Monday, March 22, 2010

More on "Meaningful" Information


In the ongoing debate about meaningful information at Uncommon Descent, one of the commentators asked:
"Does an of arrangement of nucleobases ‘adenine-cytosine-adenine’ in DNA mean anything?"
This is a surprisingly interesting and revealing question. To attempt to answer it, I would first like to put a limit on the question: let us consider the answer if the nucleotide sequence "adenine-cytosine-adenine" is in DNA (i.e. not RNA). If "meaningful" information is necessarily analogical, as I have suggested in the previous post on "Evolution, Information, and Teleology", then the answer to this question depends upon the circumstances in which the nucleotide sequence ACA is a part. If, for example, this sequence is part of a longer sequence of nucleotides in a longer DNA molecule, then there are several possible answers:

1) the DNA nucleotide sequence ACA could be located in a single strand of DNA that is suspended in a test tube (i.e. not in a living cell) and is therefore completely biologically inert (i.e. it is not binding to a complementary strand of DNA, nor being replicated, nor transcribed, nor translated);

2) the DNA nucleotide sequence ACA could be hydrogen bonded to the complementary sequence TGT (i.e. "thymine-guanine-thymine") in another strand of nucleotides that is anti-parallel with it and close enough to form hydrogen bonds between the nitrogenous bases;

3) the DNA nucleotide sequence ACA could be in a strand that is being replicated by DNA polymerase, which can synthesize the complementary sequence TGT in a newly synthesized strand of DNA;

4) the DNA sequence ACA could be in a strand of DNA that is being transcribed by RNA polymerase, which can synthesize the complementary sequence UGU in a newly synthesized strand of RNA;

5) the DNA sequence ACA could be in a strand of DNA that has already been transcribed by RNA polymerase into the complementary sequence UGU in a strand of mRNA that is bound to a ribosome and can be actively translated into an amino acid sequence in a polypeptide; or

6) the DNA sequence ACA could be in a strand of DNA that has already been transcribed by RNA polymerase into the complementary sequence UGU in a strand of mRNA that is bound to a ribosome and is being actively translated into an amino acid sequence in a polypeptide inside a living cell, within which the polypeptide has a biological function (i.e. participates in those biochemical reactions that maintain the cell alive/against the depredations of the second law of thermodynamics).

In case #1 the DNA nucleotide sequence ACA has no "meaning", in that it is not analogically related to anything. It also has no Shannon information nor Kolmogorov information nor Orgel information either, as it is not in the process of being transmitted or compressed, nor is it "specifying" anything.

In case #2 the DNA nucleotide sequence ACA has no "meaning" because its bonding with its complementary sequence is purely chemical, not analogical. Like the bonding together of water molecules in a snowflake (i.e. the regular crystalline solid form of water), the hydrogen bonding of the nitrogenous bases in complementary DNA sequences is wholly determined by "natural laws", and is therefore neither analogical nor meaningful.

Cases 3 and 4 appear to be the same as in case 2; the relationships between the nucleotide sequences and the bonding patterns therein are entirely the result of chemistry, with no analogical nor meaningful information involved.

However, in cases 5 and 6 we seem to come to a radical discontinuity. In both of these cases, there can be an analogical (and therefore "meaningful") relationship between the nucleotide sequence ACA in DNA and the corresponding amino acid sequence in a translated polypeptide, either in vitro or in a cell. What makes this difference possible (and what may make it necessary) is the analogical relationship between the nucleotide sequence and the corresponding amino acid sequence (if one exists). If the DNA sequence ACA is located in the template strand of an actively transcribed DNA sequence (i.e. a DNA sequence beginning with a promoter to which RNA polymerase can bind) and furthermore its complementary RNA analog is located in an mRNA molecule following the "start" codon AUG but not following a "stop" codon (either UAA, UAG, or UGA, assuming a three-base reading frame), then that the DNA sequence does indeed contain "meaningful" information: it is encoded in one medium, is translated into another medium, and has a function in the system of which it is a part.

It is not yet clear from current research whether or not the amino acid that is "translated" from the DNA sequence ACA (i.e. from the mRNA sequence UGU, assuming that the DNA sequence ACA is in a template strand) is necessarily related to that mRNA sequence. That is, we do not know with confidence whether the relationship between mRNA codons and the amino acids for which they code is purely arbitrary (i.e. the result of a "frozen accident") or if there is some as-yet-undetected necessary (i.e. "natural") relationship between them.

What we can say with reasonable assurance is that what distinguishes "meaningful" information from any other kind of information is not the material into which it is encoded, but rather the relationship between the information encoded in one physical medium and its decoded complement in a related physical medium. As Gregory Bateson pointed out many years ago, meaning is entirely in the relationship between material things; it is not the things themselves. Or, as Alfred Korzybski pointed out,
"The map is not the territory"
In the same way, meaningful information is not the medium in which it is encoded, transmitted, and decoded.

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As always, comments, criticisms, and suggestions are warmly welcomed!

--Allen

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