Cell differentiation is the process by which an unspecialised cell becomes specialised to carry out a particular function. A human body contains over 200 different types of specialised cell, all produced from a single fertilised egg.Differential gene expression
Almost all body cells contain the same genes, because they were all produced by mitosis from the zygote. Cells differentiate because they express different genes. In each cell type:
• some genes are switched on, so their mRNA is transcribed and the proteins are made
• other genes are switched off, so their proteins are not made.
The proteins made determine the structure and function of the cell. This is called differential gene expression.How genes are switched on and off
Signals from outside the cell, such as hormones or chemicals from neighbouring cells, activate transcription factors that switch on particular genes. Epigenetic changes such as DNA methylation and histone modification then keep genes permanently switched on or off, so the cell stays specialised. These patterns are copied to daughter cells.Example: red blood cells
As a red blood cell develops in the bone marrow, the genes for haemoglobin are switched on, so the cell fills with haemoglobin to carry oxygen. The cell then loses its nucleus and most organelles, making more room for haemoglobin and giving the cell its biconcave shape.Other examples
• Muscle cells express genes for the proteins actin and myosin.
• Cells in the pancreas express the gene for insulin.In most animals, differentiated cells cannot usually go back to being unspecialised.
Almost all body cells contain the same genes, because they were all produced by mitosis from the zygote. Cells differentiate because they express different genes. In each cell type:
• some genes are switched on, so their mRNA is transcribed and the proteins are made
• other genes are switched off, so their proteins are not made.
The proteins made determine the structure and function of the cell. This is called differential gene expression.How genes are switched on and off
Signals from outside the cell, such as hormones or chemicals from neighbouring cells, activate transcription factors that switch on particular genes. Epigenetic changes such as DNA methylation and histone modification then keep genes permanently switched on or off, so the cell stays specialised. These patterns are copied to daughter cells.Example: red blood cells
As a red blood cell develops in the bone marrow, the genes for haemoglobin are switched on, so the cell fills with haemoglobin to carry oxygen. The cell then loses its nucleus and most organelles, making more room for haemoglobin and giving the cell its biconcave shape.Other examples
• Muscle cells express genes for the proteins actin and myosin.
• Cells in the pancreas express the gene for insulin.In most animals, differentiated cells cannot usually go back to being unspecialised.