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Your NCERT chapter on the molecular basis of inheritance says mRNA carries the genetic code from DNA to the ribosome. That sentence was once an open question, and one experiment in June 1960 did much to settle it. This post explains the puzzle, the experiment Sydney Brenner designed with Francois Jacob and Matthew Meselson, how to reason through its result, and what a student can learn from the way Brenner worked.
Who was Sydney Brenner?
His Nobel Prize facts page records that he was born on 13 January 1927 in Germiston, South Africa, and died on 5 April 2019 in Singapore. In his Nobel autobiography he writes that his father was a shoe repairer who never learnt to read or write, and that he started university in Johannesburg at fifteen. He went to Oxford in October 1952 for a PhD, saw the Watson and Crick DNA model in Cambridge in April 1953, and in December 1956 left South Africa to join the Medical Research Council unit in Cambridge. He shared an office with Francis Crick for 20 years.
What was the puzzle about protein synthesis?
By the late 1950s biologists knew that proteins are made on ribosomes in the cytoplasm, while genes sit in DNA. A common idea was that stable RNA templates in the cytoplasm, such as the RNA of ribosomes, each carried the instructions for a particular protein. Francois Jacob explains in his 1965 Nobel lecture why that failed. Ribosomes were stable and all much alike, yet bacteria could switch on new proteins almost at once. The only hypothesis left was "a third species of RNA, the messenger, a short-lived molecule charged with transmission of genetic information to the cytoplasm". Ribosomes would then be "non-specific structures", machines that translate whatever messenger binds to them.
That was a prediction, not yet a result. Jacob writes that he and Brenner "decided to spend the month of June 1960 with M. Meselson" at the California Institute of Technology, "hunting for the messenger".
How did the experiment work?
The best candidate was RNA that appears in bacteria after infection by the virus T2. After T2 infects a bacterium, virus proteins are made inside it. So the question became: does the virus build new ribosomes, or does it use the old ones?
James Watson's 1962 Nobel lecture describes the method. The team grew bacteria in a medium with heavy isotopes, carbon-13 and nitrogen-15, so every ribosome made before infection was heavy. They then infected the bacteria with T2 in a light medium (carbon-12 and nitrogen-14), so anything made afterwards was light. Caesium chloride density-gradient centrifugation separated heavy particles from light ones, the same technique used in the Meselson-Stahl experiment on DNA replication. Radioactive labels marked the new RNA and, with a short pulse of labelled amino acids, the new protein.
Worked example: reading the result
Treat it as a prediction problem. Write what each hypothesis predicts before looking at the answer.
Hypothesis A, one gene per ribosome. Virus proteins need virus-specific ribosomes, so they must be built after infection. New ribosomes would be light. Prediction: new RNA and new protein appear in a light ribosome band.
Hypothesis B, messenger plus general ribosomes. The old heavy ribosomes are reused with a new virus messenger. Prediction: new RNA and new protein appear in the heavy ribosome band.
The result. Watson reports that "most of the T2 messenger RNA was indeed attached to 'old' ribosomes, as was all the ribosomal bound nascent protein". Hypothesis B wins.
Jacob draws the conclusion: "The same ribosomes can thus make either phage or bacterial proteins, depending on the messenger with which they associate." The paper appeared in Nature in May 1961 as "An unstable intermediate carrying information from genes to ribosomes for protein synthesis".
Was Brenner the only discoverer?
No. Matthew Cobb's history, Who discovered messenger RNA? (2015), points out that no Nobel Prize was given for mRNA and that "simple claims of priority may not reflect how science works". At Harvard, Francois Gros worked with Watson's group and published a companion paper in the same issue of Nature, showing unstable RNA in uninfected bacteria. Watson's lecture adds that T2 messenger was about 2 to 4% of total RNA, with most molecules lasting less than several minutes.
Brenner's own Nobel Prize came much later. In 2002 he shared it with H. Robert Horvitz and John Sulston "for their discoveries concerning genetic regulation of organ development and programmed cell death". In his Nobel lecture he explains why he picked a 1 mm worm, *Caenorhabditis elegans*, with a life cycle of 3 and a half days: he needed an animal small enough, with few enough cells, for its nervous system to be mapped with an electron microscope.
Where does this sit in your syllabus?
The NCERT Class 12 chapter on molecular inheritance covers transcription, the genetic code and translation. Our guide to the molecular basis of inheritance walks through the chapter itself. If the steps from DNA to protein still blur together, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise the one idea that is blocking you.
What can a student learn from how Brenner worked?
Brenner chose tools to fit questions. Phage was ideal for genetics, he said in his Nobel lecture, because "scoring was digital – yes or no". When his question turned to how genes build a nervous system, he went looking for a new organism. In June 1960 he reduced a big question, how genes make proteins, to one he could test in weeks: are the ribosomes old or new? Jacob credits "the extraordinary intellectual and experimental agility of Sydney Brenner" for getting the answer so quickly.
Try it now: write the two predictions from the worked example in your own words, without looking, and say which band each would put the new protein in.
