Chromosomes: Chromosomes: the cellular basis of the the cellular basis of the

instructionsinstructions

Mitosis - MeiosisMitosis - Meiosis

• Diploid organism: two sets of chromosomes ( one from each parent)

• 2n

• Haploid organism: one set of chromosomes

• n

• Germ cells: the cells that provide the continuity of life from generation to the next

• Somatic cells: all the others

meiosis

Haploid egg Haploid sperm

mitosis

Fertilization

n n

2n

MitosisMitosis

The cell cycle

Prophase

Metaphase Anaphase

Telophase

Interphase

Prophase

Metaphase

Anaphase

Telophase

2N

2N2N

Mitosis2C

4C

4C

2C

2C

Mitosis: Sister chromatids separate at anaphase

Homologous chromatids

Diploid cells

Diploid cells2C

4C

4C

2C

2C

Meiosis

Prophase I

2N 2C

2N 4C

2N 4C

2N 4C

1N 2C

1N 2C

1N 1C

1 .Leptotene

2.Zygotene

3.Pachytene 4.Diplotene 5.Diakinesis

Prophase I

Prophase I

5.Diakinesis4.Diplotene

Crossing overPairing of

homologous chromosomes

Chiasma formation

ChromosomesBreakage and joining

Recombinant chromosomes

Metaphase I Anaphase I

Telophase IProphase II

Prophase I

Meiosis anaphase I :Homologous chromosomes separateReduction from diploid to haploid

Meiosis 1st divisionMeiosis 1st division: : reduction from diploid to haploid reduction from diploid to haploid

number of chromosomesnumber of chromosomes

two haploid cellstwo haploid cells

Meiosis 2Meiosis 2ndnd division: division: like mitosis with haploid setlike mitosis with haploid set

Prophase II

Metaphase II

Anaphase II

Telophase II

4 Haploid cells Gametes4 Haploid cells Gametes

Meiosis begins by replicating the chromosomes

This step is identical to mitosis

The homologous chromosomes then pairs

CentromeresDo not splitAt meiosis I

CentromeresDo not splitAt meiosis I

Meiosis Iis reductional

2n = 2C

2n = 4C

n =2C

2n = 4C

CentromeresDo splitAt meiosis II

Meiosis 1st divisionMeiosis 1st division: : reduction from diploid to haploid reduction from diploid to haploid

number of chromosomesnumber of chromosomes

two haploid cellstwo haploid cells

Crossing over

Homologous chromosomes

Diploid to haploid

Haploid cells

Diploid cells

Homologous chromatids

2n1 cell division: 2 daughter cells

2 cell divisions: 4 product of meiosis

2n

2n

2n

n

n n

n

n=chromosome number

Somatic cellsSomatic cells

Cells in sexual cycleCells in sexual cycle

MITOSISMITOSIS

MEIOSISMEIOSIS

MitosisMitosisConservative process:Conservative process:Daughter cells genotypes identical Daughter cells genotypes identical to parental genotypesto parental genotypes

MeiosisMeiosisPromotes variation among the product Promotes variation among the product of meiosis (crossing over)of meiosis (crossing over)

30 mitotic divisions

4 mitotic divisions

2 meiotic divisions

spermatozoa

spermatids

15 years

Puberty

I

II

2n

2n2n

2n2n 2n2n

2n 2n 2n 2n 2n 2n 2n 2n

2n

2n2n

2n2n 2n2n

egg X Bearing spermZYGOTE

Mitoses

Growth and differentiation of organs and tissues

Somatic cells

Female germ line

Proliferation of gonial cells

oogonia

2n

n

Meiosis I

Meiosis II

Primary oocytes

Secondary oocytesPolar body

Egg + 3 polar bodies

fertilization

Female meiosis

• Birth: diplotene of prophase I

• At puberty every month: maturation = progress to metaphase II, release from the ovary

• Penetration of the sperm: continue to anaphase II

Final result

 Meiosis :Four genetically different haploid cells from one diploid parent cell

Mitosis :two genetically identical diploid cells from one diploid parent cell