meiosis (chapter 13). mitosis two identical daughter cells interphase cell growth, preparing for...
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Meiosis (Chapter 13)
Mitosis Two identical daughter cells Interphase Cell growth, preparing for cell
division Prophase, Metaphase, Anaphase,
Telophase Cytokinesis
Fig. 12-4 0.5 µm Chromosomes
Chromosomeduplication(including DNAsynthesis)
Chromo-some arm
Centromere
Sisterchromatids
DNA molecules
Separation ofsister 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 homologousreplicated chromosomes
Centromere
Sisterchromatids
Metaphasechromosome
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 ofhomologouschromosomesin diploidparent cell
Pair of duplicatedhomologouschromosomes
Chromosomesduplicate
Diploid cell withduplicatedchromosomes
Sisterchromatids
Homologouschromosomesseparate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
Haploid cells with unduplicated chromosomes
1
2
Meiosis I
Meiosis II
Homologouschromosomesseparate
Haploid cells withduplicated chromosomes
Sister chromatidsseparate
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
DNAbreaks
CohesinsCentromere
DNAbreaks
Pair ofhomologouschromosomes:
Paternalsisterchromatids
Maternalsisterchromatids
Synaptonemalcomplex forming
Chiasmata
Crossover Crossover
1
2 4
3
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 I Metaphase I
Centrosome(with centriole pair)
Sisterchromatids Chiasmata
Spindle
Centromere(with kinetochore)
Metaphaseplate
Homologouschromosomes
Fragmentsof nuclearenvelope
Microtubuleattached tokinetochore
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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