Q.No.30. Har Gobind Khorana and Marshall Nirenberg elucidated the genetic code by using a cell-free protein synthesizing system. It was töund that and poly(C) result in synthesis of poly(L-Phe) and poly(L-Pro) respectively. On the basis of these observations, which one of the following conclusions is correct?

1. Codon CCG specifies L-Phe and codon AAA specifies L-Pro
2. Codon GGG specifies L-Phe and codon UUU specifies L-Pro
3. Codon CCC specifies L-Phe and codon UUU specifies L-Pro
4. Codon UUU specifies L-Phe and codon CCC specifies L-Pro

Correct Answer: (D)

Har Gobind Khorana and Marshall Nirenberg used cell-free protein synthesis to decipher the genetic code, showing poly(U) directs poly(L-Phe) and poly(C) directs poly(L-Pro).

Option Analysis

A: Codon CCG specifies L-Phe and codon UAA specifies L-Pro
CCG codes for proline (Pro), not phenylalanine (Phe), while UAA is a stop codon, not specifying Pro. This mismatches experiments where poly(U) yielded Phe and poly(C) yielded Pro.

B: Codon CCC specifies L-Phe and codon AAA specifies L-Pro
CCC codes for Pro, not Phe; AAA codes for lysine (Lys), not Pro. Poly(C) produced poly-Pro, and poly(A) produced poly-Lys, confirming these assignments.

C: Codon AAA specifies L-Phe and codon GGG specifies L-Pro
AAA codes for Lys, not Phe; GGG codes for glycine (Gly), not Pro. No synthetic polynucleotides linked these to Phe or Pro polymers.

D: Codon UUU specifies L-Phe and codon CCC specifies L-Pro
UUU codes for Phe, matching poly(U) synthesis of poly-Phe; CCC codes for Pro, matching poly(C) synthesis of poly-Pro. This directly aligns with the observations.

Cell-free protein synthesis revolutionized genetics by enabling Har Gobind Khorana and Marshall Nirenberg to decode how RNA specifies amino acids. Their work showed poly(U) RNA templates produce poly(L-phenylalanine), proving UUU codes for L-Phe, while poly(C) yields poly(L-proline), confirming CCC for L-Pro.

Historical Breakthrough

Nirenberg and Matthaei initiated this in 1961 with E. coli extracts, where synthetic poly(U) directed only phenylalanine incorporation, establishing the codon concept. Khorana advanced it by synthesizing defined polymers, earning the 1968 Nobel Prize alongside Nirenberg and Holley for interpreting the genetic code.

Experimental Evidence

• Poly(U): Repeating UUUUU… translates to poly-Phe, as ribosomes read UUU as Phe codon.

• Poly(C): Repeating CCCCCC… translates to poly-Pro, identifying CCC as Pro codon.
  These cell-free systems bypassed living cells, allowing precise RNA-protein mapping without cellular complications.

Codon Assignments Confirmed

Standard genetic code tables verify: UUU/UUC for Phe, CCU/CCC/CCA/CCG for Pro. No other options match, as AAA=lysine, GGG=glycine, UAA=stop. This laid the foundation for all 64 codons.