Meiosis Notes PDF

Meiosis & Sexual Reproduction AP Biology Cell division Review (Asexual reproduction) ▪ Mitosis ◆ produce cells with same information ▪identical daughter cells ◆ exact copies ▪clones ◆ same amount of DNA ▪ same number of chromosomes ▪ same genetic info...

Meiosis & Sexual Reproduction AP Biology Cell division Review (Asexual reproduction) ▪ Mitosis ◆ produce cells with same information ▪identical daughter cells ◆ exact copies ▪clones ◆ same amount of DNA ▪ same number of chromosomes ▪ same genetic information Aaaargh! I’m seeing AP Biology double! Asexual reproduction ▪ Single-celled eukaryotes ◆ yeast (fungi) ◆ Protists ▪ Paramecium ▪ Amoeba ▪ Simple multicellular budding eukaryotes budding ◆ Hydra What are the disadvantages of asexual reproduction? What are the AP Biology advantages?  How about the rest of us? ▪ What if a complex multicellular organism (like us) wants to reproduce? ◆ joining of egg + sperm ▪ Do we make egg & sperm by mitosis? No! What if we did, then…. 46 + 46 92 egg sperm zygote AP Biology Doesn’t work!  Mitosis & Meiosis Learning Goal: ▪Describe similarities/differences between the phases & outcomes of mitosis & meiosis ◆ Similar ▪ the way chromosomes segregate ◆ Differ ▪ number of cells produced ▪ Genetic content of daughter cells ▪ Meiosis involves 2 rounds of steps (meiosis I & meiosis II) AP Biology  How do we make sperm & eggs? ▪ Must reduce 46 chromosomes → 23 ◆ must reduce the number of chromosomes by half zygote 46 23 23 meiosis egg 46 23 46 23 fertilization sperm AP Biology gametes  Meiosis: production of gametes ▪ Alternating stages ◆ chromosome number must be haploid reduced ▪ diploid → haploid ▪ 2n → n ⬥ humans: 46 → 23 ▪ meiosis reduces chromosome number ▪ makes gametes ◆ fertilization restores chromosome number ▪ haploid → diploid ▪ n → 2n diploid AP Biology  Sexual reproduction lifecycle ▪ 2 copies ▪ diploid ▪ 2n ▪ 1 copy ▪ 1 copy fertilization meiosis ▪ haploid ▪ haploid ▪ 1n In the next ▪ 1n generation… We’re mixing things up here! A good thing? gametes gametes AP Biology  Karyotype ▪ Images of the 46 chromosomes can be arranged in pairs in order of size to produce a karyotype ◆ 2 chromosomes have same length, centromere position & staining pattern ◆ Homologous chromosomes carry genes that control the same inherited characters AP Biology  Homologous chromosomes ▪ Paired chromosomes ◆ both chromosomes of a pair carry “matching” genes ▪ control same inherited characters ▪ homologous = same information diploid single stranded homologous 2n chromosomes 2n = 4 double stranded AP Biology homologous chromosomes  Meiosis ▪ Reduction Division ◆special cell division for sexual reproduction ◆ reduce 2n → 1n ◆ diploid → haploid ▪ “two” → “half” ◆ makes gametes ▪ sperm, Warning: eggs meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are AP radically Biology different. Do not confuse the two!  I.P.M.A.T.P.M.A.T Overview of meiosis 2n = 4 interphase 1 prophase 1 metaphase 1 anaphase 1 n=2 n=2 prophase 2 metaphase 2 anaphase 2 telophase 2 n=2 telophase AP Biology 1 AP Biology  Double division of meiosis DNA replication Repeat I can’t Meiosis 1 after me! hear you! 1st division of meiosis separates homologous pairs Meiosis 2 2nd division of meiosis separates AP Biology sister chromatids  Preparing for meiosis ▪ 1st step of meiosis ◆ Duplication of DNA 2n = 6 ◆ Why bother? single stranded ▪ meiosis evolved after mitosis ▪ convenient to use “machinery” of mitosis ▪ DNA replicated in S phase of interphase preceding MEIOSIS I (just like in mitosis) 2n = 6 double AP Biology M1 prophase stranded Meiosis 1 2n = 4 ▪ 1st division of meiosis separates single stranded homologous pairs to ensure each gamete receives a haploid set of 2n = 4 chromosomes prophase 1 double stranded synapsis 2n = 4 metaphase 1 double stranded tetrad reduction telophase 1 Repeat I can’t 1n = 2 after hear double AP Biology stranded you! me!  Meiosis 2 ▪ 2nd division of meiosis separates 1n = 2 sister double stranded chromatids prophase 2 What does 1n = 2 this division double metaphase 2 look like? stranded 1n = 2 single stranded 4 AP Biology telophase 2  Cell Division ▪ Interphase 1st division of ▪ Meiosis 1 meiosis separates homologous pairs ◆ prophase 1 ◆ metaphase 1 (2n → 1n) ◆ anaphase 1 “reduction division” ◆ telophase 1 ▪ Meiosis 2 2nd division of ◆ prophase 2 meiosis separates sister chromatids ◆ metaphase 2 ◆ anaphase 2 (1n → 1n) ◆ telophase 2 AP Biology * just like mitosis *  Meiosis 1 & 2 AP Biology  Cell Division ▪ Interphase- Replication of genome and growth occur. Centrosomes replicate ▪ Meiosis I stages: ◆ Prophase I- 90% of total time of meiosis ▪ Chromosomes begin to condense ▪ Homologous chromosomes pair and match up by gene (forming a tetrad = 4 chromatids) ▪ Crossing over- Wherever these homologous chromosomes match up, genetic information will switch to opposite chromosome ▪ Centrosome movement toward poles of cell ▪ Spindle formation and attachment