Re: What causes genes to be expressed for cell specialization?

  
 
    

      
The differentiation or 
specialization 
        of cells during development is driven by two interacting processes. 
The 
        first is the initial state of the cell -- which genes are being 
expressed, 
        which proteins are present, and how they are modified. This 
determines 
        how the cell responds to signals from neighboring cells. For 
example, 
        if the receptor for a signaling protein is not present in the cell, 
the 
        cell will ignore the presence of the signalling molecule.
      
Based on the initial 
state 
        of the cell, and the signals presented to it by its neighbors, there 
will 
        be change in protein activity and gene expression. The genes within 
a 
        cell form an interactive, often cross-regulating, network.

    
  


  

    
 
      

        
 
          

            
Genes consist of 
two 
              functionally distinct components. There are regulatory sites 
("reg. 
              sites") that bind specific proteins (known 
generically 
              as 'transcription factors'). 
            
The combination of 
transcription 
              factors bound to a gene's regulatory sites will determine 
whether 
              the gene will be recognized by DNA-dependent RNA polymerase.

          
        
        
 
          


            reg.sites 
.............coding 
            region
        
        

          
RNA polymerase makes 
an 
            mRNA copy of the gene's "coding region", which 
is 
            generally translated into a polypeptide. 
        
      
      

      Signals from outside the 
cell 
      (the arrows on the left of the figure) interact with cellular 
receptors 
      and lead to changes in transcription factor activities). 
    

      
input
      

      
output
    
    
 
      
Some of the genes 
expressed 
        in response to external factors make transcription factors, or lead 
to 
        altered transcription factor activity. 
      
This leads to a global 
change 
        in gene expression within the target cell (the output or 
target 
        genes are shown on the right side of the figure). 
      
 
    
  



  

    

      
The end result is a 
change 
        in the pattern of proteins that accumulate within the "target" 
        cell. The process is complicated by the fact that the cell that 
receives 
        signals from its neighbors also emits signals that can alter its 
neighbors 
        behavior.  The strength of the signals is also critical, so 
the 
        number of neighbors can alter a cell's fate. 
      
You can think of the differentiated state of a cell as the pattern 
of gene expression. In most cases, there are "feed-back" and "feed-forward" 
interactions that make that state relatively stable and difficult to alter.