what is the role of DNA, RNA, and environmental factors in cell differientation
This is an incredibly subtle and interesting problem. I really love studying these discrete decisions - like whether to become cell type A or B - and how cells make them given that their inputs are all continuous variables. Environmental factors: In very early development, cell differentiation occurs primarily in response to chemical signals from the inter-uterine environment (read, "chemical signals from Mom's body"). It is affected to a lesser extent by so called 'stochastic' or random factors, like whether a certain mRNA in the first fertilized cell ends up in cell A or cell B after it goes through the first round of cell division - but those are less well understood at the moment. DNA: The switch from an undifferentiated cell to a fully differentiated cell is not instantaneous. Cells can have different degrees of potency - that is, the range of possible cell types they can gets smaller and smaller as they become more and more differentiated. This limiting happens as the DNA is chemically modified (actual chemical groups like methyl and acetyl are covalently attached to the DNA itself) and as it is wrapped up and packaged away on clumped proteins called histones. This has to do with differentiation because, in muscle cells, the genes which are necessary to make the cell anything except a muscle cell are packaged away and inactivated. As a cell becomes more and more differentiated, more and more of the DNA becomes modified and inactivated, so the range of possible cell types becomes smaller and smaller. RNA: This is more complicated because there are different kinds of RNA (mRNA, siRNA, microRNA) which do different things. And their affects on cell differentiation are subtle: when a cell divides, all the RNA molecules in the cytoplasm end up in one daughter cell or the other - so essentially the daughter cells inherit all their parent's RNAs and only as they degrade those RNAs and replace them with their own do they come to resemble their own cell types. The mRNAs continue to carry instructions for making proteins and the siRNAs have to do with disposing of unwanted mRNAs - so the effects on cell differentiation depend on what the balance is between mRNAs and siRNAs, and what genes specifically the mRNAs and siRNAs came from.
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