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a.
b.
c.
Figure 1.4.
Three recombinational processes:
a)
homologous recombination;
b)
site-specific recombination; and
c)
non-homologous recombination. Note that
the transforming power of site-specific recombination and non-homologous
recombination is much more profound than homologous recombination.
this sense, note that GEP recombination has no counterpart in biology despite
its apparent resemblance to homologous recombination.
Transposition
Transposable genetic elements consist of genes that can move from place to
place within the genome. There are three classes of transposable elements
with different structures and mechanisms of transposition. They exist both
in eukaryotes and prokaryotes and cause different effects on the chromo-
some they move to. For instance, they can inactivate genes by interrupting
the coding sequence of a gene; they can activate an adjacent gene by provid-
ing a promoter or transcriptional activator; or they can restructure a chromo-
some by producing homologous sequences that might afterwards be used in
homologous recombination. The existence of these “jumping genes” which
move from one chromosome to another without respect for boundaries, in-
cluding species boundaries, greatly altered our views of evolution, as some-
thing smoothly driven by neutral mutations alone. The effects of transposi-
tion are too drastic to be apparently of any use. And so are the effects of non-
homologous recombination or the effects of nonsense or frameshift muta-
tions. Nonetheless, transposition (and non-homologous recombination) is not
only fairly frequent but also widely distributed throughout the living world,
and many are the marks it left behind in living organisms. Notwithstanding,
most of the times, the effect of transposition and some kinds of non-homolo-
gous recombination is deleterious. Very occasionally, though, some
