Why people fear GMOs? Can't we map a plant composition?
People are not always rational when it comes to what they eat, especially when the compounds that make up the food have long, arcane-sounding names. 

It is indeed possible to analyse the amino acid composition of a food substance. However, this has no bearing whatsoever on the safety of the food product. 

For example, the following are two truncated protein sequences. One of them is a fragment constituent of cobra venom, the other is a fragment of harmless DNA polymerase. Without looking up or otherwise referring to the PDB IDs, it is not obvious which is which, even to an experienced molecular biologist. 


  3HRZ:A|PDBID|CHAIN|SEQUENCE
  ALYTLITPAVLRTDTEEQILVEAHGDSTPKQLDIFVHDFPRKQKTLFQTRVDMNPAGGMLVTPTIEIPAKEVSTDSRQNQ
  
  1KLN:A|PDBID|CHAIN|SEQUENCE
  VISYDNYVTILDEETLKAWIAKLEKAPVFAFATETDSLDNISANLVGLSFAIEPGVAAYIPVAHDYLDAPDQISRERALE


Furthermore, even if scientists can prove beyond reasonable doubt that the two products are chemically identical, people will still irrationally purchase them. A clear example is the different prices commanded by "natural" versus "artificial" flavourings, as seen in this Scientific American article. 


  Consumers pay a lot for natural flavorings. But these are in fact no better in quality, nor are they safer, than their cost-effective artificial counterparts.