The Reading Frame, Codons and Anticodons

The mechanics of reading mRNAs to build proteins is an orchestration of several interacting molecules. The mRNAs, tRNAs, ribosomes and amino acids are the key players.

The mRNA has the codons which signal to the ribosome when to start the protein building process and when to end it. The codons in the middle known as the reading frame, determine which amino acids will be placed into the protein. The AUG start codon establishes the beginning of the reading frame on a mRNA. The ribosome must follow this reading frame to build the correct protein. How are the appropriate amino acids placed into the protein as it is built? The amino acids need to be transferred to the ribosomes with the assistance of tRNAs.

The tRNAs are able to perform their transfer and placement function because of their structure (Fig. 17). The tRNA molecule is small, only 70-80 nucleotides in length. Those sequences promote hairpin loops to form, giving tRNA a stable secondary structure. The structure of the tRNA is recognized by special enzymes in the cell that attach the proper amino acid to the tRNAs. The tRNA also has a sequence of three nucleotides called the anticodon. Anticodons on the tRNA will complement and bind to the codon on the mRNA to specify the correct amino acid placement in the growing protein chain. Let's describe how the key players get protein building started, how they keep it going, and how they end it.

Figure 17. The ribosome allows two tRNAs with complementing anticodons to enter the 'A' and 'P' sites and bring in their amino acids. (Image by D. Namuth-Covert)