Encyclopedia of Evolution.pdf - Online Reading Center

noncoding DNA
Some regulatory molecules, such as small interfering RNA
molecules (RNAi), come from noncoding DNA; and some
regulatory molecules bind to noncoding DNA sequences.
Third, some noncoding DNA is important in the functions
of the chromosomes themselves: for example, centromeres,
to which the protein strands attach during cell division, and
telomeres, the caps at the ends of the chromosomes that may
be important in protecting them from age-related damage.
Noncoding DNA has been very important in evolution.
First, many instances of gene duplication (making an extra
copy of a gene) have resulted from transposable elements taking
genes with them when they transpose. Second, the vertebrate
immune system produces up to a million different kinds
of antibodies even though the nucleus does not have a million
antibody genes. Antibodies result from the cutting and
splicing of a smaller number of genes. This process resembles
what happens during transposition and, suggests geneticist
Alka Agrawal, may have evolved from it.
Transposable elements not only move from one place to
another within the nucleus of a eukaryotic cell but can sometimes
move from one cell to another, in a different individual
of the same species, or even in a different species (see horizontal
gene transfer). For example, old world monkeys
(see primates) have a virus gene that they share only with six
species of cats and which must have resulted from horizontal
gene transfer.
Genes whose promoters cease to function become pseudogenes,
which are also an important component of noncoding
DNA. Pseudogenes are no longer used but persist as
evidence of evolutionary ancestry. Mammals have many genes
that allow the detection and discrimination of scents; mice
have and use 1036 of these genes. Primates, in contrast, rely
more heavily on sight than on scent. Humans, for example,
use only 347 scent genes. Human chromosomes still retain
many of the other scent genes, but they are pseudogenes.
The genome (the stored genetic information) of a eukaryotic
cell has been compared to a library, or a computer database.
But when considering the vast amount of noncoding DNA
in the nucleus, some geneticists consider the genome to more
closely resemble an attic, with useful items buried amid junk.
Further Reading
Agrawal, Alka, Q. M. Eastman, and D. G. Schatz. “Transposition
mediated by RAG1 and RAG2 and its implications for the evolution
of the immune system.” Nature 394 (1998): 744–751.
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York:
Garland Science, 2002.
Lynch, Michael, Britt Koskella, and Sarah Schaack. “Mutation pressure
and the evolution of organelle genomic architecture.” Science
311 (2006): 1,727–1,730.
Ochman, Howard, and Liliana M. Davalos. “The nature and dynamics
of bacterial genomes.” Science 311 (2006): 1,730–1,733.