Chromosomes: the cellular basis of the instructions Mitosis - Meiosis

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

Mitosis

The cell cycle Prophase Metaphase Anaphase Telophase Interphase

Prophase Metaphase Anaphase Telophase 2N Mitosis 2C 4C 4C 2C 2C

Mitosis: Sister chromatids separate at anaphase

Homologous chromatids Diploid cells 2C 4C 4C Homologous chromatids 2C 2C Diploid cells

Meiosis

2N 2C Prophase I 2N 4C 2N 4C 2N 4C 1N 2C 1N 2C 1N 1C

Prophase I 1. Leptotene 2.Zygotene 3.Pachytene 4.Diplotene 5.Diakinesis Prophase I

Prophase I 5.Diakinesis 4.Diplotene

Crossing over Pairing of homologous chromosomes Chiasma formation Breakage and joining Recombinant

Metaphase I Anaphase I Telophase I Prophase II Prophase I

Meiosis anaphase I :Homologous chromosomes separate Reduction from diploid to haploid

Meiosis 1st division: reduction from diploid to haploid number of chromosomes two haploid cells

Meiosis 2nd division: like mitosis with haploid set Prophase II Metaphase II Anaphase II Telophase II

4 Haploid cells Gametes

Meiosis begins by replicating the chromosomes This step is identical to mitosis

The homologous chromosomes then pairs Centromeres Do not split At meiosis I

Centromeres Do not split At meiosis I Meiosis I is reductional 2n = 2C

Centromeres Do split At meiosis II

Meiosis 1st division: reduction from diploid to haploid number of chromosomes two haploid cells

Crossing over Homologous chromosomes Diploid to haploid Haploid cells Diploid cells Homologous chromatids

MITOSIS Somatic cells MEIOSIS Cells in sexual cycle 1 cell division: 2 daughter cells 2n 2n MEIOSIS Cells in sexual cycle 2 cell divisions: 4 product of meiosis n n 2n n n n=chromosome number

Mitosis Conservative process: Daughter cells genotypes identical to parental genotypes Meiosis Promotes variation among the product of meiosis (crossing over)

2 meiotic 30 mitotic 15 years divisions Puberty 4 mitotic spermatids spermatozoa spermatids 15 years Puberty I II

oogonia X Bearing sperm egg ZYGOTE 2n 2n 2n Mitoses Growth and differentiation of organs and tissues 2n 2n 2n 2n 2n 2n 2n 2n 2n 2n 2n 2n Somatic cells 2n Female germ line 2n 2n Proliferation of gonial cells 2n 2n 2n 2n oogonia

Primary oocytes Meiosis I Secondary oocytes Polar body Meiosis II fertilization n Egg + 3 polar bodies

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