2.4.2 Increase and Decrease in the DNA Content of Individual Cells
27
because of this ability they also can not be
counted as "junk DNA" (see p. 20).
2.4.2 Increase and Decrease in the DNA
Content of Individnal Cells
Drastic changes in the DNA content take place
during the development of somatic cells. Mitosislike processes lead to the appearance of giant
chromosomes (polyteny) or nuclei with multiple
chromosomes (polyploidy). Enlargement of the
DNA content can also come about by the multiplication of single genes (gene amplification).
DNA multiplication in somatic eumetazoan cells
is comparable with the formation ofthe (somatic)
macronucleus in the Ciliophora. There are also
several examples of reduction in the DNA content of eumetazoan cells (chromatin diminution).
Somatic polyploidy is widespread in animals
[50], and high C values and somatic polyploidy
can be understood as alternative evolutionary
strategies. The ploidy level varies with the tissue;
thus, in the waterbug Gerris lacustris, one finds
diploid and tetraploid nuclei in muscle, 16-ploidy
in the testis septa, and 32- or 64-ploidy in the
Malpighian tubules. Also in mammals, for example, the liver, kidney and pancreas cells are often
tetra- or octaploid, and the human megakaryocytes are up to 32-ploid. In particular cases,
extremely high ploidy levels are found:
approximately 500000 (corresponding to 17-18
duplications) in the silk glands of Bombyx mori,
200000 and 4096 in the giant neurons of the snails
Aplysia and Lymnaea, and 4000 in rat trophoblasts [196, 316]. In the salivary gland cells of the
Diptera, the mitotically doubled chromatids
remain together and so form the giant polytene
chromosomes [50]. At anyone time, in Drosophila melanogaster these contain 1024 chromatids
and 16000 in Chironomus chromatids (corresponding to 9 and 13 replications, respectively);
they are arranged so regularly that the individual
chromatid loops lead to a cross-banding pattern.
In Drosophila, about 5000 bands are recognizable
in the haploid genome and each is equivalent to
between 3 and 300 kb of DNA [268]. Somatic
endomitosis in the Diptera produces different
effects in different cell types: polyteny is the
result in salivary glands, intestine, fat bodies,
Malpighian tubules and circular glands; and polyploidy in the ovary, pericardium cells and nerve
cells [316]. These processes do not lead to equal
multiplication of all parts of the DNA; underreplication of specific satellite DNAs is found, for
example, in polytene and polyploid cells of Drosophila [268].
There are basically two possibilities for ensuring large amounts of specific gene products at particular time-points in development; these are
accumulation of the products and gene amplification. Stable mRNAs accumulated over a long period are to be found, for example, for globins in
the reticulocytes or for ovalbumin in the oviduct
cells. The large egg cells of the clawed frog, Xenopus laevis, require about 10 12 ribosomes and corresponding amounts of tRNA for the protein synthesis that is initiated by fertilization. The immature oocytes contain reserves of 5S rRNA and
tRNA in 7S and 42S nucleoprotein particles. In
the smaller 7S particles, about half of the total 5S
rRNA is assiociated with one protein species, and
in the large 42S particles, the other half of the 5S
rRNA and almost the total tRNA is associated
with two further proteins. The protein of the 7S
particle is identical to the transcription factor
TFIIIA that is necessary for the transcription of
the 5S rRNA. Of the two proteins in the 42S particle, the smaller one (43 kDa) binds to 5S
rRNA; the larger one (48 kDa), on the other
hand, is structurally and functionally very similar
to the translation factor EF-1a [457]. The suddenly increased requirement for other rRNAs
(28S, 18S and 5.8S rRNA) at the onset of development is satisfied by the existence of about 1 million extra copies of rRNA genes produced by
gene amplification. This is in addition to the usual
diploid complement of 1000, which exist in the
form of extrachromosomal circular DNA molecules; each of these molecules carries 15-100 rRNA
genes. Amplification of the rRNA genes is also
found in other amphibians, fish, many insects,
and to a lesser extent in other animals [420].
A different amplification mechanism occurs in
the ovary follicle cells of Drosophila; within
about 5 h these produce a secondary egg membrane (chorion) from a series of specific proteins.
The chorion protein genes lie in two clusters on
different chromosomes. The cluster on the X
chromosome is amplified about 20 times and is
expressed earlier than the 60- to 80-fold amplified
cluster on chromosome 3. The region of DNA
around the chorion protein locus is amplified by
multiple replication events, starting at the same
point, such that many parallel DNA helices are
produced (Fig. 2.6). By the use of drugs, mutants
with amplification of specific genes can be isolated from cell cultures of mammals or Drosophila. Up to now, about 20 such cases are known,
e.g. the amplification of dihydrofolate-reducing
27
because of this ability they also can not be
counted as "junk DNA" (see p. 20).
