A ribosome is the cellular machine that produces protein. It has two primary functional units, or subunits, known as the large subunit and the small subunit. Eukaryotic ribosomes are referred to as 80S ribosomes. The large subunit is called the 60S ribosome and the small subunit is the 40S ribosome. In prokaryotic 70S ribosomes, the larger subunit is 50S and the smaller subunit is 30S.
Smaller ribosomal subunits require transfer RNAs (tRNAs), which are vital for the initiation of translation. The tRNAs are able to bind the smaller ribosomal subunits with the assistance of initiation factors. They are very specific and are rejected if they do not encode for the desired amino acid.
The tRNAs are 70 to 80 nucleotides long. They fold over on themselves to form a cloverleaf structure that presents a three nucleic acid anticodon at one end, which is complementary to the mRNA codon for its amino acid. The amino acid is linked at the other end of the tRNA.
As soon as the tRNA is docked, the small ribosomal subunit begins scanning the sequence, looking for the AUG start codon (nucleic acid triplet). Once the start sequence has been identified a complete ribosome can be formed by the union of the larger and smaller subunits.
The codon AUG encodes the amino acid methionine. Now that the first amino acid in the peptide chain is in place, protein synthesis proceeds through the process of elongation. The codon next to the AUG start codon is the next amino acid in the peptide chain. Its corresponding tRNA enters the ribosome and if the match is correct the ribosome links it to the initial methionine.
Once the second amino acid is linked, the ribosome moves forward by one nucleic acid triplet at a time and the elongation process continues.
Protein synthesis terminates once a stop codon has been encountered. The new polypeptide chain is released with the help of termination sequences. Upon release of the newly made chain, the larger and smaller ribosomal subunits break apart from each other. The smaller component of the ribosome will go on to find another tRNA and AUG codon to start over the process.
Prokaryotic ribosomes function in a very similar way. However, instead of using the start codon AUG to initiate translation, there is a set of nucleic acids called the Shine-Delgardo sequence that precedes the initiation codon. The Shine-Delgardo sequence aligns the mRNA on the ribosomes by pairing with a sequence on 16S rRNA, a part of the prokaryotic ribosome. Often, more than one ribosome is working on the same mRNA. Another ribosome can bind to the initiation site on the mRNA as soon as the first one moves along.
Protein synthesis may be carried out by free ribosomes in the cytosol or by membrane bound ribosomes on the endoplasmic reticulum (ER). Proteins destined for use within the cell are usually translated by free ribosomes. Proteins meant for export out of the cell or for use in lysosomes are synthesized by ribosomes bound to the ER. Targeting sequences on the mRNA direct the ribosome to the ER where it becomes bound and the protein is translocated into the ER as it is translated.