2.3.3 Transposition of Middle Repetitive Sequences
19
sequences [73]. In cattle, there are eight main
satellites that together make up 23 % of the
DNA. These are apparently all composed of similar 23-bp repeats that probably arose from the
same 11- to 12-bp ancestral sequence [430]. The
domestic rat, Rattus rattus, possesses two satellite
DNAs: type I is a 370-bp dimer, whose 185-bp a
and b subunits show 60 % sequence agreement
and apparently arose by duplication of a common
92- to 93-bp ancestral sequence. The other satellite, type I, is a tandem of 185-pb repeats (a') that
show 85 % agreement with a. Both types are
polymorphous. The brown rat, R. norvegicus has
only type I [125].
2.3.2 Middle Repetitive DNA
and Genome Organization
Middle repetitive DNA sequences are widely dispersed in the genome and show two patterns of
organization. Xenopus shows the typical shortperiod interspersion pattern. The largest part of
the euchromatin consists of single-copy sequences of about 1-2 kb alternating with short repetitive sequences of 0.1-0.3 kb. The Xenopus pattern is apparently widespread and, in addition to
its occurrence in fish, amphibians, reptiles and
mammals, is found in acrania, sea urchins and
various insects [385]. The long-period interspersion pattern, of which the Drosophila pattern is
typical, consists of repeated sequences of average
length 5 kb interspersed with single-copy sequences of up to 35 kb. This pattern is possibly an
adaptation to a relatively small genome [153].
The Drosophila pattern is also present in the dipterans Chironomus, Anopheles and Sarcophage
as well as in the honey bee, Apis mellifera; in contrast, one finds the short-period interspersion
pattern in the dipterans Aedes, Musca and Stomoxys, in the butterfly Antherea and in the Thysanura Thermobia. An intermediate pattern occurs
in the mosquito, Culex sp. [81]. Unlike other vertebrates, all birds that have been examined show
the Drosophila pattern [456]. There are regions of
the genome in all animals that deviate from the
basic type. Thus, short dispersed repeats are to be
found in Drosophila, although less often than in
mammals [153]. In humans, DNA sections of
more than 50 kb, with the basic type of short
repetitive sequences, alternate with sections in
which long repeats are also to be found [422].
2.3.3 Transposition of Middle Repetitive
Sequences
The existence of mobile genetic elements Gumping genes) was postulated in the 1940s by Barbara
McClintock on the basis of experiments with
maize. The molecular-genetic evidence was first
obtained from Drosophila, where the sequence
"copia" and similar sequences can be detected at
various positions in the genomes of different
natural populations or laboratory strains, and
even in different individuals of the same population or on the homologous chromosomes of individuals [220]. This shows that the transposition
and deletion of mobile elements are extraordinarily frequent events, much more frequent even
than base substitutions.
The transposition rate of copia and similar
sequences in Drosophila melanogaster is about
10- 3 per generation, whilst the insertion frequency
of inducer-(I)-factors and hobo elements is even
higher [176]. Extreme environmental factors, for
example, heat shock in Drosophila can increase
the transposition rate [221]. For various technical
reasons, the direct detection of a transposition in
mammals is more difficult. However, in the
region of the B-globin cluster of the mouse extensive reorganization due to insertion of an L1 element is quite convincing, as is the allelic polymorphism resulting from the insertion of an Alu
element in the region of the prolactin gene of the
rat [56,389].
The duplication, transposition and dispersion
of mobile elements and the resulting changes in
genome organization (rearrangements) are
important mechanisms in molecular evolution as
well as the main cause of the dynamic state of the
eukaryotic genome [428]. On the other hand, the
transposition of mobile sequences may, in many
cases, be of no consequence for the phenotype.