2.4.2 Increase and Decrease in the DNA
Content of Individnal Cells
Drastic changes in the DNA content take place
during the development of somatic cells. Mitosislike processes lead to the appearance of giant
chromosomes (polyteny) or nuclei with multiple
chromosomes (polyploidy). Enlargement of the
DNA content can also come about by the multiplication of single genes (gene amplification).
DNA multiplication in somatic eumetazoan cells
is comparable with the formation ofthe (somatic)
macronucleus in the Ciliophora. There are also
several examples of reduction in the DNA content of eumetazoan cells (chromatin diminution).
Somatic polyploidy is widespread in animals
[50], and high C values and somatic polyploidy
can be understood as alternative evolutionary
strategies. The ploidy level varies with the tissue;
thus, in the waterbug Gerris lacustris, one finds
diploid and tetraploid nuclei in muscle, 16-ploidy
in the testis septa, and 32- or 64-ploidy in the
Malpighian tubules. Also in mammals, for example, the liver, kidney and pancreas cells are often
tetra- or octaploid, and the human megakaryocytes are up to 32-ploid. In particular cases,
extremely high ploidy levels are found:
approximately 500000 (corresponding to 17-18
duplications) in the silk glands of Bombyx mori,
200000 and 4096 in the giant neurons of the snails
Aplysia and Lymnaea, and 4000 in rat trophoblasts [196, 316]. In the salivary gland cells of the
Diptera, the mitotically doubled chromatids
remain together and so form the giant polytene
chromosomes [50]. At anyone time, in Drosophila melanogaster these contain 1024 chromatids
and 16000 in Chironomus chromatids (corresponding to 9 and 13 replications, respectively);
they are arranged so regularly that the individual
chromatid loops lead to a cross-banding pattern.
In Drosophila, about 5000 bands are recognizable
in the haploid genome and each is equivalent to
between 3 and 300 kb of DNA [268]. Somatic
endomitosis in the Diptera produces different
effects in different cell types: polyteny is the
result in salivary glands, intestine, fat bodies,
Malpighian tubules and circular glands; and polyploidy in the ovary, pericardium cells and nerve
cells [316]. These processes do not lead to equal
multiplication of all parts of the DNA; underreplication of specific satellite DNAs is found, for
example, in polytene and polyploid cells of Drosophila [268].
There are basically two possibilities for ensuring large amounts of specific gene products at particular time-points in development; these are
accumulation of the products and gene amplification. Stable mRNAs accumulated over a long period are to be found, for example, for globins in
the reticulocytes or for ovalbumin in the oviduct
cells. The large egg cells of the clawed frog, Xenopus laevis, require about 10 12 ribosomes and corresponding amounts of tRNA for the protein synthesis that is initiated by fertilization. The immature oocytes contain reserves of 5S rRNA and
tRNA in 7S and 42S nucleoprotein particles. In
the smaller 7S particles, about half of the total 5S
rRNA is assiociated with one protein species, and
in the large 42S particles, the other half of the 5S
rRNA and almost the total tRNA is associated
with two further proteins. The protein of the 7S
particle is identical to the transcription factor
TFIIIA that is necessary for the transcription of
the 5S rRNA. Of the two proteins in the 42S particle, the smaller one (43 kDa) binds to 5S
rRNA; the larger one (48 kDa), on the other
hand, is structurally and functionally very similar
to the translation factor EF-1a [457]. The suddenly increased requirement for other rRNAs
(28S, 18S and 5.8S rRNA) at the onset of development is satisfied by the existence of about 1 million extra copies of rRNA genes produced by
gene amplification. This is in addition to the usual
diploid complement of 1000, which exist in the
form of extrachromosomal circular DNA molecules; each of these molecules carries 15-100 rRNA
genes. Amplification of the rRNA genes is also
found in other amphibians, fish, many insects,
and to a lesser extent in other animals [420].
A different amplification mechanism occurs in
the ovary follicle cells of Drosophila; within
about 5 h these produce a secondary egg membrane (chorion) from a series of specific proteins.
The chorion protein genes lie in two clusters on
different chromosomes. The cluster on the X
chromosome is amplified about 20 times and is
expressed earlier than the 60- to 80-fold amplified
cluster on chromosome 3. The region of DNA
around the chorion protein locus is amplified by
multiple replication events, starting at the same
point, such that many parallel DNA helices are
produced (Fig. 2.6). By the use of drugs, mutants
with amplification of specific genes can be isolated from cell cultures of mammals or Drosophila. Up to now, about 20 such cases are known,
e.g. the amplification of dihydrofolate-reducing