Biology Reference
In-Depth Information
example, in
Tribolium madens
, two satellite DNAs were characterized: one sat-
ellite DNA is a 225-bp monomer comprising 30% of the genome and the other
satellite DNA is a monomer of 711 bp, constituting 4% of the genome (
Durajlija
Zinic et al. 2000
).
The role of highly repetitive sequences in genome evolution is not well under-
stood (
Pardue and Hennig 1990, Ohno and Yomo 1991
), but such sequences are
associated with heterochromatin in the centromeres and telomeres, and they
are important in chromosome pairing. The sequence (TTAGG)
n
is found at the
extreme terminal region of all
Bombyx mori
chromosomes, as well as being asso-
ciated with the ends of chromosomes in the Isoptera, Orthoptera, Hymenoptera,
Trichoptera, Mecoptera, some Coleoptera, Hemiptera, and Lepidoptera (
Okazaki
et al. 1993
). The sequence (TTAGG)
n
seems to be found only in arthropod telo-
meres, although not all arthropods have this telomeric sequence.
Drosophila
melanogaster
and some coleopterans have different telomeric structures. The
telomeres of
Drosophila
have TEs called HeT-A and TART in the subtelomeric
region (
Biessmann et al. 1993, Mason et al. 2000
).
4.11 Producing Large Amounts of Protein in a Short Time: Gene
Ampliication and Gene Duplication
When it is desirable to produce large amounts of gene product (protein) in a
short time, several mechanisms could be used, including duplication of chroma-
tids resulting in polyteny, polyploidy (the duplication of whole chromosomes),
hypertranscription of the gene, gene amplification, and gene duplication.
Hypertranscription
involves producing large amounts of gene product from a
single copy of genes on a chromosome, and it is the mechanism by which
D.
melanogaster
males, which have only one X chromosome, produce as much
gene product as females with two X chromosomes (for details, see Chapter 10).
Definitions of gene amplification and gene duplication can be confusing (
Edgar
and Orr-Weaver 2001
).
Gene amplification
usually is defined as the replication of a gene (at a single
locus) so that multiple copies can be transcribed at once. One way to visualize
gene amplification is to imagine that gene amplification occurs by an “onion-
skin model” in which a
segment
of the chromosome is replicated and multiple
copies of that segment are transcribed, but the rest of the chromosome retains
its normal structure. Gene amplification originally was coined to describe the
production in mammalian cell cultures of multiple copies of cancer-drug resis-
tance genes; gene amplification is associated with an initial low drug concen-
tration of the drug, with the surviving cells subjected to multiple rounds of
