With the advent of genetic engineering technology during the last decade, much valuable insight has been gained into the structure, function, and regulation of eukaryotic genes, especially those that code for proteins. Incisive experimental studies have established that (1) most eukaryotic genes are interrupted or split, and (2) they have sequences at their 5′ regions that are responsible for the regulation of their transcription. A generalized representation of a eukaryotic gene is given in Figure 4.1.

Exons and introns

In prokaryotes, the entire nucleotide sequence of a gene codes for a messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA), as the case may be. In eukaryotes, however, the corresponding genes are considerably longer than their final products. The protein-coding genes are transcribed into pre-mRNAs (hnRNAs) of equal length, but are then processed into shorter and mature mRNAs in the nucleus. The mRNAs are translocated into the cytoplasm where they are translated into proteins. Likewise, pre-tRNAs and pre-rRNAs are transcribed from their corresponding genes and are processed into shorter and mature tRNAs and rRNAs in the nucleus. They are then translocated into the cytoplasm to carry out their functions in the translation of mRNAs.