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Steps Leading from Gene to Protein

   Copyright 2004 by Alberts, Bray, Johnson, Lewis, Raff, Roberts, Walter.
Garland Publishing: Taylor Francis Group.

Steps Leading from Gene to Protein


Procaryotes and eucaryotes handle their RNA transcripts somewhat differently. (A) In eucaryotic cells, the initial RNA molecule produced by transcription contains both intron and exon sequences. Its two ends are modified, and the introns are removed by an enzymatically catalyzed RNA splicing reaction. The resulting mRNA is then transported from the nucleus to the cytoplasm, where it is translated into protein. Although these steps are depicted as occurring one at a time, in a sequence, in reality they occur simultaneously. For example, the RNA cap is typically added and splicing typically begins before the transcript has been completed. Because of this coupling, complete primary transcripts do not typically exist in the cell. (B) In procaryotes, the production of mRNA molecules is simpler. The 5′ end of an mRNA molecule is produced by the initiation of transcription by RNA polymerase, and the 3′ end is produced by the termination of transcription. Because procaryotic cells lack a nucleus, transcription and translation take place in a common compartment. In fact, translation of a bacterial mRNA often begins before its synthesis has been completed. The amount of protein in a cell depends on the efficiency of each of these steps and on the rates of degradation of the RNA and protein molecules.  Fair Use and Copyright info

 
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