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B5: Growth
and Development
By Anokhi Patel…read more

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Cell Specialisation
· 'Unspecialised' stem cells can develop into any other type of cell. Stem
cells are found in animal zygotes (very young embryos), and in plants of all
· A zygote is a structure that forms when a sperm fertilises an egg.
· Human embryonic stem cells can come from the eight-cell stage of
embryo development.
· The zygote then divides many times by mitosis to form an embryo. The
first division of the zygote forms two cells, the next four, the next eight,
and so on.
· Up to the eight-cell stage, all of the cells are identical. They are
called embryonic stem cells. It is possible for embryonic stem cells to
develop into any other specialised type of cell that the growing embryo
needs - for example, nerve cells, blood cells and muscle cells. However,
once the embryonic stem cells become specialised, they can't change into
any other type of cell.
· The specialised cells can form all the different types of tissue that the
embryo needs. Groups of different types of tissues are arranged together
to form organs.…read more

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Switching Genes On/Off
· Cells become specialised because the genes
that are not required are switched off. Only
the genes needed to make a particular type of
cell work are switched on. So muscle cells only
have the genes needed to make muscle cell
proteins switched on. All the other genes,
such as those needed to make blood cell
proteins and nerve cell proteins, are switched
off.…read more

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Cell Specialisation in plants
· Unspecialised stem cells also exist in plants.
They can become specialised into the cells of
roots, leaves or flowers.
· Unlike animal cells, some plant cells
can remain unspecialised and develop into
any type of plant cell.…read more

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Cloning Plants
· Whenever you eat a Golden Delicious apple, you are eating
a clone. In fact, most of the fruits that we buy in shops and
supermarkets are clones. This is because the plants they come
from have been grown from cuttings.
· Cuttings develop much bigger root systems if they are dipped in
hormone rooting powder or planted in rooting compound
containing growth hormone.
These hormones cause unspecialised stem cells to grow and
develop. They turn into tissues such as xylem and phloem,
and organs such as roots, leaves, stems and flowers, thereby
forming a complete new plant.
· This makes it possible to clone plants quickly and cheaply.
· All cuttings are genetic copies of their parents. This means they
are clones.…read more

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· Plants cells are different to animal cells in another
way. Unspecialised stem cells in plants are
grouped together in structures called meristems.
Cells produced by meristems ensure plants
continue to grow in height and width throughout
their life. Animals stop growing in size once they
become adults.
· Plant meristems divide to produce cells that
increase the height of the plant, length of the
roots and girth of the stem. They also produce
cells that develop into leaves and flowers.…read more

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