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Biological and Biochemical Foundations of Living Systems


Roles of mRNA, tRNA, rRNA

Translation is the process whereby RNA transcribed from DNA is used as the template for stringing together a specific sequence of amino acids for a new polypeptide.

mRNA (messenger RNA) is the template RNA transcript whose sequence of nucleotides is read in three-nucleotide portions (codons), each corresponding to either an amino acid or the directive to stop elongation of the polypeptide. rRNA (ribosomal RNA), assists at the site of translation, moving across the mRNA transcript and hosting the association of tRNA (transfer RNA) molecules that hold a specific amino acid and an anticodon region for binding to their matching mRNA codon.

In this manner, with mRNA providing a sequence of codons, tRNAs providing an anticodon binding region and corresponding amino acid, and rRNA providing active sites, these three classes of RNAs work in concert to translate a polypeptide product.

Role and structure of ribosomes

Ribosomes are a type of organelle that acts as the site of protein translation. Ribosomes free-floating in the cytosol will produce cytosol proteins, while ribosomes attached to the endoplasmic reticulum (forming rough ER) will produce proteins destined to be transported to an organelle or the cell membrane or outside of the cell. Attachment of the ribosome to the ER is determined by the presence of a signal peptide translated at the beginning of the growing polypeptide.

Each ribosome has a small subunit and a large subunit made of rRNA and proteins, which in eukaryotes are produced in the nucleolus of the cell.

The small and large subunits come together to form three distinct active sites in the ribosome — E site, P site, and A site. Each site has space for a tRNA molecule. As the ribosome moves across the mRNA transcript in the 5' → 3' direction it effectively aligns the transcript's codons with the ribosomal sites and therefore affording each opportunity to interact with a tRNA.

Translation occurs in three stages (similar to transcription) of initiation, elongation, and termination.

The A (aminoacyl) Site

The A site provides space for a new approaching tRNA with attached amino acid and corresponding anticodon to match the next codon in the mRNA sequence. (The start codon is an exception, as it is matched to its tRNA at the P site during initiation, while all of the other codons are matched to tRNAs in the A site.)

The P (peptidyl) Site

With two amino acids held adjacent to each other, one attached to a tRNA in the A site and one attached to a tRNA in the P site, the ribosome's ribozyme-acting rRNA catalyzes peptidyl transferase activity which transfers the P site amino acid (as a single Met if it was the first or as a growing peptide chain) onto the A site amino acid. At the same time, the ribosome advances across the mRNA transcript, moving the former P site occupying tRNA into the E site, positioning the elongating polypeptide attached to a tRNA from the A site into the P site, and thereby freeing space in the A site for a new tRNA to enter for the next codon.

The E (exit) Site

A tRNA moved into the E site, having just released its amino acid (and growing polypeptide chain if it was not the first tRNA), is then free to dissociate from the ribosome and mRNA (exit).

Initiation, termination co-factors

While the ribosome translates the elongating polypeptide by proceeding along the mRNA transcript 5' → 3', this process is setup in initiation by initiation factors helping to bring together the 5' end of the mRNA, the ribosomal subunits, and the correct tRNA (5'-CAU-3' anticodon) into the P site.

When the ribosome advances such that a stop codon arises in the A site, release factors will bind instead of a new tRNA molecule. This signals termination, and the growing polypeptide is finished on its c-terminus and released from the ribosome, which breaks apart into its subunits.

Post-translational modification of proteins

A polypeptide can begin folding as it is being translated. Chaperone proteins can assist in this process to assure correct folding. Post-translational modifications may include the addition of sugars, lipids, or phosphate groups to the amino acids of the polypeptide. Additionally, proteins may be cleaved with removal of leading sequences to produce their final form.

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