Meiosis Chapter 13 Why meiosis n n n

Meiosis (Chapter 13)

Why meiosis? n n n Produces haploid cells. Genetic diversity Evolution

Mitosis n n n Two identical daughter cells Interphase Cell growth, preparing for cell division Prophase, Metaphase, Anaphase, Telophase Cytokinesis

Fig. 12 -4 0. 5 µm Chromosomes Chromosome arm Centromere DNA molecules Chromosome duplication (including DNA synthesis) Sister chromatids Separation of sister chromatids Centromere Sister chromatids

Fig. 13 -3 b 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome

Meiosis n n n 2 successive rounds of cell division No replication of DNA between rounds Meiosis I Half the number of chromosomes Meiosis II 4 haploid cells

Meiosis n n n Each round of division has a Prophase Metaphase Anaphase Telophase

Fig. 13 -7 -3 Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes Meiosis I 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II 2 Sister chromatids separate Haploid cells with unreplicated chromosomes

Meiosis n n n n Synapsis: Homologous chromosomes Closely associated Synaptonemal complex: Homologous chromosomes are paired Protein complex between them Occurs in prophase I

Meiosis n n n Crossing-over: Homologues exchange chromosomal information Genetic recombination: Chromosomes from one parent carry info from both Chiasma “cross” (plural: Chiasmata) Site where crossing-over happens

Cross-over n n Between non-sister chromatids Stabilized by sister chromatids

Prophase I n n n DNA coils tighter DNA already duplicated Sister chromatids joined at centromeres

Prophase I n n n Sister chromatid cohesion: Sister chromatids closely associate Homologous chromosomes line up next to each other Crossing over happens Non-sister chromatids

Prophase I n n Crossing over ends 4 chromatids (2 homologs) stay close due to 1. Sister chromatid cohesion 2. Chiasmata where crossover occurs

Metaphase I n n n Homologous pairs align beside each other Metaphase plate (center) Chiasmata holds homologous chromosomes together Maternal homologue orients towards one pole Paternal homologue orients towards other pole

Metaphase I

Fig. 13 -8 b Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore

Anaphase I n n n 90% meiosis is spent in Prophase & Metaphase Spindle fibers begin to shorten Pull apart homologous chromosomes Go to separate poles Sister chromatids remain together Mitosis-sister chromatids separate

Anaphase I n n n Each pole has a complete haploid set of chromosomes Each pole has one member of the homologous pair Either a maternal or paternal homologue

Anaphase I

Telophase I n n n Homologues cluster at the poles Nuclear membrane reforms Each daughter cell contains half the # of chromosomes Sister chromatids Different due to crossover

Telophase I n n Cytokinesis may occur Second division occurs after variable length

Meiosis I

Prophase II n n Nuclear membrane breaks down New spindles form

Metaphase II n Spindle fibers bind to both sides of the centromere

Anaphase II n n Spindle fibers contract Sister chromatid cohesion is released Splits the sister chromatids Move to opposite poles

Telophase II n n Nuclear envelope reforms 4 haploid cells

Meiosis II

Meiosis

Sexual reproduction n n n Gametes: Egg & sperm Half the number of chromosomes Zygote: Egg and sperm combine Fertilization or syngamy: Fusion of gametes to form a new cell

Sexual reproduction n n Life cycles alternate Diploid & haploid chromosome numbers Alternates between meiosis & fertilization Offspring inherit chromosomes from both parents Variations occur producing 3 types of sexual life cycles

1. Animals n n n Majority of time as diploids Haploids do not under go mitosis Germ-line cells: Cells that will under go meiosis Produce gametes

2. Fungi and some algae n n n Spend majority of time as haploid Zygote undergoes meiosis Then mitosis

3. Plants n n Alternate between multicellular haploid Multicellular diploid phase

Evolution n n Asexual reproduction: Inherit chromosomes from one parent Identical to parent Protists reproduce asexually Plants reproduce asexually

Sexual reproduction n n Generates genetic diversity Evolutionary adaptation depends on a population’s genetic variation

Genetic diversity n n n 1. Independent assortment 2. Crossover 3. Random fertilization

Independent assortment n n n Genes on different chromosomes Orient independently Homologous pairs line up as a matter of chance

Independent assortment

Crossover n n Recombinant chromosomes Carry information from 2 different parents

Random fertilization

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