Meiosis is a type of cell division that produces gametes, such as sperm and egg cells, with half the normal number of chromosomes. It involves two cell division phases: Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and separate, resulting in two daughter cells each with half the original number of chromosomes. Meiosis II then separates the sister chromatids, resulting in four haploid daughter cells, each with a single set of chromosomes. This process introduces genetic variation that is important for evolution and sexual reproduction.
4. Meiosis
Cell division to form the gametes:
sperm (male gamete) and
egg (female gamete)
Characteristic of eukaryotes only
Normal cells are diploid: 2 copies of every gene
Gametes are haploid: 1 copy of every gene
Need to choose 1 copy of each gene randomly
Why have sexual reproduction?
Shuffling of alleles between parents and offspring
leads to new combinations.
5. Overview
Start with a diploid cell (2n), with 2 copies of
each chromosome, one form each parent
The two copies are called homologues
Each chromosome with 2 chromatids
attached at the centromere
Meiosis consists of 2 cell divisions:
Meiosis I separate the homologues
Meiosis II separate the 2 chromatids
Meiosis I is unusual and needs a bit of
study, but meiosis 2 is just like mitosis.
6. Meiotic Division
Meiosis I
• Prophase I
• Leptotene
Meiosis I
• Zygotene
Leptotene
Zygotene
Pachitene
Diplotene
Diakinesis.
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II.
7. Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
Prophase I is virtually identical to
prophase in mitosis, involving
the appearance of the
chromosomes…
the breakdown of the nuclear
membrane (envelope)
the development of
the spindle apparatus.
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
8. Prophase I
Meiosis I
Leptotene:
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
initial phase of
condensation
appears as thin threads
with irregular dense
granules (chromomeres)
chromomeres have a
characteristic
size and number for a
given chromosome.
9. Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Zygotene:
lateral pairing of
homologous
chromosomes
(synapsis)
bivalent: synapsed
homologous
chromosome.
10. Prophase I
Meiosis I
• Prophase I
• Leptotene
Pachytene:
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
further chromosome
condensation
tetrad: pairing of 4
chrommatids
crossing over begins
crossing over: event of
genetic exchange
between chromosomes
chiasma: cross connection
of two chromosomes caused
by breakage and rejoining
between chromatids
11. Prophase I
Diplotene:
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Kiasma
chromosomes begin to
separate, crossing over
visible.
12. Prophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Diakinesis:
the centromeres
move away from
each other
the chromosomes
remain joined only at
the tips of the
chromatids.
13. Metaphase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Homologous
chromosomes align at
the equatorial plate.
14. Anaphase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Homologous pairs
separate with sister
chromatids remaining
together.
15. Telophase I
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Two daughter cells are
formed with each
daughter containing only
one chromosome of the
homologous pair.
16. (Interphase/Interkinesis)
Meiosis I
Similar to interphase of
mitosis
Without DNA replication.
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphse I
• Anaphse I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
17. Prophase II
Meiosis I
the nuclear envelope is
dissolved again
the spindle is set up
again
Prophase II is identical
to prophase of mitosis
except that there is half
the amount of
chromosomes
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
18. Metaphase II
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphase I
• Anaphase I
• Telophase I
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Chromosomes line up
individually on the
equator plate.
19. Anaphase II
Meiosis I
At anaphase the
centromeres divide,
splitting the 2
chromatids
The one-chromatid
chromosomes are
pulled to opposite poles.
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphse I
• Anaphse I
• Telophase I
Meiosis II
• Prophase II
• Metaphse II
• Anaphse II
• Telophase II
20. Telophase II
Meiosis I
• Prophase I
• Leptotene
• Zygotene
• Pachitene
• Diplotene
• Diakinesis
• Metaphse I
• Anaphse I
• Telophase I
Meiosis II
• Prophase II
• Metaphse II
• Anaphse II
• Telophase II
Cell division is complete
Four haploid daughter
cells are obtained
spindle fibers disappear
nuclear membrane forms
around chromosomes at
each end of cell
each nucleus has half the
# of chromosomes as the
original (haploid)
now there are 4 sex cells
(daughter cells).
21. Summary of Meiosis
2 cell divisions
Start with 2 copies of each chromosome
(homologues), each with 2 chromatids
In meiosis I, crossing over in prophase mixes
alleles between the homologues
In metaphase of meiosis I, homologues pair
up, and in anaphase the homologues are
separated into 2 cells
Meiosis II is just like mitosis
The centromeres divide in anaphase, giving
rise to a total of 4 cells, each with 1 copy of
each chromosome, and each chromosome
with only 1 chromatid.
25. Meiosis creates genetic variation
During normal cell growth, mitosis
produces daughter cells identical to
parent cell (2n to 2n)
Meiosis results in genetic variation by
shuffling of maternal and paternal
chromosomes and crossing over
No daughter cells formed during
meiosis are genetically identical to
either mother or father
During sexual reproduction, fusion of
the unique haploid gametes produces
truly unique offspring.
28. In humans
23 chromosomes in haploid
2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!
29. Crossing over
Chiasmata – sites of crossing over,
occur in synapsis. Exchange of
genetic material between non-sister
chromatids.
Crossing over produces
recombinant chromosomes.
30. Meiosis and sexual life cycles
Life cycle = sequence
of stages in organisms
reproductive history;
conception to
reproduction
Somatic cells = any
cell other than
gametes, most of the
cells in the body
Gametes produced by
meiosis.
Generalized animal life cycle