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.

1. Meiosis

2. Reproduction
Reproduction:

Asexual (Vegetative)

Many single-celled organisms
reproduce by splitting, budding,
parthenogenesis

Some multicellular organisms can
reproduce asexually, produce clones
(offspring genetically identical to
parent).

3. Reproduction
Reproduction:

Sexual (Generative)

Fusion of two gametes to produce a
single zygote

Introduces greater genetic variation,
allows genetic recombination

Zygote has gametes from two
different parents.

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.

22. Summary of Meiosis

23. Mitosis vs meiosis
Meiosis KM
23

24. Mitosis vs meiosis
Meiosis KM
24

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.

26. Independent assortment
Meiosis KM
26

27. Independent assortment
Number of combinations: 2n
e.g. 2 chromosomes in haploid
2n = 4; n = 2
2n = 22 = 4 possible combinations

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