On this page
NCERT's chapter on the molecular basis of inheritance has one line that many students skim: the "Severo Ochoa enzyme (polynucleotide phosphorylase)" helped to make RNA "in a template independent manner". Behind that line is a Nobel Prize, a mistaken first interpretation, and an enzyme that turned out to be useful for a different reason. This post explains the story and works through the kind of probability calculation that synthetic RNA made possible.
Who was Severo Ochoa?
The Nobel Prize facts page records that Ochoa was born on 24 September 1905 in Luarca, Spain, and died on 1 November 1993 in Madrid. His Nobel biography traces a long apprenticeship. After his medical degree in Madrid in 1929 he worked under Otto Meyerhof in Heidelberg, then in London, Plymouth and Oxford, and from 1941 with Carl and Gerty Cori in St Louis. In 1942 he joined New York University, where he became professor of biochemistry in 1954. He became an American citizen in 1956.
The 1959 Nobel Prize in Physiology or Medicine went jointly to Ochoa and Arthur Kornberg "for their discovery of the mechanisms in the biological synthesis of ribonucleic acid and deoxyribonucleic acid". Kornberg's share was for work on how DNA is made; you can read about him in our post on Arthur Kornberg and DNA polymerase.
What did the enzyme do?
In his Nobel lecture, Ochoa says that in 1955 his group isolated a bacterial enzyme that builds long RNA chains from nucleoside diphosphates, releasing phosphate. The reaction needs magnesium and runs in both directions, so the enzyme can also break RNA down. It was found with Marianne Grunberg-Manago in the bacterium *Azotobacter vinelandii*, by tracking radioactive phosphate. Their first report appeared in the Journal of the American Chemical Society in 1955.
The surprising feature was that the enzyme did not need a template. Give it all four nucleotides and it made RNA. Give it only one, such as uridine diphosphate, and it made an RNA of one repeated base, poly-U.
Was the first interpretation right?
No. The Nobel facts page says that it was first thought this enzyme assembled RNA using information from DNA, but that this turned out to be wrong, although the enzyme "proved to have other important functions nonetheless." Cells copy DNA into RNA with a different enzyme, the DNA-dependent RNA polymerase described in NCERT's chapter. Ochoa's enzyme mattered because it let chemists make RNA of a chosen composition.
How did it help crack the genetic code?
Marshall Nirenberg's Nobel lecture explains. Poly-U added to a cell-free system made a protein of only phenylalanine, showing that U in RNA corresponds to phenylalanine. In the next phase, randomly ordered RNA chains with different mixes of bases were made "with the aid of polynucleotide phosphorylase that had been discovered by Grunberg-Manago, Ortiz and Ochoa". Nirenberg's table, combining his data with that of "Ochoa and associates", shows that poly-U, poly-C and poly-A direct phenylalanine, proline and lysine. Ochoa's own group published its results in 1961 in Synthetic polynucleotides and the amino acid code.
A worked example: codons in a random RNA
This is a practice problem, not data from the original experiments. Assume the enzyme is given uridine and adenosine diphosphates so that 3/4 of the bases in the RNA are U and 1/4 are A, placed at random. What fraction of triplets are UUU, and what fraction contain two U and one A?
P(U) = 0.75, P(A) = 0.25. Bases are independent, so multiply.
P(UUU) = 0.75 × 0.75 × 0.75 = 0.421875, about 42.2%.
Two U and one A can be UUA, UAU or AUU, so there are 3 arrangements. P = 3 × 0.75² × 0.25 = 3 × 0.140625 = 0.421875, also about 42.2%.
One U and two A: 3 × 0.75 × 0.25² = 0.140625, about 14.1%.
AAA: 0.25³ = 0.015625, about 1.6%.
Check: 0.421875 + 0.421875 + 0.140625 + 0.015625 = 1.
Since UUU codes for phenylalanine and AAA for lysine, you would expect plenty of phenylalanine and very little lysine in the protein made from this RNA.
Common mistakes:
Forgetting the arrangements. Two U and one A is three different codons, so multiply by 3.
Mixing up template-free and template-dependent. Polynucleotide phosphorylase needs no template; RNA polymerase copies DNA.
Giving Ochoa the whole code. NCERT credits Khorana's chemical synthesis and Nirenberg's cell-free system too. See our post on Har Gobind Khorana and the genetic code.
What can a student take from his way of working?
Train broadly. Ochoa spent over a decade in labs in several countries before 1955, learning enzymology, which is why he recognised an unusual reaction when he saw one.
Let the evidence correct you. The first idea about the enzyme's role was wrong. The useful part was what it could actually do.
Credit your collaborators. The enzyme was found with Grunberg-Manago, and Ochoa's lecture says so.
If counting codon arrangements still trips you up, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can see where each factor comes from.
Next step: redo the example with 2/3 U and 1/3 C, and check that your four answers add up to 1.
