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Friday, 17 November, 2000, 00:27 GMT
Life molecule's chemical origins

By BBC News Online's Helen Briggs

Scientists have constructed a nucleic acid with a similar but "simpler" structure to that of RNA, the genetic material which, together with DNA, allows cells and viruses to replicate themselves.

This problem is so difficult that only very serious and stepwise research can bring science nearer to an acceptable answer

Albert Eschenmoser
The researchers believe the new chemical structure could yield important clues to the early origins of life on Earth.

The polymer is known as TNA, after the chains of tetrose, or four-carbon sugar units, that comprise its backbone.

To the scientists' surprise, TNAs are able to form stable double helices when paired with RNA or with DNA (which is akin to two strands of RNA joined together). The suggestion being made is that TNAs could have been the precursors of RNA.

Primitive Earth

Some scientists believe that RNA and DNA are too complex to have arisen from the soup of chemicals that prevailed on the primitive Earth. If this is the case, simpler nucleic acids, such as TNAs, could have been used as an alternative genetic material by the Earth's earliest life-forms.

One approach to solving this mystery is to make chemical cousins of DNA and RNA in the lab and test their properties.

A group of researchers in Switzerland and the US synthesised nucleic acid analogs that are similar to RNA but have a backbone constructed of a different sugar.

Albert Eschenmoser, who led the team, said: "The molecule resemble RNA in its molecular structure and also in its shape.

"But it's not identical to it. The molecular structure is simpler than that of RNA.

"If one were to ask how such molecules could have formed under natural (chemical) conditions, one would consider the possibility that it is easier to see such a system (molecule) forming than RNA itself."

Genetic blueprint

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are molecules that allow genetic information to be stored, retrieved and copied.

The question of how you get from the pre-biotic world to RNA molecules still remains something of a mystery

Geoff Baldwin
DNA is composed of two strands of long chains of molecules called nucleotides, which pair up to form a double helical structure. RNA molecules direct the cell to make proteins using the genetic information stored in DNA.

The polymers synthesised in the lab - TNAs (Threo-furanosyl-oligonucleotides) - belong to a new class of nucleic acids. They all contain a four-carbon sugar in their sugar-phosphate backbone in place of the five-carbon ribose sugar found in RNA and DNA.

The four-carbon sugar ring is significant because it could have been assembled in the chemical soup of the primitive Earth by joining two identical two-carbon units.

Writing in the journal Science, the researchers propose that TNA may have been the template upon which RNA formed.

Stepwise research

Commenting on the research, Geoff Baldwin, of the department of Biochemistry at Imperial College, London, said it could solve one of the great mysteries of life.

He told BBC News Online: "One of the precursors to our current RNA-and-DNA world was most likely an RNA-only world and RNA molecules were probably capable of being self-replicating.

"The question of how you get from the pre-biotic world to RNA molecules still remains something of a mystery.

"What this research does is perhaps gives us an idea as to how alternative self-complementary types of nucleic acids may have provided a template upon which the more complicated nucleic acids that we see today have formed."

The US and Swiss researchers plan to carry out further experiments to test their theory. But they urge caution about reaching any premature conclusions.

Albert Eschenmoser told BBC News Online: "This problem is so difficult that only very serious and stepwise research can bring science nearer to an acceptable answer."

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24 Sep 98 | Sci/Tech
Clues to life's origins
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