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Biology F4
6 : Cell Division
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Before
cell division
occurs, chromosomes appears as
thread-like
structures knows as
chromatin.
Interphase
consists of three stages:
G1 phase
,
S phase
, and
G2 phase.
The process of
mitosis
involves four stages:
prophase
,
metaphase
,
anaphase
, and
telophase.
Chromatin
consists of single
DNA strand
and
proteins.
Chromatin
cannot be seen under
microscope.
Karyokinesis
involves the
division
of the
nucleus.
Cytokinesis
involves the
division
of the
cytoplasm.
Somatic
cells are produced through
mitosis
process.
Gametes
are produced through the
meiosis
process.
Somatic
cells contains a
diploid
number of chromosomes,
2n
=
46
( in human )
Gametes
contains
haploid
number of chromosomes, n = 23 ( in human ).
A pair of
paternal
and
maternal
chromosomes are called
homologous
chromosomes.
Interphase
is the
longest
phase in the cell cycle.
What happens during
G1
phase?
Cell
grows
Cell
multiplies
the number of
organelles
in the
cytoplasm
What happens during S phase?
DNA replication
Each
chromosome multiplies
into
two identical chromosomes
(
sister chromatids
)
Both
chromatids
are
joined
at the
centromeres
Multiplication
of
centriole
What happens during G2 phase?
Cell
continues to
grow
More
proteins
and
enzymes
are
synthesised
Cell
pepares to
divide
What happens during M phase?
Cell division
—
prophase
,
metaphase
,
anaphase
,
telophase
Mitosis
is defined as the
division
of the
nucleus
of
parent
cell into
two
division in
somatic
nuclei.
In
prophase
, chromatin starts to
shorten
and
thicken.
The chromosome in
prophase
is made up of two identical threads called
sister chromatids.
During
prophase
, the
nucleus membrane disintegrates
,
nucleolus disappears
,
centriole
moves to the
opposite poles
and
spindle fibres
start to form.
In
metaphase
, the
chromosomes
lined up on the
equatorial plane.
During
metaphase
,
spindle fibres
maintain the chromosomes at the
equatorial plane.
Metaphase
ends when the
centromere
begins to divide.
The
centromere
divides into two and the sister
chromatids
separate during
anaphase.
In
anaphase
, the spindle fibres
shorten
,
contract
and pull the sister chromatids to
opposite poles.
Anaphase ends
when the
chromatid
arrives at the
pole
of the cell.
Chromosomes
are shaped again as fine chromatin threads at the start of
telophase.
Nucleoli
reforms and spindle fibres disappear during
telophase.
New
nucleus membrane
reforms to surround the new daughter chromosomes in
telophase.
The
telophase
stage is followed by
cytokinesis
to form two
daughter
cells.
Mitosis
in
plant
cells does not require the presence of
centrioles
for formation of
spindle fibres.
Instead, they
separate
using a new
cell wall.