a huge gene to produce a highly complex set of different proteins that function in different parts of the
anatomy of an organism.
6.2 Gene Families
Most organisms, particularly the more complex ones, contain more than one copy of a given gene. For
example, even simple prokaryotes such as E. colicontain several genes encoding rRNA. This is probably
necessary to ensure that sufficient amounts of the gene products are produced. In eukaryotes it is more
common to find multi-copy genes than true single copy genes. Human DNA has nearly 300 rRNA genes,
located in five clusters on different chromosomes.13,14Each of these repeated sequences is virtually iden-
tical to the others. rRNA gene clusters(Figure 6.5) are comprised of tandemly repeated units, each unit
containing a 28S, 5.8S and 18S gene, all driven from a single promoter. This structure resembles the
operons of prokaryotes (Figure 6.1).
There are examples of high copy number for other types of gene. The silk moth Bombyx morihas hun-
dreds of genes encoding the chorion (the egg shell). In this case however, there is far more complexity in
the sequences of the genes (Figure 6.6). It seems that an ancestral gene pair has proliferated and diversi-
fied to produce the multiplicity of different but related genes seen today.^15 This is an extreme example of
a very common process in gene and genome evolution. A comparison of haemoglobin genes across the
vertebrates shows a gradual process of duplication and diversification (Figure 6.7). Haemoglobin contains
Genes and Genomes 213
Figure 6.4 The Ubx gene of Drosophila – an example of complex gene structure
Figure 6.5 Structure of a tandemly repeated gene family, the human rRNA genes. Red shaded regions are expressed as
mature RNA, white boxed regions are transcribed and removed from the RNA precursor by RNA processing