Deciphering the Genetic Code — NEET Biology
📚 Practice ConceptMarshall Nirenberg's cell-free system for protein synthesis finally helped the code to be deciphered. Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful in polymerising RNA with defined sequences in a template independent manner (enzymatic synthesis of RNA). Finally a checker-board for genetic code was prepared which is given in Table 5.1.
Which of the following statements about the properties of the genetic code are correct, particularly in the context of its deciphering using synthetic RNAs? S1. The triplet nature of the code allowed scientists like Har Gobind Khorana to synthesize RNA molecules with defined combinations of bases. S2. The degeneracy of the genetic code, where some amino acids are coded by more than one codon, simplifies the process of protein synthesis in cell-free systems. S3. The non-overlapping and contiguous reading of the code ensures that each nucleotide is part of only one codon, which was crucial for accurate translation of synthetic homopolymers. S4. The universality of the genetic code allowed Marshall Nirenberg's cell-free system (from bacteria) to be used to decode codons that are applicable across most organisms. S5. Severo Ochoa enzyme (polynucleotide phosphorylase) was used for template-dependent synthesis of RNA, aiding in decoding of the genetic code.
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Correct answer: C — S1, S3, and S4 are correct
Let's evaluate each statement regarding the genetic code and its deciphering: S1. The triplet nature of the code allowed scientists like Har Gobind Khorana to synthesize RNA molecules with defined combinations of bases. This statement is correct. George Gamow's proposition of a triplet code was a foundational idea, and Khorana's chemical method for synthesizing RNA with defined sequences (like homopolymers and copolymers) built upon this understanding, allowing for the experimental determination of codons. For example, a homopolymer of UUU could only mean UUU was coding for one amino acid. S2. The degeneracy of the genetic code, where some amino acids are coded by more than one codon, simplifies the process of protein synthesis in cell-free systems. This statement is incorrect. While degeneracy is a property of the genetic code, it doesn't necessarily 'simplify' the process of protein synthesis in cell-free systems. In fact, it adds complexity by meaning multiple codons could result in the same amino acid, though it offers a buffering effect against mutations in vivo. S3. The non-overlapping and contiguous reading of the code ensures that each nucleotide is part of only one codon, which was crucial for accurate translation of synthetic homopolymers. This statement is correct. The genetic code is read in a contiguous fashion without punctuations. This non-overlapping nature means that synthetic RNAs like poly-U (UUUUUU...) would be read as a repeating sequence of UUU codons, each coding for Phenylalanine, leading to a polypeptide of only Phenylalanine, which was experimentally verified. S4. The universality of the genetic code allowed Marshall Nirenberg's cell-free system (from bacteria) to be used to decode codons that are applicable across most organisms. This statement is correct. The near universality of the genetic code means that a codon like UUU codes for Phenylalanine in bacteria, just as it does in humans. This property was critical for Nirenberg's work using bacterial cell-free systems to decipher the code and for the findings to be generally applicable. S5. Severo Ochoa enzyme (polynucleotide phosphorylase) was used for template-dependent synthesis of RNA, aiding in decoding of the genetic code. This statement is incorrect. Severo Ochoa enzyme (polynucleotide phosphorylase) was helpful in polymerising RNA with defined sequences in a *template-independent* manner (enzymatic synthesis of RNA), which was distinct from template-dependent transcription. Therefore, statements S1, S3, and S4 are correct. The tested concept is the properties of the genetic code and their implications for its experimental deciphering.
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