I notice the terms “information flow” and “biology” mixed together, often in terms of the “central dogma” of biology, or information theory as applied to biology. Google searches will turn up all kinds of examples.

I find that a difficulty arises because the terms associated with information theory are often loosely conflated with general biological processes, often above the individual molecule level. For instance, gene processing from DNA to protein in the “central dogma” of biology is often termed as an information flow.

The term “information flow” begs a deeper question: what, if anything, can be measured as ” biological information?” What sends that information, what receives the information, and what is the process that allows information to “flow”?

I like how Dr. Thomas D. Schneider (at NIH) defines information:

“Information is always a measure of the decrease of uncertainty at a receiver (or molecular machine).”

Using this definition, the concepts of sender and receiver are critical to understanding the message being passed.

The term “information flow” in biology implies an implicit understanding of a process between a sender and a receiver, and the nature of the message being passed. The idea of “sender” and “receiver” are difficult to map onto a biological system, and there will be very few instances where this could be cleanly applied.

One case of a clearly discernable message might be the molecular interaction of the MHC-antigen system with the T-cell receptor. It’s a fairly clean system to postulate “information flow” at the level of hydrogen bonds and molecular recognition. The players in the process are clearly defined. The information is presented and received by specific mechanisms. However, the ultimate sender and receiver of the information aren’t necessarily the receptors themselves, as the actual information needs to propagate to the immune response—in a sense, the receiver of that specific molecular information is actually another molecular process. In larger systems, or systems that cross multiple scales, the receiver of the information may be blurred in its distinction between the roles of sender, message, and receiver.

Without a good definition of a receiver, we cannot understand the character of the message being transmitted. Without a technical understanding of a message, we lack a definition of the actual information, and (obviously) without a definition of the information, there’s no “flow” we can define with any applicable certainty.

In the abstract, “biological information flow” sounds good to the ear. I’ll stop short of saying that it’s meaningless, but unless the term is applied to specific systems under specific technical conditions, I don’t see its applicability to general cases.

Dr. Scheinder’s primer on information theory for biology can be found here:

http://www.ccrnp.ncifcrf.gov/~toms/paper/primer/primer.pdf