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Cell Division
III) Meiosis
A) Introduction
A) Introduction
Cell Division
III) Meiosis
Meiosis
 a type of cell division
 results in the formation of sex cells, or gametes.
 in humans it occurs in the testis and ovaries.
 involves two stages of cell division


both have similarities to the phases in mitosis
the chromosome number of the daughter cells is half that of
the parent cell (haploid number or n)
A) Introduction
Cell Division
III) Meiosis



involves two stages of cell division
 both have similarities to the phases in mitosis
the chromosome number of the daughter cells is half that of the parent cell (haploid
number or n)
each parent provides half the genetic information to their
offspring.
 each of the 23 chromosomes you receive from your father is
matched up with 23 chromosomes from your mother.
 the paired chromosomes are called homologous
chromosomes.
 there are 22 pairs of homologous chromosomes
 the 23rd pair are the sex chromosomes and are only
partially homologous.
A) Introduction
Cell Division
III) Meiosis


the paired chromosomes are called homologous chromosomes.
 there are 23 pairs of homologous chromosoms
 the 23rd pair are the sex chromosomes and are only partially
homologous.
during fertilization, a haploid (n = 23) sperm cell unites with a
haploid (n = 23) egg cell to produce a diploid (2n = 46) zygote.
 the zygote will begin dividing by mitosis and become a
multicellular organism.
A) Introduction
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis




meiosis
 involves two nuclear division.
 Meiosis I
 Meiosis II
 produces four haploid cells.
meiosis I is called reduction division because the diploid (2n)
chromosome number is reduced to the haploid (n)
chromosome number
meiosis II is marked by the separation of the two chromatids.
DNA synthesis occurs prior to the two nuclear divisions.*
B) Stages
Cell Division
III) Meiosis
Cell Division
III) Meiosis


meiosis II is marked by the separation of the two chromatids.
DNA synthesis occurs prior to the two nuclear divisions.
Meiosis I
 Stages:





Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase I


nuclear membrane begins to dissolve
centriole splits, parts move to opposite poles and spindle fibres form.
B) Stages
Cell Division
III) Meiosis

Prophase I
 nuclear membrane begins to dissolve
 centriole splits, parts move to opposite poles and spindle fibres form.

chromosomes come together in homologous pairs.

each chromosome of the pair is a homologue and is composed of a pair of
sister chromatids.

the whole structure is referred to as a tetrad (because there are four
chromatids)
B) Stages
Cell Division
III) Meiosis


each chromosome of the pair is a homologue and is composed of a pair of
sister chromatids.
 the whole structure is referred to as a tetrad (because there is four
chromatids)
each pair of homologous chromosomes align side by side
(non-sister chromatid to another)
 this aligning is called synapsis.
 as the chromosomes synapse they often intertwine
 the intertwined chromatids from different homologous
break and exchange segments in a process called
crossing over.
B) Stages
Cell Division
III) Meiosis


this aligning is called synapsis.
as the chromosomes synapse they often intertwine
 the intertwined chromatids from different homologous break and
exchange segments in a process called crossing over.
B) Stages
Cell Division
III) Meiosis


this aligning is called synapsis.
as the chromosomes synapse they often intertwine
 the intertwined chromatids from different homologous break and
exchange segments in a process called crossing over.

DNA information is exchanged during the crossing
over event.
 promotes variation within the species.
B) Stages
Cell Division
III) Meiosis

Metaphase I

homologous chromosomes attach themselves to the spindle fibres and
line up along the equatorial plate
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis

Metaphase I
 homologous chromosomes attach themselves to the spindle
fibres and line up along the equatorial plate

Anaphase I
 the homologous chromosomes move toward
opposite poles.
 this is called segregation
 reduction division occurs
 One member of each homologous pair will
be found in each of the new cells.
B) Stages
Cell Division
III) Meiosis
this is called segregation
reduction division occurs

One member of each homologous pair will be found in each
of the new cells.



