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

E:\Chapter_13\A_PowerPoint_Lectures\13_Lecture_Presentation\1312GeneticVariationA.html