The process of translation occurs in the specialized structures known as ribosomes and requires a mRNA for initiating the process. Protein or a peptide is an arrangement of amino acids in a chain form. Hence, it is nothing but a linear arrangement of amino acids . The ribosome gets attached to the mRNA for initiating the process of translation. The process of translation is slightly different in eukaryotes. Unlike prokaryotic methionine, the eukaryotic methionine does not undergo modification. Also, the eukaryotic mRNAs do not have a Shine Dalgarno sequence. There is a different way of finding the AUG initiation codon.

Initiation:

A eukaryotic initiation factor known as Eif-4F gets involved in the process. A cap-binding protein (CBP) known as Eif-4E binds to the cap at 5’ end of mRNA. Scanning or finding an initiation codon across the mRNA in eukaryotes involves Met-tRNA, eIF proteins, and GTP. A short sequence known as a Kozak sequence consists of AUG codon. The process of scanning the initiation codon is known as a scanning model of initiation. The 40S ribosomal subunit binds to the AUG codon. Then the 60S subunit binds and displaces the eIFs. It produces an 80S initiation complex with the initiator tRNA bound to the mRNA in the P site of the ribosome. Since a mRNA molecule has a poly (A) tail binding protein (PABP) bound to a poly (A) tail, also binds to eIF-4G, thereby forming a loop close to the 5’ end. It is a sign of translation initiation in eukaryotes.

The initiation complex consists of mRNA, elongation factors (eIF4, eIF2, eIF3, and eIF5), 40S ribosomal subunit, and an initiator tRNA. The large subunit (the 60S) gets attached to a small subunit. The initiator tRNA forms a hydrogen bond with the AUG codon of mRNA.

The elongation and translocation steps are similar to that of prokaryotes. However, a small difference involves the number and the properties of the elongation factors and the sequence of the events. Both prokaryotes and eukaryotes employ a polyribosome or a polysome. Several ribosomes translate each mRNA simultaneously. A complex between mRNA and the ribosomes keeps translating simultaneously. It is known as a polyribosome or a polysome. Each ribosome in a polysome helps in producing a complete polypeptide chain.





Elongation:

The small ribosomal subunit (the 40S) moves one codon further on mRNA. The newly activated tRNA places itself in the large subunit. The new amino acid attaches to the previous amino acid under the influence of peptidyl transferase.





Termination:

The release factor binds with the ribosome on finding a stop codon. No new tRNA binds to the ribosome. Termination leads to detaching the polypeptide chain and releasing the factors.





Image: Translation in the endoplasmic reticulum





Transfer RNA (tRNA):

It helps in transferring an amino acid to a growing polypeptide chain. A yeast tRNA contains 77 nucleotides. It is folded back upon itself and kept in a cloverleaf configuration. It involves a characteristic pairing of the bases G to C and A to U. Four important sites in a tRNA include a recognition site, an amino acid attachment site, codon recognition site, and ribosome recognition site. A recognition site is a specific base sequence recognizing a correct amino acid. The amino acid attachment site helps in the attachment of specific amino acid. A codon recognition site consists of three bases with a sequence complementary to a mRNA codon. The ribosomal recognition site helps the tRNA to recognize ribosome for the attachment. The tRNA attaches to the amino acid at the 3’ end. It contains a terminal adenylic acid followed by two cytidylic acids. Adjacent to the dihydrouridine loop lies a recognition site for a tRNA synthetase. The tRNA consists of certain rare bases formed after the transcription process. Examples include pseudo-uridylic acid (s), ribothymidylic acid (T), Dihydrouridylic acid (Ud), methyl guanylic acid (Gm), Dimethyl guanylic acid (Gd), Inosinic acid (I), and methyl inosinic acid (Im). A class of tRNA molecules known as Isoacceptor tRNA accepts the same amino acid but possesses different anti-codons. There are 2-4 Isoacceptor tRNA molecules in the higher organisms. They are specific to each amino acid.

Consider an example of a translation process occurring in the endoplasmic reticulum. The ribosome consists of the 50S and 30S subunits. Each ribosomal 50S subunit attaches to the membraneous component of the endoplasmic reticulum. The messenger RNA binds to the adjacent 30S subunit. The ribosomal 50S subunit consists of a cavity or a condensing site at the middle. The site gets occupied by a tRNA attached to a nascent polypeptide. Another site known as the entrance or the decoding site occupies both the subunits. It provides a site or a place for the aminoacyl tRNA which occupies the roof of the cavity. The uncharged tRNA gets removed after contributing its amino acid to the polypeptide. Suppose the sixth codon of the mRNA occupies a decoding site on the ribosome at the time T 0 . The CUG codon pairs with GAC anticodon of tRNA. After half a second, the mRNA advances to the next codon. It leads to an addition of a new amino acid to the nascent polypeptide chain.

References:

[1] Dictionary of Genetics, Himanshu Arora, 2007 edition.

[2] Crash Course: Cell Biology and Genetics E-Book, Matthew Stubbs, Narin Suleyman.

[3] Gene Regulation: A Eukaryotic Perspective, David S. Latchman



