Are All Somatic Cells Identical?

[gracy]

Are all somatic cells same?As they all are product of mitosis of zygote ,all somatic cells should be identical.If yes then what makes eye cell different from say heart cell i think it is switching on and off mechanism i.e these two cells are exactly the same.They will contain same 46 chromosomes but gene for eye will be on in eye cell but off in heart cell,right?

 

[Drakkith]

I believe the difference between the different cells types is which genes are being expressed. Immune cells don't express the same genes as neurons do. See this article: http://en.wikipedia.org/wiki/Cellular_differentiation

In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simplezygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly controlled modifications in gene expression. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome.

(underlining mine)