Translation is the complex process of protein synthesis. Prior to this, the genetic information in DNA is converted into messenger RNA (mRNA) molecules by transcription. These single-stranded gene copies are the template for translation.
Translation takes place in the cytoplasm by which the information in the transcribed mRNA is converted into the corresponding amino acids which are later joined in a polypeptide chain.
The genetic code in the mRNA is translated to an amino acid sequence that finally leads to the formation of a unique protein. Translation involves a series of interactions between many components including different types of RNAs and other proteins.
Components of Translation
The key components required for translation are mRNA, tRNA, ribosomes, and aminoacyl tRNA synthetases. These four components are briefly explained below:
This is a complex organelle present in the cytoplasm that serves as the site of action for protein synthesis. It provides the enzymes needed for peptide bond formation.
The genetic information in mRNA is read as nucleotide triplets called codons. The ribosome moves along the mRNA recognizing the codons and adding their corresponding amino acid sequences until it reaches a "stop" codon. On encountering the stop code, the ribosome stops translation and releases the mRNA as well as the polypeptide.
Ribosome during protein synthesis. The Interaction of a Ribosome with mRNA. Process of initiation of translation. Image Copyright: Designua / Shutterstock
Messenger RNA (mRNA)
This is a single-strand of RNA complementary to the DNA template and is a product of transcription. It carries the genetic information from the DNA to the ribosome for protein synthesis. During translation, mRNA nucleotide bases are read as codons or nucleotide triplets . In the process, each codon is designated to a particular amino acid however, the genetic code is described as degenerate because a single amino acid may be coded for by more than one codon. There are also specific mRNA codons that signal the start and stop of translation.
This is a single strand of RNA composed of approximately 80 ribonucleotides. Each tRNA is read as a ribonucleotide triplet called an anticodon that is complementary to an mRNA codon. tRNAs act as an interpreter between amino acid sequences and nucleic acids by matching the amino acids to their cognate mRNA codons.
Aminoacyl tRNA synthetases
These are enzymes that link each amino acid to their corresponding tRNA with the help of a two-step process. Each amino acid has a unique synthetase and the active site of each enzyme fits only one specific combination of the amino acid and tRNA.
Key Steps in Translation
Similar to transcription, there are 3 steps in translation. These are namely; initiation, elongation, and termination. These steps are briefly discussed below:
Small ribosomal subunits bind to mRNA. The initiator tRNA which is equipped with the anticodon UAC also binds to the start codon AUG in the mRNA. The resulting large complex forms a complete ribosome and initiates protein synthesis.
Following initiation, a new tRNA-amino acid complex enters the codon next to the AUG codon. If the anticodon of the new tRNA matches the mRNA codon, base pairing occurs and the two amino acids are linked by the ribosome through a peptide bond. On the other hand, if the anticodon does not match the codon, base pairing cannot happen and the tRNA is rejected. Then, the ribosome moves one codon forward making space for a new tRNA-amino acid complex to enter. This process is repeated several times until the entire polypeptide has been translated.
As the ribosome moves along the mRNA, it encounters one of the three stop codons for which there is no corresponding tRNA. Terminator proteins present at the stop codon bind to the ribosome and trigger the release of the newly-synthesized polypeptide chain. The ribosome then disengages from the mRNA. On release from the mRNA, the small and large subunits of the ribosome dissociate and prepare for the next round of translation. The polypeptide chains produced during translation undergo some post-translational modifications before becoming a fully active protein.
Reviewed by Afsaneh Khetrapal, BSc (Hons)