Telophase I
 a membrane begins to form around each nucleus.
 the chromosomes in the nuclei are not
identical because each of the daughter nuclei
contains one member of the homologous
chromosome pair.
 homologous chromosomes are similar but
not identical.
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis

the chromosomes in the nuclei are not identical because each of the
daughter nuclei contains one member of the homologous
chromosome pair.
 homologous chromosomes are similar but not identical
Meiosis II
 Stages:
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II
 the stages occur at approximately the same time for
each of the haploid daughter cells.
 there is no replication of chromosomes prior to
meiosis II.
B) Stages
Cell Division
III) Meiosis

the stages occur at approximately the same time for each of the
haploid daughter cells.
there is no replication of chromosomes prior to meiosis II.

Prophase II


the nuclear membrane dissolves and spindle
fibres form.
B) Stages
Cell Division
III) Meiosis

Prophase II
 the nuclear membrane dissolves and spindle
fibres form.

Metaphase II
 arrangement of each chromosome,
each with two chromatids, along the
equatorial plate.

chromatids are held together by the
centromere
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis



Prophase II
 the nuclear membrane dissolves and spindle
fibres form.
Metaphase II
 arrangement of each chromosome, each with two
chromatids, along the equatorial plate.

chromatids are held together by the
centromere
Anaphase II
 breaking of the attachment between
the two chromatids.
 migration of chromatids (now called
chromosomes) to opposite poles.
B) Stages
Cell Division
III) Meiosis




Prophase II

the nuclear membrane dissolves and spindle fibres form.
Metaphase II

arrangement of each chromosome, each with two chromatids, along
the equatorial plate.

chromatids are held together by the centromere
Anaphase II

breaking of the attachment between the two chromatids.

migration of chromatids (now called chromosomes) to opposite poles.
Telophase II
 second nuclear division is completed
 second division of cytoplasm occurs (cytokenesis)
 four haploid daughter cells are produced
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis
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B) Stages
MITOSIS
Cell Division
III) Meiosis
Prophase
MEIOSIS
Parent cell
Chromosome
replication
Chiasma
Chromosome
replication
Prophase I
Homologous
chromosome
pair
2n = 6
Replicated chromosome
MEIOSIS I
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
2n
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anahase, and telophase
Two, each including prophase, metaphase, anaphase, and
telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many chromosomes
as the parent cell; genetically different from the parent
cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes by half
and introduces genetic variability amoung the gametes
B) Stages
Cell Division
III) Meiosis
B) Stages
Cell Division
III) Meiosis
Cell Division
III) Meiosis
C) Genetic
Recombination
Cell Division
III) Meiosis
Genetic Recombination
 the formation of new combinations of genes
 comes about by
 independent assortment
 crossing over
C) Genetic Recombination
Cell Division
III) Meiosis

comes about by

independent assortment

crossing over

Independent Assortment
 during metaphase I chromosome arrange in homologous pairs along the
equator of the cell.



the chromosome of maternal origin is orientated toward one
pole of the cell while the chromosome of paternal origin is
oriented towards the other pole
this orientation is “independent” of other homologous pairs
it results in gametes having different combinations of parental
chromosomes.
C) Genetic Recombination
Cell Division
III) Meiosis

Independent Assortment

during metaphase I chromosome arrange in homologous pairs along the equator of the
cell.

the chromosome of maternal origin is orientated toward one pole of the cell while the
chromosome of paternal origin is oriented towards the other pole

this orientation is “independent” of other homologous pairs

it results in gametes having different combinations of parental chromosomes.

http://www.sumanasinc.com/webcontent/animations/cont
ent/independentassortment.html
C) Genetic Recombination
Cell Division
III) Meiosis

Independent Assortment

during metaphase I chromosome arrange in homologous pairs along the equator of the
cell.

