Meiosis Sexual Reproduction AP Biology 2007 2008 Cell

Meiosis & Sexual Reproduction AP Biology 2007 -2008

Cell division / Asexual reproduction § Mitosis u produce cells with same information § identical daughter cells u exact copies § clones u same amount of DNA § same number of chromosomes § same genetic information AP Biology Aaaargh! I’m seeing double!

Asexual reproduction § Single-celled eukaryotes yeast (fungi) u Protists u § Paramecium § Amoeba § Simple multicellular eukaryotes u Hydra What are the disadvantages of asexual reproduction? AP Biology What are the advantages? budding

How about the rest of us? § What if a complex multicellular organism (like us) wants to reproduce? u joining of egg + sperm § Do we make egg & sperm by mitosis? What if we did, then…. 46 egg AP Biology + 46 92 sperm zygote Doesn’t work!

Human female karyotype 46 chromosomes 23 pairs AP Biology

Human male karyotype 46 chromosomes 23 pairs AP Biology

Homologous chromosomes § Paired chromosomes u both chromosomes of a pair carry “matching” genes § control same inherited characters § homologous = same information diploid 2 n 2 n = 4 AP Biology single stranded homologous chromosomes double stranded homologous chromosomes

How do we make sperm & eggs? § Must reduce 46 chromosomes 23 u must half the number of chromosomes 23 46 meiosis 46 AP Biology zygote 23 egg 46 23 23 sperm gametes fertilization

Meiosis: production of gametes § Alternating stages u chromosome number must be reduced § diploid haploid § 2 n n w humans: 46 23 § meiosis reduces chromosome number § makes gametes u AP Biology fertilization restores chromosome number § haploid diploid § n 2 n

Sexual reproduction lifecycle § 2 copies § diploid § 2 n § 1 copy § haploid § 1 n fertilization meiosis We’re mixing things up here! A good thing? AP Biology § 1 copy § haploid § 1 n

Meiosis § Reduction Division u u u special cell division in sexually reproducing organisms reduce 2 n 1 n diploid haploid § “half” u makes gametes § sperm, eggs Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are AP radically Biology different. Do not confuse the two!

Overview of meiosis I. P. M. A. T 2 n=4 interphase 1 prophase 1 metaphase 1 anaphase 1 n=2 prophase 2 metaphase 2 anaphase 2 telophase AP Biology 1

Double division of meiosis DNA replication Repeat I can’t after hear you! me! AP Biology 1 st division of meiosis separates homologous pairs 2 nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2

Preparing for meiosis § 1 st step of meiosis Duplication of DNA u Why bother? u § meiosis evolved after mitosis § convenient to use “machinery” of mitosis § DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) AP Biology M 1 prophase 2 n = 6 single stranded 2 n = 6 double stranded

Meiosis 1 § 1 st division of meiosis 2 n = 4 single stranded separates homologous pairs prophase 1 2 n = 4 double stranded metaphase 1 2 n = 4 double stranded synapsis tetrad telophase 1 IRepeat can’t AP Biology after hear you! me! 1 n = 2 double stranded

Meiosis 2 § 2 nd division of meiosis 1 n = 2 double stranded separates sister chromatids prophase 2 What does this division look like? 1 n = 2 single stranded AP Biology 1 n = 2 double stranded metaphase 2 4 telophase 2

Steps of meiosis § Meiosis 1 interphase u prophase 1 u metaphase 1 u anaphase 1 u telophase 1 u § Meiosis 2 prophase 2 u metaphase 2 u anaphase 2 u telophase 2 AP Biology u 1 st division of meiosis separates homologous pairs (2 n 1 n) “reduction division” 2 nd division of meiosis separates sister chromatids (1 n 1 n) * just like mitosis *

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Trading pieces of DNA § Crossing over u synapsis tetrad AP Biology during Prophase 1, sister chromatids intertwine § synapsis § homologous pairs swap pieces of chromosome w DNA breaks & re-attaches prophase 1

Crossing over § 3 steps What are the advantages of sexual reproduction? cross over u breakage of DNA u re-fusing of DNA u § New combinations of traits AP Biology

Meiosis 1 AP Biology

Meiosis 2 AP Biology

Mitosis vs. Meiosis AP Biology

Mitosis vs. Meiosis § Mitosis u u u AP Biology 1 division daughter cells genetically identical to parent cell produces 2 cells 2 n produces cells for growth & repair no crossing over § Meiosis u 2 divisions daughter cells genetically different from parent produces 4 cells 2 n 1 n produces gametes u crossing over u u

Putting it all together… meiosis fertilization mitosis + development gametes 46 meiosis 46 23 egg 23 23 23 zygote fertilization AP Biology sperm 46 46 46 mitosis & mitosis development

The value of sexual reproduction § Sexual reproduction introduces genetic variation u genetic recombination during meiosis § independent assortment of chromosomes w random alignment of homologous chromosomes in Meiosis 1 u crossing over § mixing of alleles across homologous chromosomes u random fertilization § which sperm fertilizes which egg? § Driving evolution u metaphase 1 AP Biology variation for natural selection

Variation from genetic recombination § Independent assortment of chromosomes meiosis introduces genetic variation u gametes of offspring do not have same combination of genes as gametes from parents u § random assortment in humans produces 223 (8, 388, 608) different combinations in gametes AP Biology from Mom from Dad offspring new gametes made by offspring

Variation from crossing over § Crossing over creates completely new combinations of traits on each chromosome u creates an infinite variety in gametes AP Biology

Variation from random fertilization § Sperm + Egg = ? u AP Biology any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations

Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Michael & Kirk Douglas AP Biology Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) Vas deferens spermatids (haploid) spermatozoa § Spermatogenesis continuous & prolific process Cross-section of seminiferous tubule u each ejaculation = 100 -600 million sperm AP Biology u MEIOSIS II

Egg production § Oogenesis u u eggs in ovaries halted before Anaphase 1 Meiosis 1 completed during maturation Meiosis 2 completed unequal division after fertilization 1 egg + 2 polar bodies Meiosis 1 completed during egg maturation What is the advantage of this development system? Meiosis 2 completed AP Biology triggered by fertilization ovulation

Oogenesis germinal cell (diploid) primary follicles fallopian tube fertilization primary oocyte (diploid) MEIOSIS I secondary oocyte (haploid) first polar body MEIOSIS II after fertilization second polar body AP Biology ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum

Differences across kingdoms § Not all organisms use haploid & diploid stages in same way u u which one is dominant (2 n or n) differs but still alternate between haploid & diploid § must for sexual reproduction AP Biology