Question

Explain the dual function of AUG codon. Give the sequence of bases it is transcribed from and its anticodon.

Answer 1

Hint: The universal genetic code is made up of triplet bases or several codons. The standard code has evolved over the time to minimize coding errors. There are 64 codons in genetic code arising from permutation and combination of the four bases in nucleic acids. In the genetic code, there are two punctuation marks known as the start codon and stop codon which signal the end of protein synthesis in all organisms.

Complete answer:
During the translation process, AUG acts as an initiator of the protein synthesis, and in eukaryotes, at internal protein positions, it acts as a codon for methionine incorporation and in prokaryotes an N-formylmethionine. However, the presence of AUG alone isn’t enough to start translation – initiation factors and a specific nearby sequence or nucleotide context are also required. Although in eukaryotes , the alternative start codons are rarely used, a recent study to monitor translation in budding yeast utilizing ribosome-profiling strategy discovered the occurrence of pervasive initiation at favorable, specific, non-AUG sites.
The dual functionality of AUG codon is somewhat artificial as there is no competition for the amino acid insertion at either initiator or internal reading AUG site. The initiator tRNA which binds prior to the ribosome assembly only works for providing N-terminal amino acid residue. Once the translation has started, AUG acts the same as any other codon and it has a specific role in inserting Methionine in internal positions of proteins. AUG is transcribed from bases in DNA; those bases are TAC (thymine, adenine and cytosine). Its anticodon is UAC.

Note: UGA is another example of codon with different functions, which is used for the selenocysteine (Sec) incorporation. Selenocysteine is also known as the 21st naturally occurring amino acid that is co-translationally incorporated into the nascent polypeptide chains. Structurally, Selenocysteine is similar to cysteine and serine, but contains a selenium atom in place of the sulfur in cysteine and oxygen in the serine. Its presence in all domains of life and conservation of several components of the Selenocysteine biosynthesis and insertion machinery suggest us an ancient origin of this amino acid.