2. At the moment of conception a potential human being is just one cell
By the time you’re an adult you will have an estimated 37.2trillion (3.72 x 1013) cells in your body
Almost all of these cells are made by mitosis
The cells in a multicellular body are not all the same
This is because as cell divide, grow and develop, they also being to DIFFERENTIATE

3. In the early development of animal and plant embryos, the cells are unspecialised
These cells are called STEM CELLS
Stem cells can then become any type of cell needed
In animals stem cells can become specialised very early in life
By the time a baby is born most of its cells are specialised i.e nerve, skim or muscle cells
They have differentiated
Some of their genes have switched on and others have been switched off

4. Most specialised cells can divides to produce more of the same cells i
Most specialised cells can divides to produce more of the same cells i.e. muscle cells divide to form more muscle cells
Some differentiated cells however such as skin cells or blood cells cannot divide at all and so ADULT STEM CELLS replace dead or damaged cells
Nerve cells do not divide once they have differentiated and they are not replaced by stem cells
As a result, when nerves cells are damaged they are not usually replaced
Plants keep growing through their lives
The plant cells produced do not differentiate until they are in their final position in the plant
Even then the differentiation is not permanent.
You can move a plant cell from one part of a plant to another.
There it can redifferentiate and become a completely different type of cell
You cannot do that with animal cells – once a muscle cell always a muscle cell

5. CLONING is when we produce identical offspring
A tiny piece of leaf can produce a huge number of cloned cells
This is because in the right conditions a plant cell will become unspecialised and undergo mitosis many times
Given different conditions these will then differentiate to form tissues such as the xylem, root hair cells etc. to make a tiny new plant
This new plant will be identical to the original parent
It is difficult to clone an animal because most animal cells differentiate permanently early in embryo development
As a result artificial animal clones can only be made by cloning embryos in some way although adult cells can be used to make an embryo

10. Divides EMBRYO ZYGOTE (Ball of cells)
EGG AND SPERM CELL FUSE
ZYGOTE
EMBRYO
(Ball of cells)
The inner cells of this ball are the EMBRYONIC STEM CELLS that differentiate to form all the specialised cell of your body

11. Even when you’re an adult some of your stem cells remain.
An adult stem cell is an undifferentiated cell and these give rise to many more
Adult stem cells are made in the BONE MARROW
Scientists now think there may be a tiny number of stem cells in most of the different tissues in your body including blood, brain, muscle and liver

12. Sometimes when people suffer from spinal injuries they can be come paralysed since their nerves cells cannot repair themselves.
People which have type 1 diabetes have to inject insulin into themselves everyday because specialised cells in their pancreases do not work.
Millions of people would benefit if we could replace damaged or diseased body parts.

13. In 1998 two scientists managed to culture embryonic stem cells capable of forming other types of cells.
Scientists are now hoping that stem cells can grow into any different type of cell needed in the body.
Already scientists have used nerve cells grown to restore movement of legs in paralysed patients. It is still early day but scientists are optimistic that stem cells will enable paralysed people to walk in the future.

14. In 2014 doctors transplanted embryonic stem cells into the eyes of people going blind as a result of macular degeneration. In the end patients could see a lot better

15. Scientists are also using stem cells to grow cells which are sensitive to blood sugar levels and produce more insulin to help treat people with diabetes
THE NEAR FUTURE
Stem cells could help develop whole organs or even help infertility and dementia

16. This can save some plants from extinction
Undifferentiated cells are formed at active regions of the stems and roots, known as MERISTEMS.
In these areas, mitosis takes place almost continuously.
The stem cells from plant meristems can be used to make clones of the mature plant very quickly and economically
This can save some plants from extinction
It can also give scientists a way of producing large populations of identical plants for research, by changing variables and observe the effects on genetically identical individuals

17. WORKSHEET 1