54
2 Nucleic Acids and Nuclear Proteins
begins in the D-loop region with the formation of
a new H-strand, during which the old H-strand is
displaced; the synthesis of the L-strand begins
only when the tRNA cluster on the old H-strand,
between ORF-COl and ND2 (URF2), becomes
exposed. The transcription of the two strands
progresses from the D-loop region and leads to
the formation of two large transcripts, the processing of which has not yet been fully described.
The mtDNA molecule shows the greatest
economy of space in that there are no introns and
practically no non-coding sequences between the
genes. The total number of nucleotides lying
between genes is 87 in man, 64 in the mouse, and
57 in the cow. The tRNA genes lying between the
individual rRNA and structural genes serve as signals to separate the transcripts. The sequence
-CAA is attached post-transcriptionally to the
3' end of the pre-tRNAs [80]. A peculiarity of
human mtDNA is a 200-nt-long sequence lying in
front of the D-loop on the L-strand; this codes for
a 7S RNA of unknown function, but sequences
homologous to this have not been found in other
mtDNAs [80]. Complete sequences are also
known for the mtDNAs of the cow, mouse,
chicken and clawed frog, Xenopus laevis. These
data, together with partial sequences and restriction patterns from the mtDNAs of amphibians
and fish, show that the organization of the genes
in the mtDNA of all vertebrates corresponds to
that of man. The single known exception is in the
chicken, where the genes for ND6 and cytochrome b have exchanged positions. Vertebrate
mtDNAs are also very similar in sequence: the
mtDNA of the cod fish, Gadus morrhua, and that
of the clawed frog, Xenopus laevis, agree in their
coding sequences by between 46 (ND4L) and
93 % (CO I) [106, 145,214,365]. In many vertebrates one finds variant mtDNAs up to 500 bp longer, and sometimes these are in combination with
the normal form. Cells containing more than one
type of mtDNA are referred to as heteroplastic.
In several species of the lizard genus Cnemidophorus, the mtDNA of certain populations or
single individuals contains duplications with a
length of 0.8-8 kb, resulting in increases in the
total length of up to 25 kb. In the teleost Alosa
sapidissima, the mtDNA can be increased in size
by two to three repeats of 1.5 kb in the D-Ioop
region. Similar length polymorphisms are known
from other fish, amphibians and reptiles [16, 29,
306].
2.9.2 The mtDNA of Invertebrates
Complete sequences are known for the dipterans
Drosophila yakuba and Anopheles quadrimaculatus, the nematodes Ascaris suum and Caenorhabditis elegans, and the sea urchins Paracentrous
lividus and Strongylocentrous purpuratus [61, 78,
82, 209]. Partial sequences, or at least the order
of the genes, are available for the migratory
locust Locusta migratoria, three species of broadnosed weevil (Curculionidae), the brine shrimp
Artemia salina, the liver-fluke Fasciola hepatica,
and the starfish Pisaster ochraceus [24, 42,
148, 405,482].
The mtDNA of Drosophila yakuba, the first
determined invertebrate mtDNA sequence, has a
length of 16019 bp. It contains the same genes as
vertebrate mtDNA, although in a rather different
order (Fig. 2.18). A peculiarity of the insects is the
(A+T)-rich region that has a length of 1 kb in D.
yakuba, D. teissieri, D. erecta and D. orena but
about 5 kb in D. melanogaster, D. simulans and
D. mauritiana; the total length of mtDNA in
Drosophila varies correspondingly between 16.0
and 19.5 kb [410]. The basic structure of Drosophila mtDNA is valid for all other insects, the
encoded protein sequences of Anopheles and
Drosophila agreeing by 72-98 % [24,42, 82]. In
the crustacean Artemia, only the tRNA genes are
organized differently from those in the insects
[24]. The mtDNA of the pig roundworm Ascaris
suum has a length of 14284 bp and contains the
genes for the two rRNAs and all the proteins that
are encoded in the mtDNAs of the vertebrates
and the arthropods, except for the ATPase subunit 8. Distributed between the rRNA and the
protein genes are genes for an aberrant type of
tRNA. These lack the T'\jJC loop and the variable
loop that are found both in the mitochondrial and
cytoplasmic tRNAs of vertebrates and insects
(Fig. 2.17); instead there is a single loop of 412 nt. A similar situation is seen in the mtDNA of
Caenorhabditis elegans and is thus typical for the
nematodes [482]. The mtDNAs of the two seaurchin species both have a gene order that is distinct from all other mtDNAs: between the genes
for the SS-rRNA and the LS-rRNA lies a cluster
of 15 of the 22 tRNAs and the genes for NDl and
ND2. The mtDNA of the starfish Pisaster ochraceus, in contrast, contains an inversion of the segment: -tRNA cluster-ND1-ND2-16S rRNA[61,209]. During the evolution of sea-urchin
mtDNA, the coding sequence for the tRNA Leu!
