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Figure 1.9 Protein-coding genes in eukaryotic organisms are divided into introns and exons. Introns
are removed from the mRNA before it is translated into a polypeptide. In this example, there are six
exons and four introns. The genetic message is present in exons I, II, III, IV, V, and VI.
gene that duplicated to produce two, or more, identical genes ( Francino 2005 ).
These identical genes could have diverged in nucleotide sequence through time
to produce (two or more) related functional genes. In some cases, the genes of
multigene families can be found at different positions on more than one chro-
mosome after large-scale rearrangements (translocations or inversions) that
occur both within and between chromosomes. Examples of multigene families
in insects include actins , tubulins , heat shock , salivary glue, chorion , cuticle, and
yolk protein genes. (Note that the name of a specific gene usually is italicized.)
Pseudogenes are DNA sequences that seem similar to those of functional
genes, but the genetic information has been altered (mutated) so that the for-
mer gene is no longer functional. Once the biological information has been lost,
a pseudogene can undergo rapid changes in nucleotide sequence and, given
sufficient time, it may degrade to the point where it is not possible to identify
the sequence as a former gene. At this point, it may be called “junk” DNA.
One of the interesting discoveries in genetics was the revelation in 1977 that
most protein-coding genes in eukaryotes are discontinuous. Discontinuous genes
contain coding and noncoding segments called exons and introns , respectively
( Figure 1.9 ). Considerable discussion of the origin, evolution, and importance of
introns has occurred previously ( Herbert 1996, Gilbert et al. 1997, Trotman 1998 ).
Introns have been maligned as examples of junk DNA because they may be con-
siderably longer than the coding sequences (exons), and they were thought to
have no function, although we now know that some introns contain regulatory
sequences. Two hypotheses have been proposed to explain the origin of introns:
the introns-early hypothesis and the introns-late hypothesis.
According to the introns-early hypothesis, many introns were present in
the common ancestor of all life, but large or complete losses of the introns
occurred in independent lineages. In addition, introns functioned in the primor-
dial assembly of protein genes by promoting the recombination, or shuffling,
of short exons, each encoding 15-20 amino acids (minigenes) into different
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