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Meiosis -
In
sexually
reproducing
organisms
cellular
fusion
followed
by
a
reduction
in
the
chromosome
number
is
an
important
feature.
The
two
cells
that
participate
in
sexual
reproduction
are
called
gametes
which
fuse
to
form
a
zygote.
This
is
followed
by
a
nuclear
fusion
and
the
zygote
nucleus
contains
two
complete
sets
of
genetic
determinants
(2N).
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In
order
to
maintain
the
original
haploid
number
in
succeeding
generations,
there
must
be
a
stage
during
which
a
reduction
in
the
chromosome
number
occurs.
This
process
which
Occurs
after
the
fusion
of
the
gametes
is
called
as
meiosis.In
brief,
meiosis
consists
of
two
meiotic
divisions.
In
the
prophase
of
the
first
meiotic
division,
the
homologous
chromosomes
become
visible
as
single
strands
which
then
undergo
pairing.
Later,
each
chromosome
becomes
visible
as
two
chromatids
and
crossing
over
occurs.
This
is
followed
by
metaphase
I,
in
which
orientation
of
paired
chromosomes
in
an
equatorial
plane
and
the
formation
of
spindle
apparatus
takes
place.
Anaphase
I
follows,
and
the
homologous
centromeres
move
to
the
opposite
poles
of
the
spindle.
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In
telophase
I
which
constitutes
the
end
of
the
first
meiotic
division
the
formation
of
two
nuclei
occurs.
After
this,
the
interphase
II
follows,
during
which
chromosomes
elongate.
Centromere
division
and
migration
of
the
homologous
chromatids
occurs
during
prophase
II
and
metaphase
II,
which
is
followed
by
anaphase
II
and
the
second
meiotic
division
resulting
in
the
formation
of
four
haploid
cells.
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In
many
eucaryotic
protists
of
higher
ploidy
(>1)
meiosis
occurs
after
the
formation
of
the
zygote
and
prior
to
spore
formation.
In
some
eucaryotes
there
may
even
be
a
well
marked
alteration
of
haploid
and
diploid
generations
as
in
the
case
of
the
yeast.
In
this,
the
diploid
zygote
gives
rise
to
a
diploid
individual
which
then
yields
haploid
cells
only
after
meiosis.
The
haploid
cell
can
either
multiply
as
a
haploid
or
fuse
with
another
haploid
of
the
opposite
mating
type
to
give
again
a
diploid.
A
typical
example
of
the
life
cycle
of the
eucaryotic
protist,
the
yeast
Saccharomyces
cerevisiae. Although
sexual
reproduction
may
be
the
only
method
of
reproduction
in
most
animals
and
plants,
it
is
not
an
obligatory
event
in
the
life
cycles
of
many
protists.
Many
protists
lack
a
sexual
stage
in
their
life
cycle
and
even
in
species
in
which
sexuality
does
exist,
sexual
reproduction
may
be
infrequent.
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