Download Cell Division - Kelso High School

The nucleus controls the process of cell
division
Cell Division
 Cell division is a means of increasing the number of cells in
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an organism.
Every species has a definite and characteristic number of
chromosomes (the chromosome complement) present in
each cell.
Humans have a chromosome complement of 46
chromosomes.
The nucleus of the cell controls cell activities, including cell
division.
Division of the nucleus is called Mitosis, and this results in
the formation of two genetically identical daughter nuclei.
Mitosis maintains the chromosome complement.
Stages of Mitosis –
 In stage 1 of mitosis, the nucleus contains long, uncoiled
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chromosomes – a bit like fine threads scattered
throughout the nucleus.
chromosomes are only visible in the cell during mitosis.
The nucleus, at this stage, is surrounded by a nuclear
membrane.
Structures called centrioles are present in the cytoplasm.
Between stages 1 and 2 each chromosome replicates, each
chromosome doubling to form two identical chromatids
joined at a point called the centromere.
At this point chromosomes are easily stained using a dye.
Stage I
Chromosomes,
look like fine
threads
Pair of
centrioles
necessary for
cell division
Nuclear
membrane
Stage2
 In stage 2 the chromosomes are shorter and thicker
and they are made up of two chromatids joined at the
centromere
 The nuclear membrane is still present
Stage 2
Centrioles
move to the
poles of the
cell
Chromosome made of 2
chromatids
centromere
Stage 3
 In stage 3,the nuclear membrane has disappeared
 Spindle fibres have formed.
 Chromosomes are attached by their centromere to
structures called centrioles which are now present at
the poles of the cell.
 Chromosomes, consisting of chromatids, line up at the
equator of the cell
Stage 3.
Chromosomes are
at the equator
Spindle fibres
Note, the nuclear membrane has
disappeared
centrioles are
at the poles
Stage 4
 In stage 4,each centromere splits and the pair
of identical chromatids become separated.
 one chromatid from each pair moves to one
pole of the cell and the other to the opposite
pole of the cell.
 During cell division, division of the nucleus is
followed by division of the cytoplasm.
Stage 4.
One
chromatid
goes to one
pole
The other
goes to
the
opposite
pole
Stage 5
 The cytoplasm starts to divide, nuclear division has
now been completed.
 In this stage, a nuclear membrane forms around each
set of chromatids
 The chromatids are now called chromosomes again
Stage 5.
Cytoplasm
starts to
divide
Nuclear division is complete, the cytoplasm starts to divide
and the nuclear membrane forms again around the
chromosomes
Stage 6
 In stage 6, the cytoplasm has now divided
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completely
Two GENETICALLY Identical Daughter Cells have
been formed.
These daughter cells contain the same number of
chromosomes as the original cell – the
chromosome complement has been maintained.
There is now cell growth prior to the process
being repeated.
Q. why is it important that new cells have the
same number of chromosomes as the original?
A. to ensure that no genetic information is lost.
Stage 6.
Chromosomes are thin
threads
Two new genetically identical daughter cells have
been formed: no genetic information has been lost