B2.3.1—Production of unspecialized cells following fertilization and their development into specialized cells by differentiation

Differentiation: –

Cell differentiation is the WAY by which cells become specialized.

All cells of an organism contain all the chromosomes, hence the same genes

But all cells are specialized

e.g. White blood cells to fight against pathogens (1)

ex. Heart’s muscle cells to pump blood out of the heart (2)

ex. Skeletal muscles’ cells for body movement (3)

ex. Neurons carry nervous impulses (4)

• A cell does not need all proteins all the time – A stomach cell needs the proteins the stomach needs – An iris cell needs the proteins the iris needs

BUT

– A stomach cell does not need all proteins needed by the iris

– An iris cell does not need all proteins needed by the stomach

Gene : Fragment of chromosome containing instructions to produce a specific protein

An organism has different types of specialized cells because each cell type “expresses” ONLY the genes that are necessary for this cell type

In humans: A few hundred of genes among the 20 500 available genes are expressed in cells