Translation requires the input of a messenger RNA (mRNA) template (produced during transcription), ribosomes made up of ribosomal RNA (rRNA) and polypetide chains, and transfer RNAs (tRNAs) that carry amino acids to the growing polypeptide chain.
Ribosomes are complex macromolecular factories, composed of four different structural rRNAs and many distinct polypeptides, that are the sites of protein translation. In eukaryotes, the synthesis and assembly of rRNAs occurs in the nucleolus. Unlike mRNA, rRNA does not encode amino acids and instead serves as the building blocks of ribosomes. The eukaryotic ribosome is composed of two subunits: a large subunit (called “60S”) and a small subunit (called “40S”). The 60S subunit is composed of three rRNAs (28S rRNA, 5.8S rRNA, 5S rRNA) and 50 proteins. The 40S subunit is composed of only the 18S rRNA and 33 proteins.
mRNA is a unique form of RNA that carries information about the sequence of a protein by encoding amino acids. mRNA is produced during transcription in the nucleus and acts as a messenger for DNA — it’s basically a copy of DNA in a slightly different chemical language. Each triplet combination of A, U, G, and C nucleotides in an mRNA sequence is called a codon. Of the 64 possible mRNA codons, 61 specify the addition of amino acids to the polypeptide chain (that’s right, there are multiple codons that encode a single amino acid), three specify the termination of protein synthesis (UAG, UAA, and UGA) and one encodes the start of translation (AUG).
On the other hand, tRNAs are structural RNA molecules that bind to specific codons on the mRNA template and add the corresponding amino acid to the polypeptide chain. Therefore, tRNAs are the molecules that actually “translate” the language of mRNA into the language of proteins. In eukaryotes, tRNAs are transcribed by RNA polymerase III. tRNAs have a cloverleaf shape when drawn as a 1D chain (see the image below on the right) but actually fold into a distinct 3D structure (see the image below on the left):
Each tRNA has a sequence of three nucleotides located in a loop at one end of the molecule (shown in grey at the bottom of the figures above) that can basepair with an mRNA codon. This is called the tRNA’s anticodon. Each different tRNA has a different anticodon. There are different tRNAs for each of the 21 different amino acids, and most amino acids can be carried by more than one tRNA. Each tRNA binds to one amino acid at the end opposite its anticodon. Aminoacyl tRNA synthetases are enzymes that load individual amino acids onto the tRNAs. When the tRNA anticodon basepairs with one of the mRNA codons, the tRNA will add an amino acid to the growing polypeptide chain or terminate translation, as described above. For instance, if the sequence CUA occurred on a mRNA template in the proper reading frame, it would bind a tRNA with an anticodon expressing the complementary sequence, GAU. The tRNA with this anticodon would be linked to the amino acid leucine.
Practice Questions
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Key Points
• rRNA, in combination with polypeptide chains, acts as the building block of ribosomes.
• mRNA molecules are transcribed from a DNA sequence.
• Each three nucleotides in an mRNA sequence is called a codon, and encodes either an amino acid, a termination signal for translation, or a start signal for translation.
• tRNAs have a loop of unbasepaired nucleotides at one end of the molecule that contains three nucleotides that act as the anticodon that basepairs to the mRNA codon.
• Aminoacyl tRNA synthetases are enzymes that load the individual amino acids onto the tRNAs.
Key Terms
RNA: A type of nucleic acid that helps in the synthesis of proteins in cells.
Ribosomal RNA (rRNA): A type of RNA that acts as the building blocks of ribosomes.
Messenger RNA (mRNA): A type of RNA that is transcribed from a DNA sequence and carries information about the sequence of a protein.
Transfer RNA (tRNA): A type of RNA that carries amino acids to a growing polypeptide chain by recognizing the codons in an mRNA sequence.
Ribosome: Macromolecules made up of rRNA and proteins that act as the sites protein synthesis.
Polypeptide chain: The sequence of amino acids making up a protein.
Amino acids: The molecules that form a polypeptide chain (and therefore proteins).
Transcription: The process by which mRNA is synthesized from DNA.
Translation: The process by which a protein is synthesized from mRNA.
Codon: A sequence of three nucleotides in mRNA that encodes an amino acid, start signal, or termination signal for translation.
Anticodon: The corresponding sequence of nucleotides in tRNA that matches the codons of mRNA.
Aminoacyl tRNA synthetases: Enzymes that add individual amino acids onto tRNAs.