Question

Which of the following is an initiation codon?
a. AUG
b. UAG
c. UGA
d. UAA

Answer 1

The relationship between the sequence of amino acids in a polypeptide chain and nucleotide sequence of DNA or m-RNA is known as a genetic code or codon. Generally the codon is a triplet .i.e., a codon contains three nitrogen bases. These codons are present on the m-RNA and attach with their complementary sequence known as the anticodon present on the t-RNA during polypeptide synthesis.

Complete answer:
- A codon is composed of three adjacent nitrogen bases which codes for one particular amino acid in a polypeptide chain.
- There are specific codons for the initiation of translation (chain initiation codons) and for the termination of translation (chain termination codons) in eukaryotes.
- The polypeptide chain synthesis is initiated by two initiation codons, AUG and GUG .
- AUG codes for methionine in eukaryotes and sometimes, GUG also functions as a start codon. When it is present at the starting position, GUG also codes for methionine.
- There are three termination codons or stop codons namely UAA, UAG and UGA. The presence of either of these codons marks the end of translation, thus, ending the polypeptide chain synthesis.
- As there are three stop codons, only 61 out of 64 codons are responsible for the synthesis of the polypeptide chain. These 61 codons are hence referred to as the sense codons. The synthesis of a polypeptide chain occurs in the cytoplasm by the combined action of ribosomes, t-RNA and m-RNA.
- As the genetic code is universal, the same genetic code is present in every living organism.

Hence, the correct answer is option (A).

Note: There are a total of 64 codons which code for 20 amino acids that is found in all organisms. Most of the amino acids can be coded by more than one codon but one codon can never code for two different amino acids except GUG.