Question

What are some ways in which the 5' UTR is involved of the process of translation regulation. How does it compare to mechanisms in transciption?

Answer 1

Answer:

Translation refers to the process of polymerization of amino acids to form a polypeptide. A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide. A mRNA also has some additional sequences that are not translated and referred as untranslated regions (UTR). The UTRs are present at both 5' end ( before the start codon) and at 3' end (after the stop codon).

The following are the ways in which the 5' UTR is involved of the process of translation regulation : UTRs are required for efficient translation process:

• For initiation, the ribosomes bind to the mRNA at the start codon (AUG) that is recognized only by the initiator tRNA.The ribosome proceeds to the elongation phase of protein synthesis.
• In elongation phase, complexes composed of amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA. Amino acids are added one by one, translated into polypeptide sequences dictated by DNA and represented by mRNA.
• Termination - At the end, a release factor binds to the stop codon, thereby terminating translation and releasing the complete polypeptide from the ribosome.

Compare to mechanisms in transcription:

• There are at least three RNA polymerase in the nucleus and there is a clear cut division of labour. The RNA polymerase I transcribes rRNAs whereas the RNA polymerase III is responsible for transcription of tRNA, 5seRNA and snRNAs. The RNA polymerase II transcribes precursor of mRNA, the hnRNA.
• The primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. hnRNA also undergoes additional processing (called as capping & tailing). In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5' end of hnRNA . In tailing, adenylate residues are added at 3' end in a template independent manner. It is the fully processed hnRNA, now called mRNA that is transported out of the nucleus for translation.