A living cell is a system far from chemical equilibrium: it has a large internal
free energy, meaning that if it is allowed to die and decay towards chemical
equilibrium, a great deal of energy is released to the environment as heat. Forthe cell to make a new cell in its own image, it must take in free energy from
the environment, as well as raw materials, to drive the necessary synthetic
reactions. This consumption of free energy is fundamental to life. When it
stops, a cell dies. Genetic information is also fundamental to life. Is there any
connection?
The answer is yes: free energy is required for the propagation of information,
and there is, in fact, a precise quantitative relationship between the two
entities. To specify one bit of information that is, one yes/no choice between
two equally probable alternatives costs a defined amount of free energy
(measured in joules), depending on the temperature. The proof of this abstract
general principle of statistical thermodynamics is quite arduous, and depends
on the precise definition of the term "free energy" (discussed in Chapter 2).
The basic idea, however, is not difficult to understand intuitively in the context
of DNA synthesis.
To create a new DNA molecule with the same sequence as an existing DNA
molecule, nucleotide monomers must be lined up in the correct sequence on
the DNA strand that is used as the template. At each point in the sequence, the
selection of the appropriate nucleotide depends on the fact that the correctly
matched nucleotide binds to the template more strongly than mismatched
nucleotides. The greater the difference in binding energy, the rarer are the
occasions on which a mismatched nucleotide is accidentally inserted in the
sequence instead of the correct nucleotide. A high-fidelity match, whether it is
achieved through the direct and simple mechanism just outlined, or in a more
complex way, with the help of a set of auxiliary chemical reactions, requires
that a lot of free energy be released and dissipated as heat as each correct
nucleotide is slotted into its place in the structure. This cannot happen unless
the system of molecules carries a large store of free energy at the outset.
Eventually, after the newly recruited nucleotides have been joined together to
form a new DNA strand, a fresh input of free energy is required to force the
matched nucleotides apart again, since each new strand has to be separated
from its old template strand to allow the next round of replication.
The cell therefore requires free energy, which has to be imported somehow
from its surroundings, to replicate its genetic information faithfully. The same
principle applies to the synthesis of most of the molecules in cells. For
example, in the production of RNAs or proteins, the existing genetic
information dictates the sequence of the new molecule through a process of
molecular matching, and free energy is required to drive forward the many
chemical reactions that construct the monomers from raw materials and link
them together correctly.