Question

Which codon is not an indicator of completion of Protein synthesis?
1. UAG
2. AUG
3. UAA
4. UGA

Answer 1

Correct Answer - Option 2 : AUG

Concept-

• A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis (Translation).
• Each codon corresponds to a single amino acid (or stop signal), and the full set of codons is called the genetic code.
• As the DNA and RNA molecules are written in a language of four nucleotides; whereas, the language of proteins includes 20 amino acids.
• A nonsense codon is a codon for which no normal tRNA molecule exists, The presence of a nonsense codon causes the termination of translation (ending polypeptide chain synthesis) and is called a Stop Codon.

Explanation-

• There are 64 codons for 20 types of amino acids.
• The start codon is the nucleotide triplet AUG. 
• Polypeptide chain synthesis is signaled by two initiation codons AUG or GUG.
• There are three nonsense codons are called amber (UAG) ochre (UAA) and opal (UGA). They encode no amino acid. 
• A nonsense codon is a codon for which no normal tRNA molecule exists. 
• The presence of a nonsense codon causes the termination of translation (ending polypeptide chain synthesis) and is called a Stop Codon.

Therefore AUG codon is not an indicator of completion of Protein synthesis.

• There are three nonsense codons are called amber (UAG) ochre (UAA) and opal (UGA). They encode no amino acid. The ribosome pauses and falls off the mRNA.
• The genetic code includes 64 possible permutations, or combinations, of three-letter nucleotide sequences that can be made from the four nucleotides.  
• Out of 64 codons, 61 represent amino acids, and three are stop signals.
• The genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon.
• The conversion of codon information into proteins is conducted by transfer RNA.  Each transfer RNA (tRNA) has an anticodon that can base pair with a codon.