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MODULE 2 - foundations in biology
2.6 - cell division, diversity and organisation
meiosis
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meiosis =
sexual reproduction
, produces
gametes
, gentetically
different
prophase 1
homologous
chromosomes pair up (
bivalent
)
bivalent
condenses
and the
chromosomes
join at several points (
chiasma
)
some genetic material is
exchanged
(
crossing over
)
centrioles start to form the
spindle
and the
nuclear membrane
begins to break down
2. metaphase 1:
bivalents
arrange themselves along the
equator
independent assortment
of
chromosomes
creates
variation
3. anaphase 1:
the
centromeres
do not
divide
- both
chromatids
are pulled towards
opposite poles
by the
spindle fibres
chromosomes
separated into
2 haploid sets
of
chromosomes
in the
daughter cells
independent assortment
creates
variation
4. telophase 1:
homologous
chromosomes are now at
opposite
ends of the cell
chromosome number has been
halved
each
chromatid
is genetically
different
nuclear membrane begins to
reform
and cell
splits
5. prophase 2:
nuclear membrane
breaks down
chromatids
shorten and
thicken
centrioles
move to
opposite poles
and form the
spindle
- often at right angle to the
first
ones
6. metaphase 2:
chromatids
arrange themselves along the
equator
7. anaphase 2:
centromeres
divide and
spindle fibres
pull to
opposite poles
-
chromatids
now called
chromosomes
independent assortment
of the
chromatids
creates genetic
variation
8. telophase 2:
chromosomes
uncoil
and
lengthen
spindle
disappears
and nuclear membrane
reforms
each nucleus now has
half
the number of chromosomes (
haploid
)
spermatogenesis = making of
sperm
cells
oogenesis = making of
egg
cells (
ovum
)
gametogenesis = formation of
gametes
(
sex cells
)