Meiosis
Gamete Production
Timothy G. Standish, Ph. D.
©2000 Timothy G. Standish
Chromosomes, Chromatids
and Centromeres
A packaged
chromosome
Chromatid
Identical
chromatid
Chromosome
arm
Centromere
Two identical
chromosomes
Replication
Anaphase
Chromosome
arm
©2000 Timothy G. Standish
In The Beginning Two
Humans and many other complex multicelled
organisms incorporate genetic recombination in
their reproduction
Reproduction in which there is a re-mixing of the
genetic material is called sexual reproduction
Two cells, a sperm and an egg, unite to form a
zygote, the single cell from which the organism
develops
Meiosis is the process of producing sperm and
eggs (gametes)
©2000 Timothy G. Standish
Fertilization Results In A
Diploid Zygote
Egg
1n
Haploid
nucleus
Sperm
2n
Haploid
nucleus
©2000 Timothy G. Standish
Fertilization Results In A
Diploid Zygote
Egg
1n
Haploid
nucleus
Sperm
2n
Haploid
nucleus
©2000 Timothy G. Standish
Fertilization Results In A
Diploid Zygote
Egg
1n
Haploid
nucleus
Haploid
nucleus
Sperm
2n
©2000 Timothy G. Standish
Fertilization Results In A
Diploid Zygote
Egg
1n
Haploid
nucleus
Haploid
nucleus
Sperm
2n
©2000 Timothy G. Standish
From Zygote to Embryo
Zygote
Zygote
2n
2n
©2000 Timothy G. Standish
From Zygote to Embryo
Cleavage
©2000 Timothy G. Standish
From Zygote to Embryo
Cleavage
©2000 Timothy G. Standish
From Zygote to Embryo
Cleavage
©2000 Timothy G. Standish
From Zygote to Embryo
Cleavage
©2000 Timothy G. Standish
From Zygote to Embryo
Morula
©2000 Timothy G. Standish
Gametes Are Haploid
Gametes must have half the genetic material of a
normal cell
If the genetic material in the gametes was not
halved, when they combined the zygote would
have more genetic material than the parents
Meiosis is specialized cell division resulting in
cells with half the genetic material of the parents
Gametes have exactly one set of chromosomes,
this state is called haploid (1n)
Regular cells have two sets of chromosomes, this
state is called diploid (2n)
©2000 Timothy G. Standish
Stages Of Meiosis
Meiosis resembles mitosis except that it is
actually two divisions, not one
These divisions are called Meiosis I and
Meiosis II
Meiosis I results in haploid cells with
chromosomes made up of two chromotids
Meiosis II is essentially mitosis on haploid
cells
Stages of meiosis resemble mitosis with two
critical differences: the first in Prophase I and
the second in Metaphase I
©2000 Timothy G. Standish
Stages Of Meiosis - Meiosis I
Prophase I - The beginning phase – DNA which was unraveled and spread all over
the nucleus is condensed and packaged
– Homologous chromosomes (each made of two
identical chromatids) come together and form
tetrads (4 chromatids)
– Crossing over, in which chromatids within
tetrads exchange genetic material, occurs
Metaphase I - Middle stage - Tetrads line up
along the equator of the cell
©2000 Timothy G. Standish
Stages Of Meiosis - Meiosis I
Anaphase I - One copy of each
chromosome still composed of two
chromatids moves to each pole of the cell
Telophase I - End stage - New nuclear
membranes are formed around the
chromosomes and cytokinesis (cytoplasm
division) occurs resulting in two haploid
daughter cells
©2000 Timothy G. Standish
Stages Of Meiosis - Meiosis II
Prophase II - Cells do not typically go into
interphase between Meiosis I and II, thus
chromosomes are already condensed
Metaphase II - Chromosomes line up at the
equator of the two haploid cells produced in
meiosis I
Anaphase II - Chromosomes made up of two
chromatids split to make chromosomes with one
chromatid which migrate to the poles of the cells
Telophase II - Cytokinesis and reformation of
the nuclear membrane in haploid cells each with
one set of chromosomes made of one
chromatid
©2000 Timothy G. Standish
Mother cell
Stages Of Meiosis:
Meiosis I
Interphase
Prophase I:
Condensing
Chromosomes
Prophase I:
Tetrad formation/
crossing over
Meiosis II
Telophase I
Metaphase I
Anaphase I
©2000 Timothy G. Standish
Stages Of Meiosis:
Meiosis II
Telophase I
Prophase II
The products of meiosis are 4
haploid cells each with a
unique set of chromosomes.
Metaphase II The products of
mitosis are 2 diploid
cells with identical
chromosomes.
Anaphase II
Telophase II
Crossing Over
Prophase I:
Tetrad formation/
crossing over
Because of crossing over, every
gamete receives a unique set of
genetic information.
Telophase II
Metaphase I
Anaphase I
Telophase I
©2000 Timothy G. Standish
©2000 Timothy G. Standish