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Which one of the following is a stop codon?
A) UAG
B) UAC
C) AUG
D) UCA

Answer
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Hint: The stop codons are also called nonsense codons. They do not code for any amino acid.

Complete Answer:
- In a DNA molecule there are four nitrogenous bases, namely A, G, T & C. The specific sequence of these four bases in the DNA strand determines the sequence of amino acids in a protein molecule. In the process of protein synthesis tRNA plays the role of an adapter.
- One part of the tRNA binds to a specific amino acid and the other part recognizes the nucleotide sequence encoding that amino acid in the mRNA.
- Thus, translation of the nucleotide sequence of an mRNA into the amino acid sequence of a polypeptide takes place. This overall process of protein synthesis is known as translation.
- This mechanism was proposed by F.H.C. Crick which holds the existence of a genetic code & its smallest unit which codes for one amino acid is known as a codon.

A codon is the sequence of three nucleotides (nucleotide triplet) in mRNA which codes for a specific amino acid.
- Since, there are four code letters of DNA (A, G, T & C) and nucleotide triplet codes for one amino acid, therefore, there will be 43 = 64 different combinations for 20 amino acids that are found in proteins. This concludes that, in most cases a single amino acid is coded by two to several different codons. This multiple system of coding is known as a degenerate system or degenerate genetic code.
- A specific first codon in the sequence establishes the reading frame of the genetic message. This is known as the initiation codon or start codon. AUG is the initiation codon, and it codes for the amino acid methionine or formyl-methionine.
- The last codon of the reading frame signals the end of polypeptide synthesis. This is known as the termination codon or stop codon. There are three termination codons, and they are – UAA, UAG & UGA.
- Among the given options, AUG is the start codon, UAC codes for Tyrosine and UCA codes for Serine, whereas UAG is the stop codon.

Thus, the correct answer is A, i.e. UAG.

Note: Stop codons are known by different names. UAA is known as ochre, UGA as opal and UAG as amber.