Biology Reference
In-Depth Information
the process of being analyzed. There are several hypotheses to explain its persis-
tence in genomes. One hypothesis suggested that the noncoding DNA performs
essential functions, such as global regulation of gene expression and develop-
ment. According to this hypothesis, the junk DNA is functional and deletions
of such DNA would have a deleterious effect. A second hypothesis was that the
noncoding DNA is useless, but that it is maintained because it is linked physically
to functional genes; the excess DNA is not eliminated because it does not affect
fitness of the organism and can be maintained indefinitely in the population.
A third hypothesis suggested that the noncoding DNA is a functionless parasite
that accumulates and is actively maintained by selection. A fourth hypothesis is
that the DNA has a structural function, perhaps for compartmentalizing genes
within the nucleus, or for maintaining a structural organization (nucleoskeleton)
within the nucleus (
Manuelidis 1990, Manuelidis and Cher 1990
). Recent studies
indicate that much of the so-called junk DNA does have a function in gene expres-
sion and development as well as playing a role in chromosome organization in
the nucleus. Other junk is probably the remnants of parasitic DNA (transposable
elements) that have been degraded but not yet eliminated from the genome.
The C-value paradox remains a topic of study because not all noncoding DNA
has been shown to have a function and, unless the noncoding DNA has a func-
tion, such DNA constitutes a “load” upon the insect and should be lost over evo-
lutionary time.
Petrov et al. (1996)
provided evidence that nonessential DNA
is lost at a higher rate in
Drosophila
species than in mammalian species, sug-
gesting that differences in genome size may result from persistent differences
between organisms.
Petrov et al. (2000)
provided additional support for this
hypothesis by comparing DNA loss in two insect genera (
Laupala
[crickets] and
Drosophila
) with different genome sizes. The crickets have a genome size an
order of magnitude larger than that of
Drosophila
and eliminate nonessential
DNA one-fortieth as quickly.
3.4 Repetitive DNA is Common in Insects
Much of the noncoding, heterochromatic junk DNA in insects is
repetitive
DNA
—speciic nucleotide sequences that are repeated several times to millions
of times. Repetitive DNA has been classified as “
highly repetitive
” (sequences
repeated several hundred to several million times per genome) or as “
moder-
ately repetitive
.” Highly repetitive DNA is found in and near centromeres, telo-
meres, and other heterochromatic regions. Middle-repetitive DNA sequences
are repeated 100-10,000 times and include genes that code for ribosomal RNA
(rRNA) and transfer RNA (tRNA). Middle-repetitive sequences are found in
euchromatic regions, as well as in heterochromatic regions of the genome.
