Transcription in Eukaryotes
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In eukaryotes transcription and the formation of messenger RNA is more complex than in prokaryotes. Eukaryote genes are typically divided into sections by segments of DNA that are transcribed into regions of transcript RNA called introns.
Intron: The introns separate sections of the transcript and are removed before translation to give the final messenger RNA which leaves the nucleus.
Exon: The sections of the transcript RNA that combine to form the messenger RNA are called exons. The introns stay in the nucleus where its possible that they provide a feed back mechanism to control production of RNA.
The basic scheme found in most eukaryotes is shown in the first figure. One of the functions of introns at least for some genes is to allow the cell to make different mRNA's and hence different proteins from the same gene. For example many cells have slightly different versions of a protein called tropomyosin. Each of these different versions is made by selecting different exons from a transcript produced from a common gene. For instance one mRNA might have exons A,C,D another A,B,C. Each of the resulting mRNA's would then be translated into a different protein depending on the cell. This process of selecting different combinations of exons to be translated is important in antibody production by the immune system.
The origin of introns is not clear. A few prokaryotes, and certain organelles such as chloroplasts have some introns and some scientists suggest that introns arose early in the evolution of the genetic system used currently by living things and then were lost in prokaryotes because of the premium placed on speed of reproduction in bacterial and other prokaryotes.
Created 03/07/00 pgd