The fact that the majority of the mobile elements
distributed throughout the genome appear to
have no biological significance led to the concept
of selfish DNA; this concept was formulated in
1980 by Doolittle and Sapienza and, in a similar
way, by Orgel and Crick, and has since been the
subject of active controversy [116]. "Selfish
DNA" is understood to mean DNA elements
that, on the basis of specific sequence characters,
show the potential for transposition; they multiply and move around the genome and so avoid
elimination. These elements "survive", replicate
and mutate in the genome like parasites in a
host. It is quite possible that later in evolution
they may attain a biological significance, just as a
19
sequences [73]. In cattle, there are eight main
satellites that together make up 23 % of the
DNA. These are apparently all composed of similar 23-bp repeats that probably arose from the
same 11- to 12-bp ancestral sequence [430]. The
domestic rat, Rattus rattus, possesses two satellite
DNAs: type I is a 370-bp dimer, whose 185-bp a
and b subunits show 60 % sequence agreement
and apparently arose by duplication of a common
92- to 93-bp ancestral sequence. The other satellite, type I, is a tandem of 185-pb repeats (a') that
show 85 % agreement with a. Both types are
polymorphous. The brown rat, R. norvegicus has
only type I [125].
2.3.2 Middle Repetitive DNA
and Genome Organization
Middle repetitive DNA sequences are widely dispersed in the genome and show two patterns of
organization. Xenopus shows the typical shortperiod interspersion pattern. The largest part of
the euchromatin consists of single-copy sequences of about 1-2 kb alternating with short repetitive sequences of 0.1-0.3 kb. The Xenopus pattern is apparently widespread and, in addition to
its occurrence in fish, amphibians, reptiles and
mammals, is found in acrania, sea urchins and
various insects [385]. The long-period interspersion pattern, of which the Drosophila pattern is
typical, consists of repeated sequences of average
length 5 kb interspersed with single-copy sequences of up to 35 kb. This pattern is possibly an
adaptation to a relatively small genome [153].
The Drosophila pattern is also present in the dipterans Chironomus, Anopheles and Sarcophage
as well as in the honey bee, Apis mellifera; in contrast, one finds the short-period interspersion
pattern in the dipterans Aedes, Musca and Stomoxys, in the butterfly Antherea and in the Thysanura Thermobia. An intermediate pattern occurs
in the mosquito, Culex sp. [81]. Unlike other vertebrates, all birds that have been examined show
the Drosophila pattern [456]. There are regions of
the genome in all animals that deviate from the
basic type. Thus, short dispersed repeats are to be
found in Drosophila, although less often than in
mammals [153]. In humans, DNA sections of
more than 50 kb, with the basic type of short
repetitive sequences, alternate with sections in
which long repeats are also to be found [422].
2.3.3 Transposition of Middle Repetitive
Sequences
The existence of mobile genetic elements Gumping genes) was postulated in the 1940s by Barbara
McClintock on the basis of experiments with
maize. The molecular-genetic evidence was first
obtained from Drosophila, where the sequence
"copia" and similar sequences can be detected at
various positions in the genomes of different
natural populations or laboratory strains, and
even in different individuals of the same population or on the homologous chromosomes of individuals [220]. This shows that the transposition
and deletion of mobile elements are extraordinarily frequent events, much more frequent even
than base substitutions.
The transposition rate of copia and similar
sequences in Drosophila melanogaster is about
10- 3 per generation, whilst the insertion frequency
of inducer-(I)-factors and hobo elements is even
higher [176]. Extreme environmental factors, for
example, heat shock in Drosophila can increase
the transposition rate [221]. For various technical
reasons, the direct detection of a transposition in
mammals is more difficult. However, in the
region of the B-globin cluster of the mouse extensive reorganization due to insertion of an L1 element is quite convincing, as is the allelic polymorphism resulting from the insertion of an Alu
element in the region of the prolactin gene of the
rat [56,389].
The duplication, transposition and dispersion
of mobile elements and the resulting changes in
genome organization (rearrangements) are
important mechanisms in molecular evolution as
well as the main cause of the dynamic state of the
eukaryotic genome [428]. On the other hand, the
transposition of mobile sequences may, in many
cases, be of no consequence for the phenotype.
The fact that the majority of the mobile elements
distributed throughout the genome appear to
have no biological significance led to the concept
of selfish DNA; this concept was formulated in
1980 by Doolittle and Sapienza and, in a similar
way, by Orgel and Crick, and has since been the
subject of active controversy [116]. "Selfish
DNA" is understood to mean DNA elements
that, on the basis of specific sequence characters,
show the potential for transposition; they multiply and move around the genome and so avoid
elimination. These elements "survive", replicate
and mutate in the genome like parasites in a
host. It is quite possible that later in evolution
they may attain a biological significance, just as a