Translation, reading the code to make proteins
The translation process requires a conversion of information from a chain of nucleotides in RNA to a chain of amino acids in protein. The translation term was coined based on the idea that we are converting to a new molecular language at this stage of gene expression. The conversion of information in translation is a little more complex than transcription and requires a number of molecules to interact and work together (Fig 15).
Ribosomes and tRNAs work together to translate a mRNA. The tRNA is a small RNA (70 to 80 nucleotides) and has a sequence that allows hairpins to form. This gives a secondary structure to the molecule.
Figure 15. The key molecules needed for translation are the mRNA, ribosome, tRNA, and amino acids. (Image by D. Namuth-Covert)
Three of the nucleotides near the middle of the tRNA are the anticodon. When the tRNA folds, the anticodon becomes one end of the molecule and the other end will be attached to a specific amino acid. In the figures for this lesson, the tRNA is shown with it’s secondary structure and only the anticodon sequence is shown (Fig. 16). Ribosomes are the molecular workbench of translation. Ribosomes are composed of rRNAs and proteins that will have a complex secondary structure. Active ribosomes are assembled from small and large subunits when the ribosome “workbench” is ready to translate a mRNA . When mRNAs are present in the cytoplasm, the small and large ribosome subunits assemble at the 5’ end of these messages and are ready to read mRNAs like blueprints to build specific proteins.
Figure 16. The ribosome translates the mRNA 5’ to 3’ and starts by finding an AUG sequence. The ribosome allows a tRNA with the UAC anticodon to bind and bring in an amino acid. (Image by D. Namuth-Covert)
The ribosome reads the mRNA quickly and accurately but it must know where to start. As the ribosome moves in the 5’ to 3’ direction along the mRNA it “looks” for the sequence AUG (Fig. 16). This sequence is called the start codon and signals the ribosome that this is where to begin building the protein sequence. From this point, the ribosome reads the continuous chain of nucleotides in groups of three and builds the protein according to the instructions in the mRNA sequence. Typically, proteins are several hundred or more amino acids in length. When a complete protein is built, the ribosome will leave the mRNA and look for the 5’ end of another mRNA to begin translation. Each time one ribosome translates an mRNA, one copy of a protein is made.
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