of fibers ▪ Breakdown of nuclear envelope AP Biology  ▪ Metaphase I- Tetrads arrange on the metaphase plate, spindle is fully attached ▪ Anaphase I- Sister chromatids move toward poles ◆ Sister chromatids remain attached ▪ Telophase I and Cytokinesis I ◆Each cell will have sister chromatids ◆ Splitting and cytokinesis ◆ Chromosomes might unwind, might not (depending on organism) ◆ Nucleus reappears AP Biology  Meiosis II ▪ Prophase II- Spindle apparatus forms, movement of sisters towards metaphase plate ▪ Metaphase II- Spindle fibers attach to sisters at metaphase plate ▪ Anaphase II- Separation and migration of individual chromosomes toward poles ▪ Telophase II and Cytokinesis II- Nuclei form and chromosomes unwind AP Biology  Trading pieces of DNA ▪ Crossing over prophase 1 ◆ during Prophase 1, sister chromatids intertwine ▪ homologous pairs swap pieces of chromosome ⬥ DNA breaks & re-attaches ⬥ Increases genetic diversity synapsis tetrad AP Biology  What are the Crossing over advantages of crossing over in ▪ 3 steps sexual reproduction? ◆ cross over ◆ breakage of DNA ▪ enzymes break the sugars & phosphate/nucleotides bonds ◆ re-fusing of DNA ▪ New combinations of traits AP Biology What makes Meiosis unique? 1) Synapses- process of attachment of homologous chromosomes Crossing Over- genetic recombination to increase genetic diversity among gametes Chiasma (plural- chiasmata) – physical manifestation of crossing over AP Biology 2) Tetrads on metaphase plate 3) Separation of homologous chromosomes in Anaphase I, but sisters stay attached to each other AP Biology Mitosis vs. Meiosis https://datbootcamp.com/biology-strategy/chromosome- and-chromatid-numbers-during-mitosis-and-meiosis/ AP Biology  Mitosis vs. Meiosis ▪ Mitosis ▪ Meiosis ◆ 1 division ◆ 2 divisions ◆ daughter cells genetically ◆ daughter cells genetically identical to parent cell different from parent ◆ produces 2 cells ◆ produces 4 cells ◆ 2n → 2n ◆ 2n → 1n ◆ produces cells for growth ◆ produces gametes & repair ◆ no crossing over ◆ crossing over AP Biology AP Biology  Putting it all together… meiosis → fertilization → mitosis + development gametes 46 23 46 23 46 46 meiosis egg 46 46 46 23 46 46 46 46 23 zygote fertilization mitosis sperm AP Biology development  The value of sexual reproduction ▪ Sexual reproduction introduces genetic variation ◆ genetic recombination ▪ independent assortment of chromosomes ⬥ random alignment of homologous chromosomes in Metaphase 1 ◆ crossing over ▪ mixing of alleles across homologous chromosomes ◆ random fertilization ▪ which sperm fertilizes which egg? ▪ Driving evolution ◆ providing variation for natural selection metaphase1 AP Biology  Variation from genetic recombination ▪ Independent assortment of chromosomes ◆ meiosis introduces genetic variation ◆ gametes of offspring do not have same combination of genes as gametes from parents ▪ random assortment in humans produces 223 (8,388,608) different combinations in gametes new gametes from Mom AP Biology from Dad offspring made by offspring  Variation from crossing over ▪ Crossing over creates completely new combinations of traits on each chromosome ◆ creates an infinite variety in gametes AP Biology  Variation from random fertilization ▪ Sperm + Egg = ? ◆ any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations ▪ Sexual reproduction allows us to maintain both genetic similarity & differences AP Biology  Sperm production Epididymis Testis germ cell (diploid) Coiled seminiferou s tubules primary spermatocyt e MEIOSIS I (diploid) secondary spermatocytes (haploid) MEIOSIS II Vas deferens spermatids (haploid) spermatozoa ▪ Spermatogenesis ◆ continuous & prolific process Cross-section of seminiferous tubule ◆ each ejaculation = 100-600 million sperm AP Biology  Egg production ▪ Oogenesis ◆ eggs in ovaries halted before Anaphase 1 ◆ Meiosis 1 completed during maturation ◆ Meiosis 2 completed after fertilization unequal divisions ◆ 1 egg + 2 polar bodies Meiosis 1 completed during egg maturation ovulation What is the advantage of this development system? Meiosis 2 completed triggered by fertilization AP Biology  Putting all your egg Oogenesis in one basket! primary follicles germinal cell (diploid) fallopian tube fertilization primary developing oocyte follicle (diploid) MEIOSIS I mature follicle with secondar secondary oocyte first polar body y oocyte ruptured MEIOSIS II (haploid) follicle (ovulation) after fertilization ovum (haploid second ) corpus luteum polar body AP Biology In Summary… Meiosis: ▪ a reduction division ▪Ensures genetic diversity (sexually reproducing organisms) ▪Each gamete receives 1 complete haploid set of chromosomes ▪Homologous chromosomes are paired: 1 from maternal parent & 1 from paternal ◆ Separation of homologous chromosomes ensures genetic diversity ◆ Crossing over increases genetic variation ◆ Fertilization increases genetic diversity & restores the diploid number of chromosomes AP Biology AP Biology