Biology Reference
In-Depth Information
can be extracted from the symbol string
here
and
now
without having to know
neither how it was generated in the past nor how it may be related to other symbol
strings with similar functions. For example, the linear sequences of amino acid
residues of proteins often carry sufficient “synchronic” information that allows
proteins to spontaneously fold into the secondary structures such as
a
-helices and
b
-sheets, if not into their tertiary structures (see Sect.
11.1
). In contrast, “dia-
chronic” information refers to the information embodied in a symbol string that
cannot be extracted or decoded from the structure of the string alone but must take
into account its past history as left behind or recorded in the form of the correlations
found among the symbol strings having similar or related functions. A good
example of “diachronic information” is provided by the information buried in
amino acid sequences of proteins belonging to a given family, e.g., the WW domain
family studied by Ranganathan and his group (Lockless and Ranganathan 1999;
Poole and Ranganathan 2006; Socolich et al. 2005; S
uel et al. 2003).
Diachronic
information
can be extracted if and only if multiple sequences of proteins belonging
to a given family are compared and the frequencies of occurrences of their amino
acid residues are measured at each position. The studies carried out on the WW
domain family proteins by the Ranganathan group using the
statistical coupling
analysis
(SCA) have revealed that only about 20% of the 36 amino acid residues
constituting the WW domain proteins has coevolved, thus carrying evolutionary
information (Lockless and Ranganathan 1999). Table
4.1
and its footnotes summarize
the characteristics of
synchronic
and
diachronic
information in a self-explanatory
manner, except for what is here referred to as the “Law of Requisite Information
(LRI),” which can be stated as follows:
It is impossible to solve any problem without the requisite prior information.
€
(4.18)
An example of the operation of LRI is provided by the well-known fact that an
algebraic equation with
n
unknowns cannot be solved without knowing the numeri-
cal values of the (
n-1
) unknowns. For example, the intracellular concentration of an
RNA molecule (z) is determined by the balance between two opposing rate
processes - the transcription (x) and the transcript degradation rates (y) (Sect.
12.3
):
z
¼
x
y
(4.19)
However, many workers in the DNA microarray field erroneously assumed that
x can be determined directly by measuring z alone (Sect.
12.6
) (Ji et al. 2009a),
which can be said to violate LRI: The information on z is not sufficient to solve
Eq.
4.19
for x, because the information on y is also required.
Another example that illustrates the operation of LRI may be the cosmogenesis
(Table
4.2
). Just as the amino acid sequences of proteins analyzed above, the physical
structure of the Universe that is observable by the astronomers of the twenty-first
century may contain two kinds of information - (1)
synchronic information
that can be
extracted fromour own Universe here and now and (2) the
diachronic information
that
is buried in (or hidden under) the structure of our observable Universe but not
recognizable until and unless the structure of our Universe is compared with the
