15.1 - Totipotency and Cell Specialisation

Although all cells contain all genes, only certain genes are expressed (switched on) in any one cell at any one time.
Some genes are permanently expressed (switched on) in all cells. For example, the genes that code for the essential chemicals, such as the enzymes involved in respiration, are expressed in all cells.

Differentiated cells differ from each other mainly because they produce different proteins.

Cells such as fertilised eggs, which can mature into any body cell, are known as totipotent cells. The early cells derived from a fertilised egg are also totipotent. These later differentiate and become specialised for a particular function. 
This means that only part of the DNA of a cell is translated onto proteins. The…