Translation

Translation of mRNA to proteins involves the complete decoding information contained on the mRNA sequence into an amino acid sequence of proteins. It requires three major RNA species (tRNA, mRNA, rRNA) for successful completion. Translation can be divided into three major phases

Initiation Phase leads to recognition of translation start site and activation of the amino acids. two modes of translation initiation mechanisms exist; 5'-7 methylguanosine cap-dependent initiation and cap-independent initiation mechanism. the former is more common. the translation initiation mechanism is well understood in eukaryotes. it begins with the ribosome dissociating into its two subunits-40S and 60S, The subunits are prevented from spontaneous reassociation by binding of initiation factor eIF-3 to the 40Sl ribosomal subunit. secondly the Initiation factor eIF-2 (a GTP-binding protein) recognises and binds to initiator tRNA () forming a ternary complex, . thirdly the tenary complex binds tto the small ribosomal subunit. eIF-4F recognizes and bind to the 5´' 7-methylguanosine cap of the messenger RNA. Binding of the initiation factor guides the activated 40S subunit, containing the initiator tRNA, to the 5´end of the mRNA. The mRNA-bound subunit travels along the 5’-untranslated end of the mRNA until it reaches the first AUG codon that serves as the start codon for most translation process. This process (scanning), utilizes energy in the form of ATP.

When the activated 40S subunit has reached the start codon, the 60S subunit binds to the 40S subunit. This reaction requires an additional initiation factor, eIF-5, which hydrolyses eIF-2-bound GTP, thereby releasing the initiation factors from the ribosome. As a result of the initiation process, the initiator becomes positioned in the ribosomal P-site. the central difference between this mechanism and that of cap-dependent translation is that cap-independent translation does not require the ribosome to start scanning from the 5' end of the mRNA cap until the start codon. The ribosome is guided to the start site by ITAFs (IRES trans-acting factors) hence removing the need to scan from the 5' end of the untranslated region of the mRNA. This method of translation has been recently discovered, and has found importance in conditions that require the translation of specific mRNAs,


Elongation: Elongation involves the addition of successive amino acids to the growing peptide chain as dictated by the mRNA sequence. Each amino acid containg tRNA is picked up by the eukaryotic elongation factor (eEF) in the presence of GTP. The complex recognizes the empty A site of the initiation complex and accurate complementarity of the anticodon on the amino acid containing tRNA is required for secure binding to the mRNA codon at the A-site. Upon binding the incoming amino acid is linked to the growing peptide through a peptide bond formation catalysed by the ribosome. This leaves an empty tRNA on the ribosomal P-site. The mRNA then moves along the mRNA in the 5’-3’ direction moving the empty tRNA to the E-site and the petidyl tRNA on the P-site and the A-site empty for the next incoming amino acid t-RNA.

Termination: The elongation cycle is repeated until one of the three stop-codons is positioned in the ribosomal A-site. These stop codons which have no corresponding tRNA anticodons are recognized by release factors like the eukaryotic release  factor 1 (eRF-1). eRF-1 is activated by eRF-3 which is a GTP-bound protein. Binding of eRF-1-eRF-3 complex to the stop codon leads to the dissociation of the peptide from the  tRNA Finally the bound factors are released with the hydrolysis of bound GTP freeing the ribosome for another round of translation
   
Activation of Amino Acids for Translation

Activation of amino acids for translation involves 2 steps catalyzed by aminoacyl tRNA synthetase

1.The enzyme attaches the amino acid to the α-phosphate group of ATP with the release of PPi

2.The enzyme then transfers the amino acid to either the 2'– or 3'–OH of the ribose portion of the 3'-terminal adenosine residue of the tRNA generating the activated aminoacyl-tRNA

Synthesis proceeds from the N-terminus to   the C-terminus of the protein.

Differences between Eukaryotic and Prokaryotic Translation

Prokaryotic initiation begins with the ribosomal recognition of the ribosome binding site on the mRNA. while eukaryotic initiation begins with the ribosomal recognition of the 5' 7-methylguanosine cap.

•The initiator codon can be either GUG or AUG for prokaryotes while In eukaryotes the initiator codon is always AUG.

Transcription and translation can occur simultaneously in prokaryotes, this are completely different processes in eukaryotes and rarely occur at the same time.