the chromosome of maternal origin is orientated toward one pole of the cell while the
chromosome of paternal origin is oriented towards the other pole

this orientation is “independent” of other homologous pairs

it results in gametes having different combinations of parental chromosomes.
C) Genetic Recombination
Cell Division
III) Meiosis

Crossing Over
 occurs when chromosomes synapse, or pair up during
prophase I.
 Non-sister chromatids exchange pieces of chromosome.



the section that is crossed over may contain hundreds or even
thousands of genes.
can create individual chromosomes with both maternal and
paternal genes.
can occur at several points along the non-sister chromatids.
C) Genetic Recombination
Cell Division
III) Meiosis

Crossing Over

occurs when chromosomes synapse, or pair up during prophase I.

non-sister chromatids exchange pieces of chromosome.

the section that is crossed over may contain hundreds or even thousands of genes.

can create individual chromosomes with both maternal and paternal genes.

can occur at several points along the non-sister chromatids.
Cell Division
III) Meiosis
Cell Division
III) Meiosis
D) Gamete
Development
Cell Division
III) Meiosis
Gametogenesis
 the formation of sex cells during meiosis.
 both females and males follow the same process of meiosis to
develop gametes but there is differences in the final products.
 the cytoplasm of the female gamete does not divide equally
after each nuclear division.
 this results:
 in one cell, called the ootid, receiving most of the
cytoplasm.
 three polar bodies that die and are absorbed by the
body
D) Gamete Development
Cell Division
III) Meiosis

this results:
 in one cell, called the ootid, receiving most of the cytoplasm.
 three polar bodies that die and are absorbed by the body
D) Gamete Development
Spermatogenesis
 the process starts with a diploid germ cell called a
spermatogonium.
 starting at puberty mitosis forms two spermatogonium
daughter cells.
 one cell replenishes the spermatogonia
 one cell develops into a primary spermatocyte
 the primary spermatocyte undergoes meiosis I to form
two secondary spermatocytes.
 the secondary spermatocytes undergo meiosis II to
form four spermatids.
D) Gamete Development


the primary spermatocyte undergoes meiosis I to form two secondary
spermatocytes.
the secondary spermatocytes undergo meiosis II to form four spermatids.
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D) Gamete Development
Oogenesis
 oogenesis starts with the diploid germ cell called an oogonium.
 each oogonium undergoes mitosis to form two primary
oocytes.
 three months after conception two million primary oocytes
can be found in the ovaries arrested in prophase I awaiting
puberty.

three months after conception two million primary oocytes can be found in the
ovaries arrested in prophase I awaiting puberty.

every month after puberty one primary oocyte undergoes
meiosis.
 unequal division of cytoplasm called asymmetrical
cytokinesis occurs.
 the cell that receives the most cytoplasm is called the
secondary oocyte, the other cell is called the first polar
body.
 the first polar body may or may not go through a
second division to produce a pair of polar bodies.
D) Gamete Development

the cell that receives the most cytoplasm is called the secondary oocyte, the
other cell is called the first polar body.
 the first polar body may or may not go through a second division to
produce a pair of polar bodies.

when sperm penetrates the secondary oocyte it
undergoes meiosis II.
 cytoplasm divides unequally again.
 the cell with the most cytoplasm becomes the
mature egg.
 the cell with the least cytoplasm becomes the
second polar body.
D) Gamete Development



cytoplasm divides unequally again.
the cell with the most cytoplasm becomes the mature egg.
the cell with the least cytoplasm becomes the second polar body.
D) Gamete Development

cool facts about oogenesis
 asymmetrical cytokinesis allows for one egg to have a large
amount of nutrients (cytoplasm) for the zygote to use prior
to implantation
 meiosis I and II is not continuous
 primary oocytes begin meiosis I before birth but stalls in
prophase I until puberty.
 secondary oocytes stalls at metaphase II until fertilization
occcurs.
 if fertilization does not occur meiosis II will not be
completed.
D) Gamete Development