Protein Synthesis
Messenger RNA
Translation is the synthesis of a polypeptide using the information in messenger RNA (mRNA). The last opportunity to control gene expression is after translation. Post-translation gene regulation is performed by regulatory proteins that can be modified by chemical group addition, transport, and protein degradation.
In eukaryote cells, the primary mRNA undergoes modification at both ends, and the introns are removed. These modifications happen before pre-mRNA exits the nucleus. A G cap is added to the 5’ end of pre-mRNA, while at the 3’end a polyadenylation, or addition of a poly-(A) tail, takes place.
Transfer RNA
In translation, a molecule that becomes a link between information in mRNA and a specific amino acid is called transfer RNA (tRNA). The function of tRNA are to:
- Interact with ribosomes
- Read mRNA codons correctly
- Deliver the amino acids that correspond to each mRNA codon.
The amino acid binds covalently at the 3’ end of tRNA. tRNA contains anticodons that will recognize codons of mRNA in an antiparallel fashion, permitting hydrogen bonds to occur between complementary bases. Translation occurs in three steps: initiation, elongation, and termination.
Ribosome
The ribosome is the site of which translation occurs. Ribosomes have a large and small subunit, each constructed from ribosome RNA (rRNA) and protein. Protein synthesis happens as a result of collaboration between tRNA, mRNA and the ribosome. The tRNA traverse along three sites of the ribosome in the order: A, P, and E.
- The A (amino acid) site is where the tRNA, with attached amino acid, bind to the mRNA codon
- The site where the tRNA adds its amino acid to the polypeptide chain is the P (polypeptide) site
- The E (exit) site is where tRNA resides before being releasesed from the ribosome
A newly formed polypeptide contains a signal sequence, usually on the N-terminus. This signal sequence indicates the place where the polypeptide belongs. Quite often, the translation process is not sufficient enough to produce a functional protein. Post-translational modification and protein folding allows the polypeptide to be functional. Modifications that are essential for functional proteins are as follows:
- Proteolysis: Cutting of the polypeptide chain by protease.
- Glycosylation: Addition of sugars to the protein, forming a glycoprotein. The process happens in the Golgi apparatus.
- Phosphorylation: Addition of a phosphate group, catalyzed by protein kinases.