Biological Information Transfer

The continued existence of a biological species requires that its genetic information be maintained in a stable form and, at the same time, expressed with very few errors. Effective storage and accurate expression of the genetic message defines individual species, distinguishes them from one another, and assures their continuity over successive generations.
Among the seminal discoveries of twentieth-century biology are the chemical nature and the three-dimensional structure of the genetic material, DNA. The sequence of deoxyribonucleotides in this linear polymer encodes the instructions for forming all other cellular components and provides a template for the production of identical DNA molecules to be distributed to progeny when a cell divides.

Genetic Continuity Is Vested in DNA Molecules

Perhaps the most remarkable of all the properties of living cells and organisms is their ability to reproduce themselves with nearly perfect fidelity for countless generations. This continuity of inherited traits implies constancy, over thousands or millions of years, in the structure of the molecules that contain the genetic information. Very few historical records of civilization, even those etched in copper or carved in stone, have survived for a thousand years (Fig. 1-15). But there is good evidence that the genetic instructions in living organisms have remained nearly unchanged over very much longer periods; many bacteria have nearly the same size, shape, and internal structure and contain the same kinds of precursor molecules and enzymes as those that lived a billion years ago.
Hereditary information is preserved in DNA, a long, thin organic polymer so fragile that it will fragment from the shear forces arising in a solution that is stirred or pipetted. A human sperm or egg, carrying the accumulated hereditary information of millions of years of evolution, transmits these instructions in the form of DNA molecules, in which the linear sequence of covalently linked nucleotide subunits encodes the genetic message.

The Structure of DNA Allows for Its Repair and Replication with Near-Perfect Fidelity

The capacity of living cells to preserve their genetic material and to duplicate it for the next generation results from the structural complementarity between the two halves of the DNA molecule (Fig. 1-16). The basic unit of DNA is a linear polymer of four different monomeric subunits, deoxyribonucleotides (see Fig. 1-3), arranged in a precise linear sequence. It is this linear sequence that encodes the genetic information. Two of these polymeric strands are twisted about each other to form the DNA double helix, in which each monomeric subunit in one strand pairs specifically with the complementary subunit in the opposite strand. In the enzymatic replication or repair of DNA, one of the two strands serves as a template for the assembly of another, structurally complementary DNA strand. Before a cell divides, the two DNA strands separate and each serves as a template for the synthesis of a complementary strand, generating two identical double-helical molecules, one for each daughter cell. If one strand is damaged, continuity of information is assured by the information present on the other strand. Genetic information is encoded in the linear sequence of four kinds of subunits of DNA. The double-helical DNA molecule contains an internal template for its own replication and repair. Changes in the Hereditary Instructions Allow Evolution Despite the near-perfect fidelity of genetic replication, infrequent, unrepaired mistakes in the replication process produce changes in the nucleotide sequence of DNA, representing a genetic mutation (Fig. 1-17). Incorrectly repaired damage to one of the DNA strands has the same effect. Mutations can change the instructions for producing cellular components. Many mutations are deleterious or even lethal to the organism; they may, for example, cause the synthesis of a defective enzyme that is not able to catalyze an essential metabolic reaction. Occasionally the mutation better equips an organism or cell to survive in its environment. The mutant enzyme might, for example, have acquired a slightly different specificity, so that it is now able to use as a reactant some compound that the cell was previously unable to metabolize. If a population of cells were to find itself in an environment where that compound was the only available source of fuel, the mutant cell would have an advantage over the other, unmutated (wild-type) cells in the population. The mutant cell and its progeny would survive in the new environment, whereas wild-type cells would starve and be eliminated. Chance genetic variations in individuals in a population, combined with natural selection (survival of the fittest individuals in a challenging or changing environment), have resulted in the evolution of an enormous variety of organisms, each adapted to life in a particular ecological niche.