Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

4. All of the above

Explanation:

The genetic code is degenerate which means that most amino acids are specified by more than one codon. As we know that there are only four nucleotides namely deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. Further, there are 20 essential amino acids that are to be encoded by these four nucleotides.

A triplet code could make a genetic code for 64 different combinations (4 × 4 × 4) genetic code. Thus, there are ample of codons to specify 20 amino acids. Wobble hypothesis is based on the observation that, the base in the first position of anti-codon on tRNA is usually an abnormal base, like inosine, pseudouridine, tyrosine etc.

These abnormal bases pair with more than one type of nitrogenous base in the third position of the codon on mRNA; Inosine (I) can pair with A, C or U. This base is called Wobble base which is present at position 1 of the anti-codon and position 3 of the codon.

According to Wobble hypothesis, the third position of the codon on mRNA and the first position of the anticodon on tRNA are bound less tightly than the other two pair and provide unusual base combinations.

Due to genetic code degeneracy, many amino acids share the third base and differ in first two bases only thereby, making the third base less important. Base triplets that do not code for amino acids are called as stop codons; there are three stop codons TAG, TAA and TGA.