meiosis in a nut shell

8/7/2019 MEIOSIS in a Nut Shell

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1903: Walter Sutton -

American

Studied Grasshoppers

Sex Cells: 12

Chromosomes

Body (somatic cells):

24 Chromosomes Observed the fusion of

the egg and sperm cell


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THEODOR BOVERIBOVERI SUTTON CHROMOSOMAL

THEORY OF HEREDITARY

1862 1915 - German

Devised the same theoryusing sea urchins

Discovered the centromere


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A special form of cell division

To form sex cells or gametes

Occurs in Reproductive tissue of plants andanimals


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PLANTS

Male Cells: Pollen

Formed in Anther Supported by Filament

Anther and Filament formthe Stamen

MALE PARTS OF PLANT


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PLANT

Female Cells: Ovules

Formed in the ovary At the base of Style A Anther (Male) B Filament (Male)

C Stigma (Female)

D Style (Female) E Ovary (Female)

FEMALE PARTS OF THE PLANT


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MALE POLLEN CELLS FEMALE OVULE CELLS


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MALE GAMETES (SEX CELLS) FEMALE GAMETES (SEX CELLS)


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Chromosomes are made of DNA

Genes are sections of DNA

Most of the time DNA is in long strands calledthe Chromatin Network

Just beforeMeiosis (andMitosis) the DNAreplicates then

Coils to form a duplicated chromosome


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Single Chromosome =5cm

46 Chromosomes in ahuman body cell 10 000 000 000 000

cells in the body

10 Trillion cells or 1013

Calculate the length Number of

chromosomes x .005mx number of cells Comes to about

2.0 1013 meters

Which is about 67 tripsto the sun and back IN ONE HUMAN


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Chromosomes occur in homologous pairs

In humans 23 homologous pairs

23 came from mother 23 came from father

Total number is 46 Same characteristics are coded for on each

pair E.g. eye colour


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The DNA replicatesduring interphase

The DNA coils To form a duplicated

chromosome Two chromatid arms

Joined by centromere Occur in pairs


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ANIMALS

Camel = 70

House Fly = 18 Chicken = 78

Chinchilla = 64

Porcupine = 34

Mosquito = 6

PLANTS

Rice = 24

Beans = 22 Lettuce = 18

Tomato = 24

Apple = 34

Pea = 14


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DIPLOID CHROMOSOMENUMBER

Somatic or body cells

contain a diploid number ofchromosomes

E.g. Humans 46

HAPLOID CHROMOSOMENUMBER

Sex cells or gametes

contain a haploid numberof chromosomes

E.g. Humans 23


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MEIOSIS 1

BeforeMeiosis DNAreplicates

Start with one Diploid cell Formation of duplicated

chromosomes Homologous pairs of

duplicated chromosomesalign on equator (crossing

over) Homologous pairs separate

and go to each pole Cell divides into two 2 Haploid cells form Chromosome number halved

MEIOSIS 2

No replication of DNA

Start with two HaploidCells Duplicated chromosomes

align individually onequator

Split at centromere Chromatid arms separate

and go to each pole Each Cell divides into two 4 haploid cells form


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INTERPHASE INTERPHASE

Formation of proteins

enzymes etc. needed forcell division

Normal cell processes

Duplication of DNA to formduplicated chromosomes.


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PROPHASE OF MEIOSIS 1

Chromosomes coil up Spindle forms

Homologous pairs alignINDEPENDANTLY along cell equator Each homologous pair of

chromosomes forms a tetrad Tetrad:two homologous

chromosomes each made of two sisterchromatids

Crossing over occurs

Exchange of genetic (DNA) materialbetween non-sister chromatids Breaks in chromatids occurs at

Chiasmata Homologous pairs no longer

genetically identical

PROPHASE OF MEIOSIS 1


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METAPHASE 1

Tetrads align on equator

Aligned in homologouspairs

Homologous pairs begin toseparate

ANAPHASE 1

Homologous pairs separate

Duplicated chromosomesremain intact

Each chromosome of ahomologous pair (tetrad)moves to the opposite pole

of the cell Each new cell receives one

chromosome from thehomologous pair


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INTERPHASE

Short interphase

DNA re-coils to formduplicated chromosomes

PROPHASE 2

Spindle forms

Duplicated chromosomesmove towards equator


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METAPHASE 2

Duplicated chromosomes

align individually alongequator of cell

Centromere on equator

Chromatid arm either sideof equator

ANAPHASE 2

Centromere splits

Chromatid arms move toopposite poles


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TELOPHASE

Nuclei reform

Cytokinesis takes place Four haploid cells result

SUMMARY

Short interphase

Duplicated chromosomesalign on equator of cell

Split at centromere

Move to opposite poles

Cell division to form 4

haploid cells


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INDEPENDENT ASSORTMENT

Each duplicated chromosome

in a homologous pair canalign in two ways

In humans 223 possibilities = 8million

At fertilisation the possibility

is 223

x 223

70 trillion (70 000 000 000)possible variations of humans

BUT there is more

CROSSING OVER

2 to 3 crossing over events

occur per chromosome Provides additional

variation

VARIATION is the rawmaterial for natural

selection to act on Accumulated changes

cause the process ofevolution


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meiosis in a nut shell

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