Living organisms produce proteins, molecules performing countless functions within cells. This process begins with DNA, which holds instructions for building cellular components. DNA does not directly construct proteins. Instead, DNA information is first copied into an intermediate molecule, which then guides protein assembly.
What Exactly Is a Codon?
A codon is a fundamental unit of genetic information, consisting of three consecutive nucleotides: adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA. Codons are primarily found on messenger RNA (mRNA) molecules, which carry the genetic message from DNA to the cell’s protein-making machinery.
Each codon acts like a specific “word” in the genetic language, providing an instruction for protein synthesis. Most codons specify which of the 20 common amino acids should be added to a growing protein chain. The sequence of these three-nucleotide units determines the precise order in which amino acids are linked.
From Codons to Proteins: The Translation Process
The process of converting genetic information from codons into proteins is called translation. This occurs primarily in the cytoplasm of a cell, at structures known as ribosomes. After DNA information is transcribed into an mRNA molecule, this mRNA leaves the nucleus and travels to a ribosome.
Ribosomes are composed of ribosomal RNA (rRNA) and proteins, where mRNA codons are “read.” As mRNA threads through the ribosome, transfer RNA (tRNA) molecules play a crucial role. Each tRNA carries a specific amino acid and contains a three-nucleotide sequence called an anticodon.
The tRNA anticodon is complementary to a specific mRNA codon, allowing for precise matching. When a tRNA anticodon correctly pairs with an mRNA codon, the amino acid it carries is brought into position. The ribosome then facilitates peptide bond formation, linking this new amino acid to the growing protein chain. This sequential addition of amino acids, guided by mRNA codons and tRNA molecules, continues until the entire protein chain is assembled.
Understanding the Genetic Code
The genetic code refers to the rules that dictate how mRNA codons are translated into amino acids. There are 64 possible combinations of three nucleotides, resulting in 64 different codons. Of these, 61 codons specify one of the 20 amino acids, while the remaining three act as “stop” signals, indicating the end of protein synthesis. For example, AUG typically serves as a “start” codon, initiating protein synthesis and coding for methionine.
The three stop codons are UAA, UAG, and UGA, which do not code for any amino acid but signal the ribosome to terminate translation. The genetic code exhibits “degeneracy” or redundancy, meaning most amino acids are specified by more than one codon. For instance, leucine can be coded by six different codons, providing resilience against minor genetic sequence changes.
The genetic code is also nearly universal, meaning the same codons specify the same amino acids in almost all living organisms, from bacteria to humans. This highlights a shared evolutionary history across life forms. While the code is robust, changes in codons, known as mutations, can occur. A single change in a codon can lead to a different amino acid or a premature stop signal, potentially affecting protein structure and function. However, due to the code’s degeneracy, some mutations, called silent mutations, may not alter the resulting amino acid sequence, having no apparent effect on the protein.