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DNA Replication
1. The DNA molecule unwinds and
unzips. The enzyme DNA
helicase causes the hydrogen
bonds between base pairs to
2. new DNA nucleotides pair up
with the exposed bases,
according to the
complementary base pairing
3. The nucleotides are joined
together by the enzyme DNA
polymerase. A phosphodiester
bond forms between the sugar
of one nucleotide and the phosphate of the next.
(d) G1 and G2 Points
G1 is the first growth phase. During this stage proteins are made.…read more

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Apoptosis is triggered cell suicide. The cell shrinks, the dna and protein in the nucleus breaks down. The
mitochondria break down. The cell breaks up into small fragments wrapped in membrane. A
phospholipid, phosphatidylserine, normally on the outside is place on the inside. Phagocytic cells such as
macrophages engulf the `blebs' or fragments.
(g) Need for Apoptosis
Apoptosis is needed to destroy cells that pose a risk to the organism such as cells infected with a virus or
a cell with damaged DNA.…read more

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Stem Cells
A cell that can remain undifferentiated and divide to form different kinds of cells are called stem cells.
(l) Differentiation
When cell types remain different they are said to be differentiated. This means that some genes within
the cell are switched off. Once a cell has differentiated it loses the ability to divide to produce a different
kind of cell.…read more


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