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Genome may be full of junk after all
By Tina Hesman Saey
Most of the human genome may actually
be junk.
In recent years scientists have
stopped dismissing as nonfunctional
the part of the genome that doesn’t produce proteins. But a new study comparing the human genetic blueprint with
those of other mammals concludes
that very little of the human genome is
really necessary.
About 7 percent of the human
genome is similar to the DNA of other
mammals, said Arend Sidow of Stan-
ford University. Because it is similar,
or “conserved,” geneticists assume this
DNA is the most integral. In all, Sidow
concludes, these important parts of the
genome comprise only 225 million of
the 3 billion chemical letters of DNA
found in the complete human genetic
instruction book.
Central dogma
thrown off-kilter
In thousands of genes, RNA
is not a faithful copy of DNA
By Tina Hesman Saey
Text messagers and computer gamers
aren’t alone in the willful misspelling
department. RNA molecules do it too.
RNA molecules aren’t always faithful
reproductions of the genetic instructions contained within DNA, a new study
shows. The finding seems to violate a
tenet of genetics so fundamental that
scientists call it the central dogma: DNA
letters encode information and RNA is
made in DNA’s likeness. The RNA then
serves as a template to build proteins.
But a study of RNA in white blood cells
from 27 different people shows that, on
average, each person has nearly 4,000
genes in which the RNA copies contain
misspellings not found in DNA.
“It’s unbelievable,” said Mingyao Li,
a geneticist at the University of Pennsylvania Medical School in Philadelphia
who presented the finding November 3.
Scientists already knew that every
now and then RNA letters
can be chemically modified
or edited — sort of the molecular equivalent of adding an
umlaut to some letters. But
such RNA editing events are
not that common.
What Li and her colleagues
discovered is quite common.
RNA molecules contained
misspellings at more than
20,000 different places in the genome,
with about 10,000 different misspellings
occurring in two or more of the people
studied. The most common of the 12 different types of misspellings changed the
chemical letter “A” in the DNA to “G”
in the RNA. That change accounted for
nearly a third of the misspellings.
Some researchers who saw Li’s pre-
sentation asked whether a virus used in
growing the white blood cells that the
researchers studied might be the source
of the shenanigans. Li and her collabo-
rators had wondered the same thing. In
order to rule out the virus, the research-
ers analyzed skin cells from
the same people and found
that RNA misspellings origi-
nally discovered in the white
blood cells were also in the
skin cells. And the misspell-
ings aren’t just rare, random
mistakes. “When DNA and
RNA differ from each other
it happens in nearly every
RNA” copy, Li said.
The researchers don’t yet know how
the RNA misspellings happen. They
could be substitutions made while the
RNA copy is being made, or the changes
could happen later. Any consequences
of the misspellings are also unknown,
though some speculate that the mis-
spellings could cause the RNA to be
degraded faster or interfere with the
molecule’s ability to make proteins. s
RNA
molecules
contained
misspellings
at 20,000
different
places in the
genome.
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December 4, 2010 | science news | 17
