Read the given passage and answer the following questions.
Determining the biochemical nature of genetic material and the structure of DNA was very exciting, the proposition and deciphering of genetic code were most challenging. In a very true sense, it required involvement of scientists from several disciplines – physicists, organic chemists, biochemists and geneticists. It was George Gamow, a physicist, who argued that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases. He suggested that in order to code for all the 20 amino acids, the code should be made up of three nucleotides. This was a very bold proposition, because a permutation combination of 43 (4 × 4 × 4) would generate 64 codons; generating many more codons than required. The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons. Some amino acids are coded by more than one codon, hence the code is degenerate. The codon is read in mRNA in a contiguous fashion. There are no punctuations. The code is nearly universal: for example, from bacteria to human UUU would code for Phenylalanine (phe). Some exceptions to this rule have been found in mitochondrial codons, and in some protozoans. AUG has dual functions. It codes for Methionine (met) , and it also act as initiator codon. UAA, UAG, UGA are stop terminator codons.
Given below are two statements:
Statement I: Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion.
Statement II: Insertion or deletion of three or its multiple bases inserts or deletes one or more codons, and the reading frame remains unaltered from that point onwards.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both Statement I and Statement II are correct.
The genetic code is read in mRNA in a contiguous fashion, three bases at a time, with no punctuations. The book uses the sentence RAM HAS RED CAP to show this. Inserting one letter shifts every later group of three and the words become meaningless, but inserting three letters (RAM HAS BIG RED CAP) adds one word and leaves the rest readable.
Statement I: Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. — True. One or two extra (or missing) bases shift every later codon, so the reading frame changes from that point. Such changes are called frameshift insertion or deletion mutations.
Statement II: Insertion or deletion of three or its multiple bases inserts or deletes one or more codons, and the reading frame remains unaltered from that point onwards. — True. Three bases make one whole codon. So one or more codons, and hence one or more amino acids, are added or lost, and the codons after them are read as before.
NCERT Extract:
Mutations and Genetic Code – “… Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. However, such mutations are referred to as frameshift insertion or deletion mutations. Insertion or deletion of three or its multiple bases insert or delete in one or multiple codon hence one or multiple amino acids, and reading frame remains unaltered from that point onwards.”