8/30/10
Meiosis
•  Nuclear
division
creating
cells
that
contain
only
one
member
of
each
pair
of
chromosomes
present
in
premeiotic
cell.
•  Takes
place
in
meiocytes
–  Primary
oocytes
–  Primary
spermatocytes
•  Reduces
cell
from
diploid
to
haploid
•  Two
successive
nuclear
divisions
–  Meiosis
I
=
reductional
division
–  Meiosis
II
=
equational
division
Meiosis
I
‐
Reductional
Division
•  Divided
into
4
stages
–  Prophase
I
•  Leptotene
•  Zygotene
•  Pachytene
•  Diplotene
•  Diakinesis
–  Metaphase
I
–  Anaphase
I
–  Telophase
I
Meiosis
II
‐Equational
Division
•  Resembles
mitotic
division
EXCEPT
the
sister
chromatids
of
a
homolog
are
not
identical
along
their
entire
length
because
of
crossing
over
1
8/30/10
Prophase
I
•  Leptotene
‐
“thin
thread”
–  As
chromosomes
condense,
chromomeres
appear
–  Characteristic
size,
position
and
number
•  Zygotene‐paired
threads
–  Period
of
synapsis
in
which
homologous
chromosomes
match
chromomeres
creating
bivalents
Prophase
I
continued
•  Pachytene
‐
“thick
thread”,
chromosomes
continue
to
condense
•  By
late
pachytene
each
bivalent
can
be
seen
to
be
a
tetrad
of
four
chromatids
•  Crossing
over
takes
place
during
this
stage
although
it
is
not
apparent
until
next
stage
2
8/30/10
Prophase
I
continued
•  Diplotene‐
“Double
thread”,
synapsed
chromosomes
begin
to
separate
•  Crossover
points
called
chiasma
(chiasmata)
result
from
physical
exchange
of
between
nonsister
chromatids
of
homologous
chromosomes
•  Each
bivalent
should
have
at
least
one
chiasma
3
8/30/10
Prophase
I
continued
•  Diakinesis
‐
“moving
apart”,
terminalization
of
chiasmata
•  Nuclear
envelope
breaks
down
•  Spindle
is
formed
Metaphase
I
•  Bivalents
position
on
opposite
sides
of
metaphase
plate
•  Genes
on
different
chromosomes
undergo
independent
assortment
because
nonhomologous
chromosomes
align
at
random
on
the
metaphase
plate
4
8/30/10
Anaphase
I
•  Homologous
chromosomes
composed
of
two
sister
chromatids
joined
at
an
undivided
centromere
separate
from
one
another
and
move
to
opposite
ends
of
the
spindle
•  The
physical
separation
of
homologous
chromosomes
is
the
physical
basis
of
Mendel’s
principle
of
segregation
5
8/30/10
Telophase
I
•  Spindle
breaks
down
•  Depending
on
the
species,
a
nuclear
envelope
may
form
•  Chromosomes
begin
to
uncoil
So
in
Review…
6