mRNA is translated into a polypeptide chain (and therefore a protein) at ribosomes, complex macromolecules composed of rRNAs and many distinct polypeptides.
Ribosomes are the sites of mRNA translation into a polypeptide. Ribosomes may consist of different numbers of rRNAs and polypeptides, depending on the organism. However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to archaea to human cells. Ribosomes exist in the cytoplasm in prokaryotes, and in the cytoplasm and on rough endoplasmic reticulum membranes in eukaryotes. Mitochondria and chloroplasts also have their own ribosomes, although these are more similar to prokaryotic ribosomes. Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation.
Prokaryotic ribosomes have a 30S small subunit and a 50S large subunit, while eukaryotic ribosomes have a 40S small subunit and a large 60S subunit. The small subunit is responsible for binding the mRNA template and the large subunit sequentially binds tRNAs. The large ribosomal subunit sits atop the small ribosomal subunit and the mRNA is threaded through a groove near the interface of the two subunits. When assembled for translation, ribosomes have three binding sites that accomodate tRNAs: The A site, the P site, and the E site. Take a look at the diagram below to see how these are arranged relative to each other:
Incoming aminoacyl-tRNAs (a tRNA with an amino acid covalently attached) enter the ribosome at the A site. The peptidyl-tRNA (a tRNA carrying the growing polypeptide chain) is held in the P site. The E site, located right next to the P site, holds empty tRNAs just before they exit the ribosome.
Each mRNA molecule is simultaneously translated by many ribosomes, all reading the mRNA from 5′ to 3′ and synthesizing the polypeptide from the N terminus to the C terminus. The complete mRNA/poly-ribosome structure is called a polysome.
Practice Questions
MCAT Official Prep (AAMC)
Practice Exam 3 B/B Section Passage 8 Question 41
Practice Exam 4 B/B Section Passage 3 Question 19
Practice Exam 4 B/B Section Passage 3 Question 21
Key Points
• Ribosomes, macromolecular structures composed of rRNA and polypeptide chains, are formed of two subunits (in bacteria and archaea, 30S and 50S; in eukaryotes, 40S and 60S), that bring together mRNA and tRNAs to catalyze protein synthesis.
• Ribosomes are present in the cytoplasm and on the rough endoplasmic reticulum in eukaryotes, as well as in organelles like chloroplasts and mitochondria.
• Fully assembled ribosomes have three tRNA binding sites: an A site for incoming aminoacyl-tRNAs, a P site for peptidyl-tRNAs, and an E site where empty tRNAs exit.
Key Terms
Ribosome: Macromolecules made up of rRNA and proteins that act as the sites protein synthesis.
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.
Polypeptide chain: The sequence of amino acids making up a protein.
Small subunit: 30s in prokaryotes, 40s in eukaryotes; binds the mRNA template.
Large subunit: 50s in prokaryotes, 60s in eukaryotes; binds to sequential tRNAs.
A site: The location where aminoacyl-tRNAs enter the ribosome during mRNA translation.
P site: The location where peptidyl-tRNAs are held.
E site: The location where tRNAs are held prior to their release from the ribosome.
Aminoacyl-tRNA: A tRNA with an amino acid covalently attached.
Peptidyl-tRNA: A tRNA carrying the growing polypeptide chain.
Polysome: The complete mRNA template/poly-ribosome structure that forms during translation.