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2.6 Cell Division, Cell Diversity and Cell Differentiation
Cell Cycle

Cell Cycle Checkpoints
There are two main checkpoints ­ the G1/S checkpoint (the restriction point) & the G2/M
There is another halfway through mitosis and another in early G1
These prevent uncontrolled division that would lead to tumours

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S (synthesis) phase Cell is committed to completing the cell
The chromosomes are cycle
unwound and DNA is DNA replicates
diffuse so every molecule When all chromosomes have been
of DNA is replicated duplicated each consists of an identical pair
of sister chromatids
The sequence for DNA
This phase is…

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The cell now contains two identical nuclei

Cytokinesis Animals the plasma membrane nips in the cytoplasm
Plants ­ an end plate forms at the equator and a cell wall
Chromatids ­ replicates of chromosomes
Cytokinesis ­ division of the cytoplasm of a cell following mitosis
Mitosis a type of…

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in favour of
straight to
prophase 2

Prophase 2 Nuclear envelopes break down again.
The chromosomes condense
The chromatids are no longer identical
Spindle forms

Metaphase 2 Chromosomes attach by their centromere to the
equator of the spindle
The chromatids of each chromosome are
randomly arranged
The random arrangement…


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