Meiosis & Sexual Life Cycles Chapter 13

What are disadvantages of asexual reproduction? Cell Reproduction Mitosis produce cells with same information identical daughter cells exact copies (clones) same amount of DNA same number of chromosomes asexual reproduction What are disadvantages of asexual reproduction? What are advantages?

Asexual Reproduction Single-celled eukaryotes reproduce asexually (mitosis + cytokinesis) Yeast, Paramecium, Amoeba Simple multicellular eukaryotes reproduce asexually by budding Hydra Budding in Hydra

Budding in Yeast Cell Division in Amoeba

Reproduction in multicellular organisms How is it possible to maintain such genetic continuity in a multicellular organism? Michael & Kirk Douglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

What has to happen to make gametes? Sexual Reproduction But how does a complex multicellular organism reproduce? joining of gametes egg (or ovum) sperm What has to happen to make gametes?

Role of Meiosis in Sexual Cycles Alternating processes fertilization & meiosis alternate meiosis reduces chromosome number diploid haploid 2n n fertilization restores chromosome number haploid diploid n 2n

Meiosis & fertilization must alternate for sexual reproduction to continue Alternation of generation dominant multicellular stage (n vs. 2n) differs amongst organisms

Multicellular Haploid (n) organism = gametophyte Multicellular Diploid (2n) organism = sporophyte Alternation of generations: life history having a multicellular haploid stage which alternates with a multicellular diploid stage Evolutionary trends: Earlier organisms have dominant gametophyte (n) individual • gametophyte generation is major stage in life of mosses, visible stage in fungi, and is an independent plant in ferns However, gametophyte is only an inconspicuous structure within flower or other reproductive organs of "higher" plants

Visualizing Chromosomes

23 pairs of chromosomes

Human Female Karyotype

Human male karyotype

What does a karyotype reveal? Mutation in chromosome number or gross chromosomal abnormalities Down’s syndrome or Trisomy 21

Cri du chat

Homologous Chromosomes Paired chromosomes both chromosomes of a pair carry genes controlling same inherited characters homologous = same information 2 copies = diploid = 2n

Sexual reproduction: Fertilization From Mom – female gamete From Dad – male gamete 1 copy Haploid 1n What would happen if meiosis didn’t occur? -2 copies -Diploid -2n Zygote

Making gametes -2 copies -diploid -2n -1 copy -haploid -1n

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

Double division of Meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd division of meiosis separates sister chromatids

1st division of meiosis separates homologous pairs (2n 1n) Steps of Meiosis Meiosis 1 prophase 1 metaphase 1 anaphase 1 telophase 1 Meiosis 2 prophase 2 metaphase 2 anaphase 2 telophase 2 1st division of meiosis separates homologous pairs (2n 1n) 2nd division of meiosis separates sister chromatids (1n 1n) *just like mitosis*

Preparing for Meiosis 1st step of meiosis Replication of DNA Why bother? meiosis evolved after mitosis convenient to use “machinery” of mitosis DNA replicated in S phase of meiosis interphase (just like in mitosis) 2n Single-stranded 2n Double-stranded

Preparing for meiosis Synapsis Tetrad 1st division of meiosis separates homologous pairs Synapsis – pairing of homologous chromosomes – forms a tetrad (4 sister chromatids) 2n single stranded 2n double stranded Prophse 1 Synapsis Metaphase 1 2n double stranded Telophase 1 Tetrad 1n double stranded

What does this division look like? Meiosis 2 2nd division of meiosis separates sister chromatids What does this division look like? 1n double stranded Metaphase 2 1n single stranded cytokinesis

Mitosis vs. Meiosis

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

New gametes made by offspring The value of meiosis Meiosis introduces genetic variation gametes of offspring do not have same genes as gametes from parents genetic recombination random assortment in humans produces 223 (8,388,608) different combinations From mom From dad New gametes made by offspring

Random Fertilization Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

And more variation During Prophase 1 homologous pairs swap pieces of chromosome sister chromatids intertwine crossing over

What are advantages of sexual reproduction? What are disadvantages? Crossing Over 3 steps cross over breakage of DNA re-fusing of DNA New combinations of traits What are advantages of sexual reproduction? What are disadvantages?

Genetic variation Meiosis & crossing over introduce great genetic variation to population - drives evolution Consider the greater variation with 23 pairs of chromosomes = mixing and matching

Sources of genetic variability Genetic variability in sexual reproduction independent assortment homologous chromosomes in Meiosis 1 sister chromatids in Meiosis 2 crossing over between homologous chromosomes in prophase1 random fertilization random ovum by a random sperm

Spermatogenesis Continuous & prolific process in adult male Each ejaculation contains 100–650 million sperm Occurs in seminiferous tubules

Structure of sperm

What is the advantage of 1 egg released at a time? Oogenesis At birth an ovary contains all of the primary oocytes it will ever have ~300 eggs released Unequal cytokinesis 1 egg 3 polar bodies polar bodies degenerate What is the advantage of 1 egg released at a time?