In eukaryotic cells, genes are more complex. It was discovered in 1977 that eukaryotic genes are functionally separated into coding segments called exons, which are interrupted by noncoding sequences of DNA called introns. The entire region between the initiation and termination sites is transcribed, including the introns, to form the primary transcript. This must then be processed by special enzymes that cut out the introns and splice together the exons to form an mRNA. The mRNA is then exported from the nucleus for translation.
The existence of introns allows for the creation of multiple proteins from one gene, by the use or exclusion of different exons. Such alternative splicing gives rise to protein "isoforms," highly similar but slightly different proteins, with functions that vary as well. Isoforms are typically tissue-specific. For example, the muscle enzyme creatine kinase exists in one form in the heart, and another form in the skeletal muscles (such as the biceps), which have different ends formed through use of different exons. Even though it codes for two or more proteins, most scientists call such a DNA sequence a single gene.
Eukaryotic genes also contain a sequence close to the termination site called the polyadenylation signal. After transcription, this sequence prompts a special enzyme, called poly-A polymerase, to cut the RNA chain and begin adding multiple adenine nucleotides, as many as 250, to the primary transcript. This poly-A tail helps transport the RNA out of the nucleus, stabilizes it in the cytoplasm, and promotes efficient transcription at the ribosome.
Thus, the eukaryotic gene consists of an RNA binding site (promoter), a transcription initiation site, the coding region including exons and introns, the polyadenylation signal, and a termination site.
Genes for RNAs are transcribed in the same way, but the RNA formed is not translated into protein. Details vary among different types, but most RNA-coding genes do not contain introns. Transcripts of the ribosomal RNA genes must be cut apart to form a number of smaller functional RNA molecules.