CUN fused with the gene for the NADH dehydrogenase subunit 5 and this now encodes a 24-
2 Nucleic Acids and Nuclear Proteins
begins in the D-loop region with the formation of
a new H-strand, during which the old H-strand is
displaced; the synthesis of the L-strand begins
only when the tRNA cluster on the old H-strand,
between ORF-COl and ND2 (URF2), becomes
exposed. The transcription of the two strands
progresses from the D-loop region and leads to
the formation of two large transcripts, the processing of which has not yet been fully described.
The mtDNA molecule shows the greatest
economy of space in that there are no introns and
practically no non-coding sequences between the
genes. The total number of nucleotides lying
between genes is 87 in man, 64 in the mouse, and
57 in the cow. The tRNA genes lying between the
individual rRNA and structural genes serve as signals to separate the transcripts. The sequence
-CAA is attached post-transcriptionally to the
3' end of the pre-tRNAs [80]. A peculiarity of
human mtDNA is a 200-nt-long sequence lying in
front of the D-loop on the L-strand; this codes for
a 7S RNA of unknown function, but sequences
homologous to this have not been found in other
mtDNAs [80]. Complete sequences are also
known for the mtDNAs of the cow, mouse,
chicken and clawed frog, Xenopus laevis. These
data, together with partial sequences and restriction patterns from the mtDNAs of amphibians
and fish, show that the organization of the genes
in the mtDNA of all vertebrates corresponds to
that of man. The single known exception is in the
chicken, where the genes for ND6 and cytochrome b have exchanged positions. Vertebrate
mtDNAs are also very similar in sequence: the
mtDNA of the cod fish, Gadus morrhua, and that
of the clawed frog, Xenopus laevis, agree in their
coding sequences by between 46 (ND4L) and
93 % (CO I) [106, 145,214,365]. In many vertebrates one finds variant mtDNAs up to 500 bp longer, and sometimes these are in combination with
the normal form. Cells containing more than one
type of mtDNA are referred to as heteroplastic.
In several species of the lizard genus Cnemidophorus, the mtDNA of certain populations or
single individuals contains duplications with a
length of 0.8-8 kb, resulting in increases in the
total length of up to 25 kb. In the teleost Alosa
sapidissima, the mtDNA can be increased in size
by two to three repeats of 1.5 kb in the D-Ioop
region. Similar length polymorphisms are known
from other fish, amphibians and reptiles [16, 29,
306].
2.9.2 The mtDNA of Invertebrates
Complete sequences are known for the dipterans
Drosophila yakuba and Anopheles quadrimaculatus, the nematodes Ascaris suum and Caenorhabditis elegans, and the sea urchins Paracentrous
lividus and Strongylocentrous purpuratus [61, 78,
82, 209]. Partial sequences, or at least the order
of the genes, are available for the migratory
locust Locusta migratoria, three species of broadnosed weevil (Curculionidae), the brine shrimp
Artemia salina, the liver-fluke Fasciola hepatica,
and the starfish Pisaster ochraceus [24, 42,
148, 405,482].
The mtDNA of Drosophila yakuba, the first
determined invertebrate mtDNA sequence, has a
length of 16019 bp. It contains the same genes as
vertebrate mtDNA, although in a rather different
order (Fig. 2.18). A peculiarity of the insects is the
(A+T)-rich region that has a length of 1 kb in D.
yakuba, D. teissieri, D. erecta and D. orena but
about 5 kb in D. melanogaster, D. simulans and
D. mauritiana; the total length of mtDNA in
Drosophila varies correspondingly between 16.0
and 19.5 kb [410]. The basic structure of Drosophila mtDNA is valid for all other insects, the
encoded protein sequences of Anopheles and
Drosophila agreeing by 72-98 % [24,42, 82]. In
the crustacean Artemia, only the tRNA genes are
organized differently from those in the insects
[24]. The mtDNA of the pig roundworm Ascaris
suum has a length of 14284 bp and contains the
genes for the two rRNAs and all the proteins that
are encoded in the mtDNAs of the vertebrates
and the arthropods, except for the ATPase subunit 8. Distributed between the rRNA and the
protein genes are genes for an aberrant type of
tRNA. These lack the T'\jJC loop and the variable
loop that are found both in the mitochondrial and
cytoplasmic tRNAs of vertebrates and insects
(Fig. 2.17); instead there is a single loop of 412 nt. A similar situation is seen in the mtDNA of
Caenorhabditis elegans and is thus typical for the
nematodes [482]. The mtDNAs of the two seaurchin species both have a gene order that is distinct from all other mtDNAs: between the genes
for the SS-rRNA and the LS-rRNA lies a cluster
of 15 of the 22 tRNAs and the genes for NDl and
ND2. The mtDNA of the starfish Pisaster ochraceus, in contrast, contains an inversion of the segment: -tRNA cluster-ND1-ND2-16S rRNA[61,209]. During the evolution of sea-urchin
mtDNA, the coding sequence for the tRNA Leu!
CUN fused with the gene for the NADH dehydrogenase subunit 5 and this now encodes a 24-