6.1.1 Conventional Eukaryotic Gene Structure – The Globin Gene as an Example
Most of the genes in eukaryotes belong to the third class described above and the great majority of these
encode proteins. The pathway from the gene to its protein product and the structural relationships between
the gene and its gene products are exemplified by the globin gene (Figure 6.2), which is found in all ver-
tebrate animals. The gene is first transcribed in the cell nucleus to produce a precursor RNA that contains
all the gene’s introns. The 5-end of the precursor RNA corresponds to the transcription start but the 3-end
extends past the eventual terminus of the mature messenger RNA (mRNA) product. Such a precursor
RNA is typically unstable and is quickly processed into a mature mRNA by removal of its introns and by
cleavage at its 3-end, followed by the addition of a few hundred adenosine bases to produce a ‘poly A
tail’.^8 The mRNA includes start and stop sites for translation. Therefore, an mRNAs always contains extra
nucleic acid sequences at both its 5- and 3-end that are not converted into protein (shown in white in the
RNA in Figure 6.2). The mature mRNA is exported from the nucleus^9 to the cytoplasm of the eukaryotic
cell where it is translated into protein.
6.1.2 Complex Gene Structures
The large majority of protein-encoding genes have the general structures shown in Figure 6.1, with vari-
ations in overall size and number of exons. However, there are many examples of more complicated gene
structure (Figure 6.3).
6.1.2.1 Alternative Promoters. It is relatively common to find a single gene, which contains more
than one promoter.^10 An example is the alcohol dehydrogenase gene of the fruit fly Drosophila (Figure 6.3a).
Typically, the different promoters function either in different tissues of the organism, at different develop-
mental stages or in response to different stimuli. Alternative promoterstherefore provide a way of varying
Genes and Genomes 211
Figure 6.2 Relationships between a typical eukaryotic protein-coding gene and its gene products. Red shaded
regions are translated into protein