Translation is the process by which ribosomes convert the information carried by messenger RNA(mRNA) to the synthesis of proteins. It can also be defined as the process in which sequence of nucleotides in mRNA is translated into the sequence of amino acids. It can also be defined as the translation of the language available in the form of mRNA into the language of proteins.
mRNA (translation) → Proteins
Translation involves the transport of amino acids from the intercellular pool to the ribosomes where they are assembled into proteins elsewhere in the cytoplasm. Transfer of amino acids to the ribosome surface is accomplished by mRNA.
Requirement of protein synthesis:
Various molecules are required for the process of protein synthesis. They are:
1)D.N.A - D.N.A is a double helical prime molecule that determines the kind of protein needed to be synthesized. The protein synthesis is initiated, guided and regulated by DNA molecule.
2)Messenger R.N.A(mRNA)- mRNA is a single-stranded molecule that carries information from D.N.A to the cytoplasm for protein synthesis. The information stored in the form of a base sequence of mRNA is complementary to the base sequence present on template D.N.A.
3)Transfer R.N.A(tRNA)-tRNA helps in protein synthesis by picking up activated amino acids from the amino acid pool and transporting them to the ribosomes where it recognizes a specific triplet codon of mRNA. Each amino acid is carried by a specific tRNA as the lowermost segment of tRNA has three base sequences anticodon loop which are complementary to the triplet codons of mRNA.
4)Ribosomes-These are the sites of protein synthesis and are found in the cytoplasm, They contain a number of enzymes responsible for the formation of the polypeptide chain. Each ribosome has two subunits- a larger subunit and a smaller subunit. Larger subunit has two sites:
I) Aminoacyl site (A site) or acceptor site
II) Peptide site (P site) or donor site
5)Amino acids-These are the building blocks of a polypeptide chain or protein. There are 20 types of amino acids which occur in cytoplasm forming an amino acid pool. These amino acids are assembled in polypeptide chain to form a protein.
6)Enzymes-A number of enzymes are responsible for the process of transcription. Aminoacyl-tRNA synthetase is one of them.
The process of translation is much more complex than that of transcription. It involves the following steps:
1)Binding of mRNA to ribosomes:
During transcription, DNA molecule synthesizes three types of RNAs inside the nucleus. Then, these RNAs migrate into the cytoplasm through the nuclear pore. Out of these RNAs, mRNA carries the genetic information and it is joined to the ribosomal subunits by the initiation codon 'AUG' found on its 5'end. This union forms mRNA ribosomal complex. [Many ribosomes lined up on a chain is known as poly-ribosomes.]
2)Activation of amino acid:
Amino acids are found in the amino acid pool in the cytoplasm in an inactive form. To form polypeptide chain, the amino acids must be activated before they are joined to the tRNA. The enzyme aminoacyl synthetase activates the amino acid in the presence of ATP and Mg++.
Amino acid + Aminoacyl-Synthetase + ATP→ Aminoacyl-AMP enzyme complex(Activated amino acid) + Ppi
3)Attachment of activated amino acid with tRNA:
The activated amino acids are joined to the 3' end of the tRNA and form amino-acyl-tRNA complex.
Activated amino acid + tRNA→ Aminoacyl-AMP enzyme complex + AMP + enzyme
There are more than 20 different enzymes and 20 tRNA molecules in the cell. So a specific amino acid attaches to a specific aminoacyl-tRNA molecule to form chained tRNA. This chain of tRNA serves as an adaptor molecule for decoding the information to mRNA till it reaches the last codon. As one ribosome moves along mRNA, the initiating part of mRNA becomes free. In this site, new ribosomes get lined up to form polyribosome.
4)Initiation of polypeptide chain:
Each mRNA molecule has initiation codon AUGm which signals the beginning of polypeptide chain. In this process, mRNA first binds to the subunits of ribosomes. The AUG codon lies near 'P' peptidyl site of the larger subunit of the ribosome. This codon codes for amino acid methionine. This means, activated methionine bearing tRNA has anticodon UAC. The second codon on mRNA leads close to 'A' site of the ribosome. Then, the 2ndaminoacyl-tRNA complex with anticodon bonds with the 2ndcodon of mRNA and occupies the 'A'-site of the ribosome.
5)Elongation of polypeptide chain:
The elongation begins with the formation of the peptide bond (-CO-NH-) between the amino acids present in 'P' and 'A' sites of the ribosomes. This is catalyzed by enzyme peptide synthetase. It causes the transfer of amino acid from 'A' site to 'P' site and formation of amino acid chain on 'A' site and releases the tRNA from P-site.
During the elongation of the polypeptide chain, ribosomes move along mRNA till it reaches the last codon. As one ribosome moves along mRNA, the initiating point of mRNA becomes free. In this site, new ribosome gets lined up to form polyribosomes.
6) Termination of polypeptide chain:
When the ribosome reaches the end of mRNA strand (3' end) the synthesis of the polypeptide chain is completed. It is signaled by the termination codon UAA, UGA, and UAG. During this process:
•One polypeptide chain or protein molecule is released from tRNA.
•Ribosomes are set free and hence dissociates into two subunits.
Thus, protein synthesis takes place in the above steps.
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Mehta, Krishna Ram.Principleof biology.2nd edition.Kathmandu: Asmita, 2068,2069.
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