Meiosis (Chapter 13)

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

Fig µmChromosomes Chromosome duplication (including DNA synthesis) Chromo- some arm Centromere Sister chromatids DNA molecules Separation of sister chromatids Centromere Sister chromatids

Why meiosis? Produces haploid cells. Genetic diversity Evolution

Eukaryotes Haploid(n): Single set of chromosomes (23 in humans) Diploid(2n): Twice the haploid number or two sets (46) Homologous chromosomes: 2 chromosomes that match up One from the mother one from the father Homologs: One of the pair of chromosomes

Human chromosomes 23 pairs 22 autosomes 1 sex chromosome pair XX female All eggs are X XY male Sperm are either X or Y Chromosomes are divided into 7 groups Based on size, shape and appearance

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

Karyotype Display of the chromosomes

Downs Syndrome

Turners syndrome

Meiosis 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 Each round of division has a Prophase Metaphase Anaphase Telophase

Interphase Meiosis I Meiosis II Pair of homologous chromosomes in diploid parent cell Pair of duplicated homologous chromosomes Chromosomes duplicate Diploid cell with duplicated chromosomes Sister chromatids Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes 1 2

Meiosis I Meiosis II Homologous chromosomes separate Haploid cells with duplicated chromosomes Sister chromatids separate Haploid cells with unduplicated chromosomes 1 2

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

Meiosis 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

DNA breaks Cohesins Centromere DNA breaks Pair of homologous chromosomes: Paternal sister chromatids Maternal sister chromatids Synaptonemal complex forming Chiasmata Crossover

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

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

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

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

Metaphase I 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. 13-8b Prophase IMetaphase 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 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 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 Homologues cluster at the poles Nuclear membrane reforms Each daughter cell contains half the # of chromosomes Sister chromatids Different due to crossover

Telophase I Cytokinesis may occur Second division occurs after variable length

Meiosis I

Prophase II Nuclear membrane breaks down New spindles form

Metaphase II Spindle fibers bind to both sides of the centromere

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

Telophase II Nuclear envelope reforms 4 haploid cells

Meiosis II

Meiosis

Sexual reproduction 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 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 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 Spend majority of time as haploid Zygote undergoes meiosis Then mitosis

3. Plants Alternate between multicellular haploid Multicellular diploid phase

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

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

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

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

Independent assortment

Crossover Recombinant chromosomes Carry information from 2 different parents

Random fertilization

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