DNA, RNA…and XNA?
Scientists create artificial genetic molecules that can carry information,
evolve
Inside your body’s cells, molecules called DNA act
as instruction booklets. DNA tells the cells within
every living thing which molecules to make.
Everything about you — your hair color, height, voice
— is determined at least partly by your DNA.
DNA and its partner molecule, RNA, are the only
molecules in nature known to carry genetic
information. Now, in a new study, scientists have
created artificial molecules that also store genetic
instructions. These new molecules — called “XNA”
— are similar to DNA and RNA but slightly tweaked.
DNA is a pair of long, intertwined chains made of
repeating chemical building blocks. Each building
block contains three parts: a sugar called
deoxyribose, a phosphate molecule and a molecule
called a base. There are four bases in DNA: adenine,
Scientists have made artificial genetic molecules
called XNA that are similar to DNA. DNA, illustrated
here, is a pair of long, intertwined chains made of
repeating chemical building blocks. Credit: Frank
Ramspott/iStockphoto
thymine, guanine and cytosine, or A, T, G, C for
short. The bases are like the letters in the instruction manual.
Cells “read” the DNA and make a molecule called RNA. RNA is made of similar repeating units, but it has
a different type of sugar, called ribose, in each building block. The cell then reads the RNA to create
molecules called proteins, which perform important functions in the body.
XNA is similar to DNA and RNA. But the researchers made an important change: They didn’t use the
sugars in DNA and RNA. Instead, they substituted different molecules. The team created six types of
XNA, each with a different molecule in the spot where a deoxyribose or ribose would be in DNA and RNA.
“What makes DNA and RNA so cool is they are the genetic molecules, they are the basis for propagating
information through generations,” Gerald Joyce told Science News. Joyce is a biochemist, someone who
studies the chemistry of living things, at the Scripps Research Institute in La Jolla, Calif. “Well, now we
have eight genetic molecules: RNA, DNA and these six.”
For genetic information to be useful, the cell needs a way to copy it. When a cell divides, it passes on one
copy of its genetic information to each daughter cell.
To create a “copy machine,” the scientists made molecules called enzymes that could read the XNA. The
enzymes built molecules of DNA that contained the same information as the XNA. The process is similar
to translating a story from English to French. The words may look different, but the underlying information
is the same.
The enzymes could also “translate” the DNA back into XNA. So the scientists had a way to make copies
of XNA molecules.
Finally, the team wanted to find out if the XNA could evolve. Evolution is the process of changing genetic
information over generations. DNA evolves when individual letters are tweaked, slightly altering the
instruction manual.
The scientists gave XNA a simple evolution test. They mixed XNA with other molecules and picked the
XNAs that were the best at attaching to those molecules. Then the researchers copied those XNAs and
repeated the process of mixing them with other molecules several times. The XNAs got better and better
at attaching to the molecules. They were evolving.
The new study hints that forms of life based on molecules other than DNA and RNA might be possible.
“We only know this one example of life — it’s what’s been on Earth for 4 billion years,” Joyce told Science
News. But perhaps there are organisms on other planets that have different types of genetic molecules
than we do.
Power Words
DNA The genetic instructions inside cells that tell them which molecules to make.
RNA An intermediate molecule in the process of “reading” DNA to create proteins in the cell. A cell’s
molecular machinery reads DNA to create RNA, then reads RNA to create proteins.
evolve To change gradually over generations.