<P> An additional layer of regulation occurs for protein coding genes after the mRNA has been processed to prepare it for translation to protein . Only the region between the start and stop codons encodes the final protein product . The flanking untranslated regions (UTRs) contain further regulatory sequences . The 3' UTR contains a terminator sequence, which marks the endpoint for transcription and releases the RNA polymerase . The 5' UTR binds the ribosome, which translates the protein - coding region into a string of amino acids that fold to form the final protein product . In the case of genes for non-coding RNAs the RNA is not translated but instead folds to be directly functional . </P> <P> The structure of eukaryotic genes includes features not found in prokaryotes . Most of these relate to post-transcriptional modification of pre-mRNAs to produce mature mRNA ready for translation into protein . Eukaryotic genes typically have more regulatory elements to control gene expression compared to prokaryotes . This is particularly true in multicellular eukaryotes, humans for example, where gene expression varies widely among different tissues . </P> <P> A key feature of the structure of eukaryotic genes is that their transcripts are typically subdivided into exon and intron regions . Exon regions are retained in the final mature mRNA molecule, while intron regions are spliced out (excised) during post-transcriptional processing . Indeed, the intron regions of a gene can be considerably longer than the exon regions . Once spliced together, the exons form a single continuous protein - coding regions, and the splice boundaries are not detectable . Eukaryotic post-transcriptional processing also adds a 5' cap to the start of the mRNA and a poly - adenosine tail to the end of the mRNA . These additions stabilise the mRNA and direct its transport from the nucleus to the cytoplasm, although neither of these features are directly encoded in the structure of a gene . </P> <Table> <Tr> <Td> Polycistronic operon Regulatory sequence Regulatory sequence Enhancer Enhancer / silencer / silencer Operator Promoter 5'UTR ORF ORF UTR 3'UTR Start Start Stop Stop Terminator Transcription DNA RBS RBS Protein coding region Protein coding region mRNA Translation Protein The structure of a prokaryotic operon of protein - coding genes . Regulatory sequence controls when expression occurs for the multiple protein coding regions (red). Promoter, operator and enhancer regions (yellow) regulate the transcription of the gene into an mRNA . The mRNA untranslated regions (blue) regulate translation into the final protein products . </Td> </Tr> </Table>

What are the components of a eukaryotic gene
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