Transcribed Ribonucleic Acid (RNA) are copies of DNA which serve as the first step towards production of molecules necessary for life. After transcription, the RNA is spliced (introns removed, exons joined) so it can produce or teach the proper protein or molecule structure. In many cases, a single RNA is respliced to produce different results; this is called Alternative Splicing and is common in all multi-cellular organisms.
While all cells utilize gene splicing, alternative splicing -- and production of a wide variety of proteins and molecules -- is most often seen in Eukaryotic cells, or complex membrane-bound cells, which are most often seen in plant and animal life, including humans. Prokaryotes, which mostly do not have membrane-bound nuclei, do not need to produce as many protein types, and so their RNA is usually only spliced for one purpose. Humans contain almost three times as many proteins as lower life forms, and also contain many more repeated proteins because of alternative splicing.