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: The untranslated regions (UTRs) of an mRNA are present at both the 5′-end (after the start codon) and the 3′-end (before the stop codon).
Statement II: The UTRs of an mRNA are required for an efficient translation process.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Statement I is incorrect but Statement II is correct.
The part of an mRNA that is translated into a polypeptide is the translational unit. It runs from the start codon (AUG) to the stop codon. An mRNA also carries extra sequences that are not translated. These are the untranslated regions (UTRs).
Statement I: The untranslated regions (UTRs) of an mRNA are present at both the 5′-end (after the start codon) and the 3′-end (before the stop codon). — False. The UTRs lie outside the translational unit: at the 5′-end before the start codon and at the 3′-end after the stop codon. A region after the start codon and before the stop codon would be inside the translational unit and would be translated. The statement swaps “before” and “after”.
Statement II: The UTRs of an mRNA are required for an efficient translation process. — True. Although the UTRs are not translated, they are needed for efficient translation of the mRNA.
NCERT Extract:
Translation – “… A translational unit in mRNA is the sequence of RNA that is flanked by the start codon (AUG) and the stop codon and codes for a polypeptide. An mRNA also has some additional sequences that are not translated and are referred as untranslated regions (UTR). The UTRs are present at both 5′-end (before start codon) and at 3′-end (after stop codon). They are required for efficient translation process.”