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Meiosis
• A type of cell division that results in
gametes(sperm and egg) being created with
half the number of chromosomes(haploid-n)
as the parent.
Purpose of Meiosis
• genetic continuity
• Genetic variability
Where does meiosis occur?
• It occurs in the gonads( testes and Ovaries)
Two Types of Meiosis
• Oogenesis
• spermatogenesis
Meiosis
Meiosis
Haploid cells
Diploid cells
Somatic cells
Gametes
Role of Meiosis
To produce haploid gametes
from diploid germ cells.
Occurs only in the gonads
• Plays an important
role in creating genetic
variability through the
processes of:
• Segregation
• Independent
assortment
• Crossing over
Meiosis
• Interphase
• S phase—replication
• Cell is now diploid with
all chromosomes being
mitotic
• Meiosis I—reductive
division
• Creates 2 haploid cells
with mitotic chromsomes
• Consists of several stages
• Prophase I—Like
prophase in mitosis.
• Synapsis—homologous
chromosomes attach to
form tetrads.
• Crossing over occurs.
• Reshuffling of genes.
Genetic recombination.
Important for creating
variability.
The stages of meiosis
• Meiosis 1
• Meiosis 2
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Four stages
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Four stages
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Stages of meiosis 1
• Meiosis one is called the reduction
division
• The process of reducing the number of
chromosomes in a cell by half (1/2).In other
words, going from diploid (2n) to
haploid(n)
• Ex. 46 chromosomes to 23 chromosomes
Prophase 1
• During prophase 1 of meiosis, homologous
chromosomes(similar chromosomes) line up
next to each other as pairs
• Pairs of homologous chromosomes are
called Tetrads.
• Crossing over of non-sister chromatids
occurs.
• Crossing over : the overlapping of nonsister chromatids resulting in an exchange
of genes.
Prophase 1
Metaphase 1
• During this phase homologous chromosomes line up at the
middle of the cell.
• tetrads are aligned in such a way that the homologous
chromosomes are situated across from each other
• The spindle from one end of the cell attaches to one pair of
sister chromatids while a spindle fiber from the other end
attaches to the other pair of sister chromatids.
• Independent Assortment—maternal and paternal
chromosomes are line up on the opposite sides of the
metaphase plate in a random fashion. When these
homologues split during anaphase I. The new nuclei
consist of a mixture of maternal and paternal
Metaphase 1
Anaphase 1
• Spindle fibers pull homologous
chromosomes to separate ends of the cell
Anaphase 1
Telophase 1
• Nuclear membrane reappears around each
set of separated chromosomes
• Spindle disappears
• Cytokinesis (division of the cytoplasm)
occurs
• Two new cells are created with ½ (haploid)
the chromosomes number of the original
cell
• 46 DS chromosomes – 23 DS
chromosomes per cell or 96 chromatids- 46
chromatids per cell
Telophase 1
Meiosis II
• Almost the same as
Mitosis.
• 2 cells are formed from
each of the cells from
Meiosis I.
• These cells are haploid
and have chromosomes
formed by only one
chromatid.
• These cells develop into
gametes
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Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
Results in 4 haploid
cells that will
eventually form
gametes
Stages of Meiosis II
• Stages of meiosis 2 are identical to the
stages of Mitosis
• Prophase II
• MetaphaseII
• Anaphase II
• Telophase II
Prophase II
• Nuclear membrane disappears
• Spindle fibers form
• Chromosomes become visible
Prophase II
Metaphase II
• Chromosomes line up at the middle of the
cell.
• Spindle attaches to centromere of each
chromosome
Metaphase II
Anaphase II
• Spindle pulls one chromatid from each
chromosome to opposite ends of cell
Anaphase II
Telophase II
• Nuclear membrane reappears around each
group of chromatids (now called SS
chromosomes)
• Spindle disappears
• Cytokinesis occurs
• two new cells are created each having the
haploid number of chromosomes
Telophase
Gametogenesis
• In males:
• spermatogenesis
• 1 diploid germ cell
goes through meiosis
to form 4 haploid
cells. These develop in
the testes to form
sperm.
• In females:
• Oogenesis
• 1 diploid germ cell split
unequally into 4 haploid
cells.
• The large cell develops
into an egg.
• The small cells are know
as polar bodies and
usually die.
• Why does this unequal
division occur?
Oogenesis
• This is the meiosis that occurs in the ovaries
of females
• Resulr in the creation of An egg with ½ the
normal number of chromosomes
Explanation of Oogensis
• Within the ovaries of females, a diploid 2n cell called an
oogonium undergoes meiosis
• Before meiosis begins the oogonium replicates(doubles) its
chromosomes so that it has 46 DS chromosomes or 92
sister chromatids
• During the first meiotic division the Homologous (similar
chromosomes are separated. During this stage cytokinesis
occurs unevenly and as a result one large and one samll
cell are created. The large cell is called the primary
Oocyte and the small one is a polar body.
• Each of these chromosomes will have 23 chromosomes or
46 SS chromatids
• During the second metiotic division the
cells divide again creating 4 cells, but this
time the new cells created have 23 SS
chromosomes, Again cytokinesis has
resulted in an unequal distribution of the
cytoplasm
• Note: The three polar bodies will die
because they do not have enough
cytoplasm to keep them running
Spermatogensis
• This is meiosis that occurs in the testes of
males
• Results in the creation of 4 sperm each with
½(haploid) the number of chromosomes as
the original cell
Explanation of Spermatogenesis
• Within the testicles of a male, a spermatogonium
replicates its chromosomes to contain 46 Ds
chromosomes or 92 chromatids.
• During the first meiotic division, the cell divides
and the homologous chromosomes separate. This
results in 2 new cells each with 23 DS
chromosomes or 46 chromatids each
• During the second division two new cells are
created, each with 23 SS chromosomes. These
cells are called Primary spermatids will mature
into sperm
• Each spermatid matures into sperm in the
Epididymis
Definitions
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Diploid
Haploid/ monoploid
Tetrad
Synapsis
Crossing Over
Nondisjunction
Reduction Division
Spermatids
Primary body
Oognium
End Result of Meiosis
• 4 new cells are created
• Each cell has ½ or haploid (n) the number
of chromosomes as the parent /original cell
• Crossing over has resulted in a new
combination of genes on chromosomes
leading to variety in orgasnisms
Meiosis / Mitosis Compared
Event
Where does it occur?
Mitosis
Meiosis
Somatic cells
Gonads
Number of cells created
per complete cycle.
2
4
Number of divisions
1
2
Same as parent
½ parent cell
Does crossing over
occur?
No
Yes possibly
Do homologous
chromosomes line up?
No
Yes
Genetic continuity,
growth
Genetic variability
Number of Chromosomes
per new cell created
Purpose