromosomes 92 Chromatids
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. 23 Chromosomes 46 Chromatids on each side!
Compare Anaphases Anaphase – Mitosis Anaphase I - Meiosis
Telophase I and Cytokinesis Chromosomes gather at cell poles. Cell divides. 23 Chromosomes 46 Chromatids in each cell!
Summary of Meiosis I in Diagrams Prophase IMetaphase 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. No crossing over this time. 23 Chromosomes 46 Chromatids in each cell!
Metaphase II [SAME AS MITOSIS] Chromosomes line up in the middle of the cell. 23 Chromosomes 46 Chromatids in each cell!
Anaphase II [SAME AS MITOSIS] Chromosomes pulled apart at centromeres, move toward poles. Chromosomes are no longer X-shaped. 23 Chromosomes in each cell!
Telophase II and Cytokinesis Nuclear envelope re-forms. Cell divides. Chromosomes return to chromatin. 4 GENETICALLY DISTINCT haploid cells result! 23 Chromosomes in each cell!
Summary of Meiosis I in Diagrams Prophase IMetaphase I Anaphase I Telophase I
Summary of Meiosis II in Diagrams Prophase IIMetaphase II Anaphase II Telophase II
Summary of Mitosis in Diagrams ProphaseMetaphase 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:
Summary of Mitosis 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) Start spermatogenesis with one diploid spermatocyte that has 46 chromosomes. Meiosis I (diploid to haploid) 23 Meiosis II (haploid to haploid) End with four haploid sperm cells that each have 23 chromosomes.
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 million sperm.
Summary of Meiosis (Females) Start oogenesis with one diploid oocyte that has 46 chromosomes. Meiosis I (diploid to haploid) 23 Meiosis II (haploid to haploid) End with one haploid ovum with 23 chromosomes and three polar bodies. 23 First polar body 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 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.
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.
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:
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 2 23 (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 (2 23 x 2 23 ) 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 G 1 or G 0 :
Closure: Mitosis Through Karyotypes Somatic cell in G 2 (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
Closure: Meiosis Through Karyotypes Somatic cell in testes/ovaries undergoing meiosis, starting in G 1 or G 0 :
Closure: Meiosis Through Karyotypes Somatic cell in G 2 (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 Necrophiliacs article