2.1.8 explain that cells in multicellular organisms differentiate to carry out the specialised functions by expressing some of their genes but not others.  Process of differentiation.  Cells specialised to perform their function.  This happens because some genes are expressed (switched on) and others are not. 

After fertilisation That’s when (in humans), a sperm cell and an egg cell (ovum) join to make a zygote. The zygote undergoes mitosis. One cell divides to form two. Each with exactly the same genetic information. They are identical.

Mitosis continues  2 cells become 4.  4 cells become 8……….  Until a ‘ball’ of cells forms.  This is known as an embryo.

The embryo At this stage, any one of these cells contains a complete set of genetic information to make a human.

These cells will specialise.  Some will become nerve cells  Some become blood cells  Others become muscle cells ……..

Going back to the embryo  These cells will go through a process called: DIFFERENTIATION

SPECIALISED FUNCTIONS Each of these cells has become specialised to carry out a particular function. e.g. Nerve cells to transmit electrical impulses Blood cells to carry O 2 and CO 2 Muscle cells to contract. Special functions require a change in cell shape.

How do these cells become specialised? Interesting: because we know that all of these cells contain the genetic information to become a:  Nerve cell or  Muscle cell or  Red blood cell…..

‘Switching on’  Genes for the nerve cell pathway are ‘switched on’ or expressed and the others are not.  Genes for the muscle cell pathway are ‘switched on’ and the others are not.  Genes for the red blood cell pathway are ‘switched on’ and the others are not.

 Cells differentiate.  Specialise.  and change shape. typical of multicellular organisms.