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11.4 – Meiosis & Sexual Reproduction Regents Biology 2007-2008 Objectives § Summarize the events of meiosis. § Contrast meiosis and mitosis. Regents Biology Cell division/Asexual reproduction § Mitosis - produce identical cells/ clones Ex: - single-celled eukaryotes (yeast, protists) - simple multicellular eukaryotes (hydra) § What might be some advantages of asexual reproduction? Disadvantages? Regents Biology http://www.youtube.com/watch?v=ofxDIS7fbCE How about the rest of us? § What if a complex multicellular organism (like us) wants to reproduce? § Do we make egg and sperm by mitosis? Regents Biology Regents Biology Regents Biology Meiosis – production of gametes § chromosome number must be reduced (2n → n) § fertilization restores chromosome number (n → 2n) Regents Biology Regents Biology Double division of meiosis DNA replication 1st division of meiosis separates homologous pairs Meiosis 1 2nd division of meiosis separates sister chromatids Regents Biology Meiosis 2 Preparing for meiosis § 1st step of meiosis: DNA replication § DNA replicated in S phase of interphase of meiosis (just like in mitosis) Regents Biology M1 prophase 2n = 4 2n = 8 Meiosis 1 2n = 4 § 1st division of meiosis separates homologous pairs prophase 1 2n = 8 metaphase 1 2n = 8 reduction telophase 1 Regents Biology 1n = 4 Meiosis 2 § 2nd division of 1n = 4 meiosis separates sister chromatids prophase 2 1n = 4 metaphase 2 1n = 2 Regents Biology telophase 2 Regents Biology Trading pieces of DNA § Crossing over - during prophase 1, sister chromatids intertwine - homologous pairs swap pieces of chromosome - DNA breaks and re-attaches § Advantages? synapsis tetrad Regents Biology prophase 1 Mitosis vs. Meiosis Regents Biology Variation from crossing over § Crossing over creates completely new combinations of traits on each chromosome – genetic recombination! § creates an infinite variety in gametes Regents Biology Variation from random fertilization § Sperm + Egg = ? § any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations Regents Biology Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity and differences Regents Biology Sperm production Epididymis Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) MEIOSIS I secondary spermatocytes (haploid) Vas deferens spermatids (haploid) spermatozoa - a continuous Cross-section and prolific process of seminiferous tubule - each ejaculation contains ~ 100-600 million Regents Biology sperm MEIOSIS II Egg production - eggs in ovaries halted before anaphase 1 - meiosis 1 completed during maturation - Meiosis 2 completed after fertilization - 1 egg + 2 polar bodies unequal divisions Meiosis 1 completed during egg maturation Meiosis 2 completed triggered Regents Biology by fertilization ovulation Egg Production 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 Regents Biology ovum (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) corpus luteum