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In between the
well-structured genes are
large sections of DNA for
which no function has yet
been identified. These areas
have been called ‘junk DNA’
or 'non-coding DNA' and make
up a large proportion of the
genomes of both plants and
animals.
But is it junk at all?
We don’t really know.
This DNA appears to act as a
filler in between genes and
a number of ideas are
starting to emerge about
what role it plays. This is
a mystery to be solved in
the next couple of decades.
Some of the ideas are:
- it is where defective
genes, or pseudogenes, are
dumped
- it is the accumulated
DNA of viruses that have
infected the body and
failed to take over the
cell
- it acts as a
protective buffer against
genetic damage and harmful
mutations, because the
area is irrelevant to the
metabolic and
developmental processes
(if a random change occurs
in the sequence, there is
no effect on the body)
- it acts as a reservoir
of sequences from which
potentially advantageous
new genes can emerge
Researchers believe that
this unknown DNA probably
plays some role in
regulating the 'coding DNA'
and therefore cellular
processes. But there is
currently very little
knowledge about the
relationship between
non-coding DNA and the DNA
of genes.
Onions contain 12
times more DNA per cell than
humans. A pufferfish’s
genome is only about one
tenth the size of the human,
yet seems to have about the
same number of genes. The
ratio of functional DNA to
‘in-between filler’ DNA of
unknown function differs
widely per species.
Chickens have a
similar number of genes to
humans: 20,000 to 23,000 for
chickens and 25,000 to
30,000 for humans. But their
genome is much smaller -
they have 1 billion DNA
bases, compared to about 3
billion in humans. The
chicken genome appears to
contain less repetitive
non-coding DNA than the
human genome. |
