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A gene is the nucleic acid sequence needed to synthesize a particular gene product.

A gene includes more than just the coding region that encodes an RNA transcript; there are also control regions controlling synthesis, processing and translation of the RNA transcript. In prokaryotes, the entire coding region encodes a continuous polypeptide sequence. In eukaryotes, coding regions contain exons (50-250 nucleotides) that encode polypeptide sequences and introns (500-50,000 nucletides, removed during RNA processing) that do not. Higher eukaryotes not only have introns within genes, but large intergenic regions. For example, a ~80 kb region in Saccharomyces cerevisiae (baker's yeast) contains 40 genes; the ~80 kb region encompassing the human β-globin cluster contains only 5 genes. This extra DNA comes from multiple repeats described here.

Exons often encode modular units that are included or excluded via RNA processing. Exons are usually highly conserved while introns are barely conserved. For example, SUR2 exons are 90% identical between mice and humans while SUR2 introns are less than 10% identical between mice and humans. A lack of inter-species sequence conservation indicates a lack of function.

diagram showing exons and introns within a gene
MonocistronicMost eukaryotic genes are monocistronic, meaning their mRNAs encode a single protein. Often, a eukaryotic primary transcript forms a single mRNA that encodes a single protein. Most eukaryotic mRNAs have a 5' cap structure that directs ribosome binding, with translation beginning only at the closest AUG codon.
PolycistronicProkaryotic genes are mostly polycistronic, with one mRNA encoding multiple proteins involved in a biological process. Along the mRNA, there is a ribosome binding site near each coding region's start site. Translation can initiate at any of these sites, allowing production of different proteins from one mRNA.

A transcription unit is a region of DNA that is transcribed under the control of a particular promoter. While a gene and a transcription unit (like the LAC operon) are distinguishable in prokaryotes, the two terms are used interchangeable in eukaryotes. There are simple and complex eukaryotic transcription units. A simple transcription unit RNA transcript is processed to yield a single mRNA encoding a single protein. Complex transcription units, which are more common, encode an RNA transcript that is processed to form different monocistronic mRNAs each encoding a different protein. A single transcript can undergo different mRNA pathways via:

Alternative SplicingmRNAs have the same 5' and 3' exons but different internal exons.
Alternative Poly(A) SitesmRNAs have the same 5' exons but different 3' exons.
Alternative PromotersmRNAs have different 5' exons but share 3' exons.
Next StepsStudy about the eukaryotic chromosome.