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The nucleus controls the process of cell division
Cell DivisionCell division is a means of increasing the number of
cells in 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
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
Nuclear membrane
Pair of centrioles necessary for cell division
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
Chromosome made of 2 chromatids
centromere
Centrioles move to the poles of the cell
Stage 3
In stage 3,the nuclear membrane has disappearedSpindle 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.
Spindle fibres
Chromosomes are at the equator
centrioles are at the poles
Note, the nuclear membrane has disappeared
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.
Nuclear division is complete, the cytoplasm starts to divide and the nuclear membrane forms again around the chromosomes
Cytoplasm starts to divide
Stage 6In stage 6, the cytoplasm has now divided
completelyTwo 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.
Two new genetically identical daughter cells have been formed: no genetic information has been lost
Chromosomes are thin threads