2.6 Cell Division, Cell Diversity and Cell Differentiation

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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
They detect and repair damage to DNA (may be caused by UV light)
The cell cycle cannot be reversed
DNA is only duplicated once during each cell cycle
Phase and checkpoints Events within the cell
G0 (gap 0) phase Cells may undergo apoptosis, differentiation
A resting phase triggered during early G1 or senescence
at the restriction point by a checkpoint Some cells stay in G0 for a long time or
chemical indefinitely (e.g. neurones)
Some cells e.g. epithelial cells in the gut do
not have this phase
G1 (gap 1) phase/growth phase Cells grow and increase in size
A G1 checkpoint control ensures the cell is Transcription of genes to make RNA occurs
ready for the S phase Organelles duplicate
Biosynthesis e.g. protein synthesis,
including making the enzymes needed for
DNA replication in the S phase
The p53 gene helps control this phase

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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 rapid because exposed DNA
replication starts with
base pairs are more susceptible to
Interphas those that are active in all mutagenic agents
e cells.…read more

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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 nuclear division that maintains the same genetic information of the parent cell
Meiosis ­ a type of nuclear division that results in the formation of cells containing half the
number of chromosomes of the…read more

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Prophase 2 Nuclear envelopes break down again.…read more


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