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As far as the genetic content of each cell is concerned I have read to my satisfaction that all cells of a person's body except the red blood cells (with no nucleus and so no genetic message) and the gametes ( with just half the genetic copy or 23 of the 46 chromosomes) have the same DNA copy and so all the 60,000-80,000 estimated genes. The only difference is that which genes are active and which are dormant. For example the gene for insulin would be present in all cells but would be active and functional only in the cells of pancreas assigned the function of insulin production.

My question is:-

What is the factor or process or molecule or information that tells a cell or helps it to activate the genes necessary for its own growth and more importantly its designated function?

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  • $\begingroup$ this assumption is not really correct - its now well understood that epigenetic factors including rearrangement of the chromosomes in the nucleus will affect the expression of genes in different cell lines. I guess you could say that the genome is not different in this case, though it behaves quite differently, but even then, in some cell lines the telomeres shorten when the cells divide, so even the sequence is not uniform in all cells in the human body. $\endgroup$
    – shigeta
    Commented Nov 19, 2013 at 6:07
  • $\begingroup$ As i understand, you mean why all cell have the same gene, but they can know what they do. It's because gene like a computer programming language. They are programmed to be converted from stem cell into different type of cells in different environment. There must be a series reaction that cause the environment different for cells itself know what they need to do. And i think this question related to how to convert stem cell into different type of cell. Scientist using different environment to do it. $\endgroup$
    – DucFabulous
    Commented Nov 19, 2013 at 12:20
  • $\begingroup$ By the way, lymphocytes do not have all the genes as they undergo recombination for creating antibodies. $\endgroup$
    – biogirl
    Commented Nov 19, 2013 at 17:10
  • $\begingroup$ Humans have about 20000 genes. But see also this http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2898077/: $\endgroup$
    – yotiao
    Commented Nov 20, 2013 at 13:03

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Your question refers to differential gene expression and methods of Eukaryotic gene regulation. This is a very broad question for there are a lot of methods to control gene expression. They can function as transcription control, mRNA processing control, translational control, post-translational control and so on in eukaryotes. In prokaryotes, the methods include the operon model etc, which is much simpler than eukaryotes and multicellualr organisms but still quite complicated when studied in its entirety. Here and Here are few places where you can study differential gene expression. As an introductory text, chapter 19 of Life:The science of Biology by Purves, Sadava, Orions and Heller(the link is to a supplementary-material site and not to the book itself), Otherwise, to elucidate all mechanisms here would become too long and in any case uncomprehensive.

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