What is Meiosis Exactly?What is Meiosis Exactly? Meiosis is a form of cell division that Meiosis is a form of cell division that

halves the number of chromosomes halves the number of chromosomes when forming specialized reproductive when forming specialized reproductive cells such as gametes or sporescells such as gametes or spores

There are 2 stages of meiosis, Meiosis There are 2 stages of meiosis, Meiosis 1 and meiosis 21 and meiosis 2

Meiosis 1 and 2 create 4 haploid cells Meiosis 1 and 2 create 4 haploid cells all togetherall together

IV. Meiosis – Reduction DivisionIV. Meiosis – Reduction Division

Stages of Meiosis 1Stages of Meiosis 1

Stages of Meiosis 2Stages of Meiosis 2

Interphase before Interphase before replicationreplication

Interphase after replicationInterphase after replication

I. Meiosis (Reduction I. Meiosis (Reduction Division)Division)A. Meiosis IA. Meiosis I

1. 1. Prophase IProphase I

a. chromosomes become distincta. chromosomes become distinct

b. nucleolus and nuclear b. nucleolus and nuclear membranemembrane

disappear and spindle fibers disappear and spindle fibers appearappear

Prophase IProphase I

c. spindle fibers appearc. spindle fibers appear

d.d.synapsissynapsis–homologous –homologous chromosomes chromosomes

Line up togetherLine up together

Prophase IProphase I

e. e. crossing overcrossing over may occur may occur

1) portions of chromatid from one1) portions of chromatid from one

parent break off and attach to a parent break off and attach to a

homologous chromatid from the homologous chromatid from the

other parentother parent

2) results in 2) results in genetic recombinationgenetic recombination

2. 2. Metaphase IMetaphase I a. chromosomes line up along a. chromosomes line up along

the midlinethe midline b. sister chromatids b. sister chromatids do notdo not

separateseparate

3. 3. Anaphase IAnaphase I

a. at random, one member of a. at random, one member of each each

homologous pair moves to homologous pair moves to the the

opposite poles (opposite poles (independent independent

assortmentassortment

4. 4. Telophase I and Cytokinesis ITelophase I and Cytokinesis I a. chromosomes reach a. chromosomes reach

opposite polesopposite poles b. cytokinesis beginsb. cytokinesis begins

Telophase ITelophase Ic. resulting cells have the c. resulting cells have the n or n or

haploidhaploid number of chromosomes number of chromosomes

1) one member of each 1) one member of each homologous pair with two homologous pair with two attached chromatids attached chromatids

d. each new cell contains ½ the d. each new cell contains ½ the

the number of chromosomes the number of chromosomes as the original cell as the original cell

B. B. Meiosis IIMeiosis II

1. 1. Prophase IIProphase II a. spindle form and a. spindle form and

chromosomes chromosomes begin to move toward the begin to move toward the

mid-line of mid-line of the cellthe cell

. . Metaphase IIMetaphase II

a. chromosomes move to the a. chromosomes move to the mid-line of the dividing cellmid-line of the dividing cell

3.3. Anaphase IIAnaphase II a. chromatids separate and a. chromatids separate and

move to the opposite poles of move to the opposite poles of the cellthe cell

4. Telophase II4. Telophase II

a. nuclear membrane forms a. nuclear membrane forms around the nucleus in each cellaround the nucleus in each cell

b. each resulting cell contains b. each resulting cell contains the the

nn number of chromosomes number of chromosomes

Meiosis 1 and Meiosis 2Meiosis 1 and Meiosis 2

Animation of meiosisAnimation of meiosis

Meiosis- video.wmv

C. Chart Comparing Mitosis C. Chart Comparing Mitosis and Meiosisand Meiosis

MitosisMitosis 2 cells result2 cells result

One divisionOne division

22nn number of number of chromosomes chromosomes in resulting in resulting cellscells

