The Genetic Code
The genetic code is the system by which three-nucleotide sequences in messenger RNA correspond to amino acids used in protein synthesis. It connects information stored in nucleic acids with the chemically different world of proteins.
The Investigative Question
How does a sequence written with four nucleotide bases specify proteins constructed from twenty different amino acids?
What We Observe
Messenger RNA contains a sequence of four kinds of nucleotide bases. Proteins, however, are constructed from twenty principal amino acids.
Cells bridge these two chemically different systems by reading messenger RNA three nucleotides at a time. Each three-base sequence is called a codon.
The relationship between codons and amino acids is known as the genetic code.
Scientific Background
Four RNA bases arranged in groups of three produce 64 possible codons. In the standard genetic code, 61 codons specify amino acids while three function as termination signals.
Because there are more amino-acid codons than amino acids, several amino acids are specified by more than one codon. This property is commonly called degeneracy or redundancy of the genetic code.
The code is nearly universal among living organisms, although a limited number of variations occur in mitochondria and some organisms.
Why It Matters
The genetic code creates a functional relationship between two different classes of molecules.
A nucleotide triplet does not simply become an amino acid. Instead, the cell contains molecular machinery that interprets the nucleotide sequence and places appropriate amino acids into a growing protein.
This means that understanding the code requires understanding not only codon assignments but also the machinery that implements those assignments.
What Is Known
The standard genetic code has been experimentally deciphered. The codon assignments used by modern cells are known in great detail.
Translation depends upon messenger RNA, transfer RNAs, aminoacyl-tRNA synthetases, ribosomes, and numerous additional factors.
The widespread similarity of the genetic code among organisms is consistent with inheritance from an ancient common ancestral system.
What Is Proposed
Several explanations have been proposed for the origin and development of the genetic code.
These include stereochemical relationships between amino acids and nucleic-acid sequences, coevolution of the code with amino-acid biosynthesis, selection for error minimization, historical contingency or frozen accident, and models combining several of these processes.
Some research also suggests that earlier coding relationships may have involved portions of tRNA different from the modern anticodon system.
What Remains Uncertain
The operation of the modern genetic code is well understood, but its historical origin is not.
Scientists continue to investigate how the first associations between nucleotide sequences and amino acids arose, how the code expanded, and how increasingly elaborate translation machinery developed.
It is also uncertain how much of the modern code reflects chemical relationships and how much reflects evolutionary history and contingency.
Key Numbers
4 RNA bases produce 64 possible three-base codons; 61 standard codons specify 20 amino acids and 3 are termination codons.
Design Relevance
The genetic code is highly relevant to Intelligent Design because it is an information-processing relationship in which nucleotide sequences are used to specify amino-acid sequences.
Design advocates argue that coded relationships of this kind deserve consideration as possible evidence of intelligent causation.
That argument should not depend upon claiming that no natural relationships exist between nucleotides and amino acids. The important question is whether proposed natural processes adequately explain the origin of the coding relationship and the machinery required to implement it.
Assessment
The genetic code is not merely a metaphor. Molecular biology actually uses a defined set of codon-amino-acid relationships during protein synthesis.
But calling the relationship a code does not by itself establish how it originated. Its origin must be investigated through chemistry, evolutionary history, and the development of the translation machinery.