Eukaryotic Chromosome Fine Structure - Protein-coding Genes

Protein-coding Genes

Regions of the genome with protein-coding genes include several elements:

• Enhancer regions (normally up to a few thousand basepairs upstream of transcription).
• Promoter regions (normally less than a couple of hundred basepairs upstream of transcription) include elements such as the TATA and CAAT boxes, GC elements, an initiator, etc.
• Exons are the part of the transcript that will eventually be transported to the cytoplasm for translation. When discussing gene with alternate splicing, an exon is a portion of the transcript that could be translated, given the correct splicing conditions. The exons can be divided into three parts
  • The coding region is the portion of the mRNA that will eventually be translated.
  • Upstream untranslated region (5' UTR) can serve several functions, including mRNA transport, and initiation of translation (including, portions of the Kozak sequence). They are never translated into the protein (excepting various mutations).
  • The 3' region downstream from the stop codon is separated into two parts:
    • 3' UTR is never translated, but serves to add mRNA stability. It is also the attachment site for the poly-A tail. The poly-A tail is used in the initiation of translation and also seems to have an effect on the long-term stability (aging) of the mRNA.
    • An unnamed region after the poly-A tail, but before the actual site for transcription termination, is spliced off during transcription, and so does not become part of the 3' UTR. Its function, if any, is unknown.
• Introns are intervening sequences between the exons that are never translated. Some sequences inside introns function as miRNA, and there are even some cases of small genes residing completely within the intron of a large gene. For some genes (such as the antibody genes), internal control regions are found inside introns. These situations, however, are treated as exceptions.