the amount of gene product produced (in this case two corresponding enzymes). Often, different promot-
ers use a different splicing site.
6.1.2.2 Alternative Exons and Optional Splicing. A single gene does not necessarily use all of its
exons to produce a gene product (Figure 6.3b). Sometimes an exon may be omitted during RNA splicing.
If this exon is protein coding, the proteins produced from the two different mRNAs differ from each other.
In this way, a single gene can produce more than one protein.^11 Another way that an encoded protein sequence
can be altered is by an intron being missed out in the RNA splicing process (Figure 6.3c). This results in an
mRNA containing an intron. This is likely to terminate the synthesised protein, either by introducing a trans-
lational frame shift or a stop codon (Section 7.3.1), leading to a protein truncated at its carboxyl terminus.
6.1.2.3 Genes Within Genes. Occasionally, two genes can be found within the same section of DNA
sequence. Most commonly, a small gene, such as a small RNA-encoding gene, can be found within the
intron of a conventional protein-encoding gene (Figure 6.3d). In such circumstances, the two genes do not
seem to be expressed in the same cells at the same time, thus avoiding the problem of head-to-head colli-
sions between RNA polymerases transcribing the two DNA strands simultaneously. Certain small RNA
encoding genes, such as small nucleolar RNA (snoRNA) genes, may be found within introns in the same
transcriptional orientation as the surrounding gene.^12 In these cases, the snoRNA is cut out of the precur-
sor RNA by RNA processing (Section 7.5.2).
6.1.2.4 The Complexity of Some Genes in Higher Eukaryotes. A significant number of the genes
in a variety of higher organisms, such as Drosophila and humans, are highly complex and very large. For
example, in some of the major RNAs encoded by the Ubxgene of Drosophila(Figure 6.4) there are mul-
tiple promoters, optional or alternative introns and multiple polyadenylation sites, which are combined in
212 Chapter 6
Figure 6.3 Complex gene structures. (a) Alternative promoters for a single gene. (b) Optional exon usage.
(c) Intron omission. (d) A gene within the intron of another gene