Genetic instructions stored in DNA are transcribed into messenger RNA (mRNA). This mRNA acts as the blueprint, which is ultimately translated into functional proteins by the ribosome. Translation is the final stage of gene expression, where the mRNA nucleotide sequence is read accurately to build a chain of amino acids. The fundamental unit of this instruction set is the codon, a sequence that controls the start, continuation, and stop of protein production.
The Genetic Code and Codons
A codon is a sequence of three consecutive nucleotides found on the mRNA molecule. Since RNA contains four types of nucleotides (Adenine, Uracil, Guanine, and Cytosine), there are 64 possible combinations of these three-nucleotide sequences. Sixty-one of these codons specify one of the twenty common amino acids, while the remaining three serve as termination signals. This mapping between the three-nucleotide sequence and the corresponding amino acid is known as the genetic code, which is nearly universal across all forms of life.
The concept of the reading frame is central to accurately interpreting the genetic code, as there are no spaces or punctuation between codons in the mRNA sequence. A reading frame is the specific way the ribosome groups the nucleotides into triplets. Since a codon is three bases long, an mRNA strand can theoretically be read in three different ways, depending on the starting point. If the ribosome shifts its reading frame by just one or two nucleotides, the entire subsequent sequence of amino acids will be altered, often resulting in a non-functional protein.
The Initiation Signal
Protein synthesis must begin at a precise location on the mRNA to ensure the correct reading frame is established. The universally recognized sequence that signals the start of translation is the AUG codon. This sequence tells the ribosome exactly where to begin reading the genetic code. Without this initiation signal, the cellular machinery would not know where to start building the protein chain.
The AUG codon serves a dual function within the cell’s genetic framework. First, it signals the ribosome to begin translation, setting the correct reading frame for all subsequent codons. Second, AUG codes for the amino acid Methionine, meaning every newly synthesized polypeptide chain initially begins with this amino acid. Although this initial Methionine is often removed or modified after the protein has been created, it is always the starting point.
The AUG sequence can appear multiple times within an mRNA molecule. Only the first AUG that is correctly positioned and recognized by the ribosome’s initiating machinery acts as the start codon. Any subsequent AUG sequences encountered simply code for the addition of Methionine to the growing protein chain.
The Termination Signals
Just as translation requires a precise start signal, it also requires distinct instructions to indicate when the protein chain is complete. Termination of protein synthesis is signaled by three specific stop codons: UAA, UAG, and UGA. These sequences are sometimes referred to as “nonsense” codons because they do not code for any amino acid. They function purely as punctuation marks in the genetic code, marking the end of the protein-coding sequence.
When the ribosome encounters one of these three stop codons, there is no transfer RNA (tRNA) molecule with a complementary anticodon to match it. This lack of a corresponding tRNA halts the elongation process and recruits specialized proteins called release factors. These release factors bind to the ribosome at the stop codon site, promoting the hydrolysis of the bond between the final tRNA and the newly synthesized polypeptide chain.
The binding of the release factors triggers the release of the completed protein from the ribosomal complex. Following this release, the entire translation complex, including the ribosome and the mRNA, dissociates. This mechanism ensures the protein is synthesized to the correct length, preventing the creation of truncated or non-functional proteins.