MeiosisMeiosis 4 cells result4 cells result

Two divisionsTwo divisions

nn number of number of chromosomes chromosomes in resulting in resulting cellscells

D. Genetic Recombination and D. Genetic Recombination and VariationVariation

1. 1. independent assortmentindependent assortment

2. 2. crossing overcrossing over

3. 3. random fusionrandom fusion of sperm and of sperm and eggegg

E. Formation of GametesE. Formation of Gametes

1. 1. spermatogenesisspermatogenesis – results in 4 – results in 4

viable spermviable sperm

2. 2. oogenesisoogenesis – results in 1 egg – results in 1 egg andand

3 3 polar bodiespolar bodies

SpermatogenesisSpermatogenesis

OogenesisOogenesis

Fertilization [sperm (n) + egg (n) Fertilization [sperm (n) + egg (n) zytote (2n) ] zytote (2n) ]

F. Sexual vs Asexual F. Sexual vs Asexual ReproductionReproduction

Asexual Asexual

One parentOne parent Identical genetic Identical genetic

materialmaterial Mitosis, budding, Mitosis, budding,

binary fission binary fission

Sexual Sexual

Two parentsTwo parents Different Different

genetic materialgenetic material Meiosis + Meiosis +

fertilizationfertilization

Meiosis creates genetic Meiosis creates genetic variationvariation

During normal cell growth, During normal cell growth, mitosismitosis produces daughter cells identical to produces daughter cells identical to parent cellparent cell (2n to 2n) (2n to 2n)

MeiosisMeiosis results in results in genetic variationgenetic variation by by shufflingshuffling of maternal and paternal of maternal and paternal chromosomes and chromosomes and crossing overcrossing over..

No daughter cells formed during No daughter cells formed during meiosis are genetically identical to either meiosis are genetically identical to either mother or father mother or father

During sexual reproductionDuring sexual reproduction,, fusion of fusion of thethe unique haploid gametes produces truly unique haploid gametes produces truly unique offspringunique offspring..

KaryotypeKaryotype

1. picture of homologous 1. picture of homologous chromo-chromo-

somes and sex chromosomessomes and sex chromosomes

2. used to detect chromosome2. used to detect chromosome

abnormalitiesabnormalities

HomologuesHomologues Chromosomes exist in homologous Chromosomes exist in homologous

pairs in diploid cells.pairs in diploid cells.

Exception: Sex chromosomes (X, Y).

Other chromosomes are known as autosomes, they have homologues.

Human ChromosomesHuman Chromosomes

In humans …In humans … 23 chromosomes donated by each parent 23 chromosomes donated by each parent

(total = 46 or 23 pairs).(total = 46 or 23 pairs). Gametes (sperm/ova):Gametes (sperm/ova):

Contain 22 autosomes and 1 sex chromosome.Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid numberAre haploid (haploid number “n” = 23 in humans “n” = 23 in humans).).

Fertilization/syngamy results in zygote with Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now 2 haploid sets of chromosomes - now diploiddiploid..

Diploid cell; Diploid cell; 2n = 462n = 46. (. (n=23 in humansn=23 in humans))

Most cells in the body produced by Most cells in the body produced by mitosismitosis..

Only gametes are produced by Only gametes are produced by meiosis.meiosis.

In humansIn humans

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

Random fertilizationRandom fertilizationAt least 8 million combinations from At least 8 million combinations from

Mom, and another 8 million from Dad Mom, and another 8 million from Dad ……

>64 trillion combinations for a diploid >64 trillion combinations for a diploid zygote!!!zygote!!!

Karyotyping- normalKaryotyping- normal

some of the previous slides some of the previous slides came from the following websitecame from the following website

http://cchs.churchill.k12.nv.us/http://cchs.churchill.k12.nv.us/marshk/Notes/marshk/Notes/meiosis.ppt#352,39,Slide 39meiosis.ppt#352,39,Slide 39

www.biology.Arizona.edu//