A combination of downward pressure AND friction. It occurs at the deeper layers of tissue resulting in cell deformation and cell death
        
             
        
        
        
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what you are doing ??? why are you asking these
 
        
             
        
        
        
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DNA and RNA are composed of nucleotides that are linked to one another in a chain by chemical bonds, called ester bonds, between the sugar base of one nucleotide and the phosphate group of the adjacent nucleotide. The sugar is the 3' end, and the phosphate is the 5' end of each nucleiotide.