Chromosomes:
the cellular basis of the instructions
Mitosis - 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
n
Haploid egg
n
Haploid sperm
Fertilization
2n
mitosis
Mitosis
The cell cycle
Metaphase
Prophase
Anaphase
Telophase
Interphase
2N
2C
4C
4C
2C
2C
Prophase
Metaphase
Anaphase
Telophase
2N
2N
Mitosis
Mitosis: Sister chromatids
separate at anaphase
2C
Diploid cells
4C
4C
Homologous chromatids
2C
2C
Diploid cells
Meiosis
2N
2C
2N
4C
2N
4C
2N
4C
1N
2C
1N
2C
1N
1C
Prophase I
1. Leptotene
2.Zygotene
4.Diplotene
5.Diakinesis
Prophase I
3.Pachytene
Prophase I
4.Diplotene
5.Diakinesis
Crossing over
Pairing of
homologous
chromosomes
Chiasma
formation
Chromosomes
Breakage
and joining
Recombinant
chromosomes
Prophase I
Metaphase I
Telophase I
Anaphase I
Prophase II
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
Anaphase II
Metaphase 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
2n = 2C
Centromeres
Do not split
At meiosis I
2n = 4C
2n = 4C
Meiosis I
is reductional
n =2C
Centromeres
Do split
At meiosis II
Meiosis 1st division:
reduction from diploid to haploid
number of chromosomes
two haploid cells
Diploid cells
Crossing over
Homologous chromosomes
Diploid to haploid
Homologous chromatids
Haploid cells
MITOSIS
1 cell division:
2 daughter cells
2n
MEIOSIS
2 cell divisions:
4 product of meiosis
2n
Somatic cells
2n
Cells in sexual cycle
n
n
n
n
2n
n=chromosome number
Mitosis
Conservative process:
Daughter cells genotypes identical
to parental genotypes
Meiosis
Promotes variation among the product
of meiosis (crossing over)
30 mitotic
divisions
15 years
Puberty
4 mitotic
divisions
I
spermatids
spermatozoa
II
2 meiotic
divisions
X
Bearing sperm
egg
ZYGOTE
2n
2n
2n
2n
2n
Mitoses
2n
2n
Growth and differentiation
of organs and tissues
2n 2n 2n 2n 2n 2n 2n 2n
Female germ line
2n
2n
2n
2n
Somatic cells
2n
2n
Proliferation of gonial cells
2n
oogonia
Meiosis I
Primary oocytes
2n
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