DNA, RNA…and XNA?

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, 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.