The message contained within an mRNA is converted to protein through translation, where the genetic code is deciphered into amino acids. The bases in mRNA are decoded in threes into codons, each of which encodes an amino acid; there are 20 amino acids. Several different codons encode the same amino acid.
Making a protein involves stringing together many amino acids into a long chain, which then folds into the shape it needs to be in to perform its function. Amino acids have different properties.
Some are hydrophobic and don’t mix with water; some are hydrophilic and mix well with water; some are acidic and others are basic; some are more subtle and don’t interact strongly with any other molecules. The different combinations of these properties create the many kinds of proteins.
There are many important players in protein synthesis, but two in particular have crucial jobs:
Ribosome: The ribosome’s job is to hold everything in place, as well as form the bonds between amino acids. All cells have ribosomes. Ribosomes are made of RNA and associated proteins, with a small subunit and a large subunit coming together during translation to catalyze protein synthesis.
Transfer RNAs (tRNAs): Transfer RNAs are small RNA molecules that are folded into a specific shape necessary for fitting into ribosomes, carrying an amino acid and reading a codon. The way that each tRNA recognizes a codon is through base paring with a complementary sequence on the tRNA called the anticodon.
The start of translation is signaled by the codon AUG, which also codes for the amino acid methionine. The end of translation is signaled by one of three stop codons (UAA, UGA, or UAG), none of which codes for an amino acid. In prokaryotes, the process works like this:
Intiation is the beginning of protein synthesis and involves assembly of the ribosome, the tRNA that recognizes the start codon, and the mRNA molecule itself, as well as other accessory proteins.
A second tRNA for the next codon enters the ribosome, and the two first amino acids are joined with a peptide bond.
Elongation happens as the ribosome moves along the mRNA so that tRNAs can enter and add the appropriate amino acids to the growing peptide chain.
Termination occurs once the ribosome has reached the stop codon. At this point, the ribosome separates into its two subunits, and the mRNA molecule and the peptide chain are released.
A peptide chain is a newly formed protein made up of amino acids covalently bonded by peptide bonds.
In eukaryotes the process is similar with a few key differences:
Eukaryotic mRNAs are recognized by the ribosome by the mRNA’s methylated cap and its poly A tail.
The ribosomes are larger and use different accessory proteins for each step of translation. The ribosomes of archaea also use some of the same accessory proteins as those in eukaryotes.
The peptide chain then folds properly either on its own or with the help of other proteins. After it’s sent to the proper location in the cell, the freshly made protein will be ready to perform its function in the cell. Some bacterial proteins need to be secreted to the periplasm (the space between the inner membrane and the outer membrane in Gram-negative bacteria) or inserted into the membrane.
Proteins to be secreted have a signal peptide that is around 10 to 15 amino acids long. The signal peptide is bound by other proteins that will shuttle them to the area in the membrane where they can be exported from the cytoplasm.