Meiosis & Sexual Reproduction 2/27/15
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
Asexual reproduction Single-celled eukaryotes u yeast (fungi) u Protists Paramecium Amoeba Simple multicellular eukaryotes u Hydra budding 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 + 46 92 egg sperm zygote
Human female karyotype 46 chromosomes 23 pairs
Human male karyotype 46 chromosomes 23 pairs
Homologous chromosomes Paired chromosomes u both chromosomes of a pair carry matching genes control same inherited characters homologous = same information diploid 2n 2n = 4 single stranded homologous chromosomes double stranded homologous chromosomes
How do we make sperm & eggs? Must reduce 46 chromosomes 23 u must reduce the number of chromosomes by half 46 46 23 egg 46 meiosis 23 sperm gametes 23 23 fertilization zygote
Meiosis: production of gametes Alternating stages u chromosome number must be reduced diploid haploid 2n n w humans: 46 23 meiosis reduces chromosome number makes gametes u fertilization restores chromosome number haploid diploid n 2n haploid diploid
Meiosis Reduction Division u special cell division for sexual reproduction u reduce 2n 1n u diploid haploid two half u makes gametes sperm, eggs Warning: meiosis evolved from mitosis, so stages & machinery are similar but the processes are radically different. Do not confuse the two!
Overview of meiosis 2n = 4 I.P.M.A.T.P.M.A.T interphase 1 prophase 1 metaphase 1 anaphase 1 n = 2 n = 2 prophase 2 metaphase 2 anaphase 2 telophase 2 n = 2 telophase 1
Meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids Meiosis 1 Meiosis 2
Preparing for meiosis 1st step of meiosis u Duplication of DNA u Why bother? meiosis evolved after mitosis convenient to use machinery of mitosis DNA replicated in S phase of interphase of MEIOSIS (just like in mitosis) 2n = 6 single stranded M1 prophase 2n = 6 double stranded
Meiosis 1 1st division of meiosis separates homologous pairs prophase 1 2n = 4 single stranded 2n = 4 double stranded synapsis tetrad telophase 1 1n = 2 double stranded metaphase 1 2n = 4 double stranded reduction
Meiosis 2 2nd division of meiosis separates sister chromatids 1n = 2 double stranded prophase 2 1n = 2 double stranded metaphase 2 1n = 2 single stranded 4 telophase 2
Meiosis 1 & 2
Trading pieces of DNA Crossing over u during Prophase 1, sister chromatids intertwine homologous pairs swap pieces of chromosome w DNA breaks & re-attaches prophase 1 synapsis tetrad
Crossing over 3 steps u cross over u breakage of DNA u re-fusing of DNA New combinations of traits
Mitosis vs. Meiosis
Mitosis vs. Meiosis Mitosis u 1 division u daughter cells genetically identical to parent cell u produces 2 cells u 2n 2n u produces cells for growth & repair u no crossing over Meiosis u 2 divisions u daughter cells genetically different from parent u produces 4 cells u 2n 1n u produces gametes u crossing over
Putting it all together meiosis fertilization mitosis + development gametes 46 23 46 23 meiosis 46 egg sperm 23 23 46 46 46 46 46 46 46 46 zygote fertilization mitosis development
The value of sexual reproduction Sexual reproduction introduces genetic variation u genetic recombination independent assortment of chromosomes w random alignment of homologous chromosomes in Metaphase 1 u crossing over mixing of alleles across homologous chromosomes u random fertilization which sperm fertilizes which egg? Driving evolution u providing variation for natural selection metaphase1
Variation from genetic recombination Independent assortment of chromosomes u meiosis introduces genetic variation u gametes of offspring do not have same combination of genes as gametes from parents random assortment in humans produces 2 23 (8,388,608) different combinations in gametes 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
Variation from random fertilization Sperm + Egg =? u any 2 parents will produce a zygote with over 70 trillion (2 23 x 2 23 ) possible diploid combinations
Sexual reproduction creates variability Sexual reproduction allows us to maintain both genetic similarity & differences. Jonas Brothers Baldwin brothers Martin & Charlie Sheen, Emilio Estevez
Epididymis Sperm production Vas deferens Testis Coiled seminiferous tubules germ cell (diploid) primary spermatocyte (diploid) secondary spermatocytes (haploid) spermatids (haploid) MEIOSIS I MEIOSIS II Spermatogenesis u continuous & prolific process Cross-section of seminiferous tubule u each ejaculation = 100-600 million sperm spermatozoa
Egg production Oogenesis u eggs in ovaries halted before Anaphase 1 u Meiosis 1 completed during maturation u 1 egg + 2 polar bodies unequal divisions Meiosis 1 completed during egg maturation What is the advantage of this development system? ovulation
Oogenesis germinal cell (diploid) primary follicles fallopian tube fertilization MEIOSIS I first polar body MEIOSIS II primary oocyte (diploid) secondary oocyte (haploid) developing follicle mature follicle with secondary oocyte ruptured follicle (ovulation) second polar body ovum (haploid) corpus luteum
Differences across kingdoms Not all organisms use haploid & diploid stages in same way u which one is dominant (2n or n) differs u but still alternate between haploid & diploid must for sexual reproduction