AUSTIN, TEXAS--The DNA that encodes genes consists of endless sequences of only four chemical "letters"--A, C, G and T--and a little old-fashioned thermodynamics may partially explain why.
Within the nucleus of each cell, long strands of DNA encode an organism's genes by combining just four chemical building blocks--adenine, guanine, cytosine, and thymine. In DNA's double helix, adenine pairs with thymine, and guanine with cytosine. The cell uses DNA, one gene at a time, to produce matching strands of RNA, from which it makes proteins that perform various tasks. Yet, scientists don't know why DNA spells out genes with four chemical letters instead of two or six.
The answer may not lie in DNA itself, however, but in the thermodynamics of tangled RNA, researchers announced here on 4 February at a meeting of the American Physical Society. For years, biologists have suspected that the very first “life” consisted not of DNA-based organisms, but of molecules of RNA that developed the knack of reproducing. RNA, too is composed of four chemical bases: adenine, guanine, cytosine, and--in a slight departure from DNA--uracil.
Compared to the way two matching strands of DNA twist into a graceful helix, RNA is a mess. Each individual strand tries to reduce its energy by twisting and folding and zippering to itself, but it also tries to increase its disorderliness--or entropy--by remaining free and untangled. Eventually, each strand strikes a compromise, settling on a shape that determines how the molecule behaves. That shape will be far more stable if the RNA contains four or six chemical instead of two, according to new calculations that Rajan Mukhopadhyay of the NEC Research Institute in Princeton, New Jersey, reported at the meeting. So life in the primordial RNA world required four chemical letters, Mukhopadhyay says, and it passed that characteristic along to more complicated DNA-based life.
But the study doesn't explain why early RNA life didn't have six or more letters, says Gary Slater, who studies DNA at the University of Ottawa, Canada. RNA with more letters would have the advantage that it can carry more information, he adds.
)
)
)
)
)
)
)
)
)
)
)
