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.
Why is the genetic code called degenerate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Some amino acids are coded by more than one codon.
The passage states that “Some amino acids are coded by more than one codon, hence the code is degenerate”.
There are 61 codons for only 20 amino acids. So several amino acids have more than one codon. This is what “degenerate” means. Each codon still stands for only one amino acid; it is the amino acid that has more than one codon. The passage lists other features of the code too, and each of them has its own name.
Option (1) It is nearly universal. — Incorrect. “Nearly universal” means that a codon stands for the same amino acid in almost all organisms, for example UUU codes for phenylalanine from bacteria to human. This is a different feature of the code.
Option (2) It has a few codons that do not code for any amino acid. — Incorrect. The three codons that code for no amino acid (UAA, UAG, UGA) are the stop codons. They end translation; they do not make the code degenerate.
Option (3) AUG acts as the initiator codon and also codes for methionine. — Incorrect. AUG has dual functions: it codes for methionine and also acts as the initiator codon. This too is a separate feature of the code.
Option (4) Some amino acids are coded by more than one codon. — Correct. When an amino acid is coded by more than one codon, the code is called degenerate.