In the synthesis or production of proteins, a process of decoding the 'messenger Ribonucleic Acid' or mRNA takes place. It is the first step in the process and is known as translation. The mRNAs decoded in translation are obtained from a process known as transcription. The translation process takes place in the cell cytoplasm, specifically where the cell organelle, ribosome is present. Translation produces polypeptides as a result of decoding of mRNA.
The process of translation starts with 'activation', which actually is not a step, but the starting point of the process. During activation, the amino acids form covalent bonds with 'transfer RNA'.
The next step in translation is 'initiation'. The components that play an important part in initiation are as follows.
- There are two subunits of ribosomes involved in the initiation process.
- Messenger RNA (mRNA).
- Aminoacyl transfer RNA (tRNA)
- Guanosine-5'-triphosphate (GTP) provides energy for the process of translation.
- Initiation factors (IF) assembles the components taking part in the initiation process.
The nucleotides - DNA and RNA is made up of strands. One end of a strand is known as the "3' end", while the other as "5' end". In the initiation process, a small subunit of ribosomes is attached or bound to the 5' end of messenger RNA. Special proteins known as 'Initiation Factors' help in the binding process. The 'eIF3' or eukaryotic Initiation Factor assists in binding the small ribosome unit to mRNA. Along with the ribosome, transfer RNA (tRNA) is also attached to mRNA. When the tRNA gets attached to the mRNA, it carries amino acids along with itself.
The mRNA is made up of a set of three nucleotides called 'codon'. The codons present on nucleic acids (DNAs and RNAs) correspond to the amino acids present in proteins. For every codon, there is a set of nucleic acids attached at the end of tRNA. These are known as 'anticodons'.
The tRNA starts searching the mRNA for the start codon. The start codon present on the mRNA is mostly 'AUG' (Adenine, Uracil, Guanine). In the cells of eukaryotic organisms, the corresponding amino acid for the start codon is methionine. The tRNA that has formed a covalent bond with methionine during the activation phase of translation becomes a part of the complex structure called 'ribosomal complex'. The protein synthesis, thus starts with methionine.
A eukaryotic Initiation Factor 2 (eIF2) brings tRNA to a small subunit of the ribosome. This carries out the hydrolysis of the GTP. It further leads to the formation of a complete ribosome followed by dissociation of its small and large subunits. This ends the initiation phase and elongation phase begins thereafter.
The elongation of polypeptide chain begins with tRNA entering the 'P' site of ribosome. The 'A' site of ribosome is now ready for receiving the aminoacyl-tRNA. In the phase of elongation, a cycle of three steps is followed for the elongation of the protein chain to take place.
- The aminoacyl is correctly placed in the ribosome.
- Formation of the peptide bond.
- After every peptide bond formation, the mRNA shifts by one codon.
It is the final phase of the translation process. If any of the termination codon enters the 'A' site of the ribosome, the translation process stops. This happens because the tRNA does not recognize these codons. These codons are recognized by the 'release factors' instead, and they trigger a reaction called hydrolysis. The protein formed as a result of this whole process, is released from the ribosome and finally the translation process ends.
In prokaryotic cells, 18 amino acids are produced per second while 1000 amino acids are generated in the bacteria. Protein synthesis is a vital process that takes place in the ribosomes of cells of living beings and translation forms an important part of it.