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NCERT says Francis Crick proposed the central dogma, "which states that the genetic information flows from DNA to RNA to Protein". That is a useful summary for an exam answer, but it is not quite what Crick said, and the difference explains why reverse transcription does not break the rule. This post goes back to Crick's own words and turns them into a way of answering questions.
Who was Francis Crick?
According to his Nobel Prize facts page, Francis Harry Compton Crick was born on 8 June 1916 in Northampton, England, and died on 28 July 2004 in San Diego. He shared the 1962 Nobel Prize in Physiology or Medicine with James Watson and Maurice Wilkins "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material". At the time he was at the MRC Laboratory of Molecular Biology in Cambridge.
What did Crick say in 1957?
Matthew Cobb's essay 60 years ago, Francis Crick changed the logic of biology reconstructs the lecture. Crick gave it on 19 September 1957 at University College London, and it was published in 1958 as "On protein synthesis". Three ideas stand out.
Information means sequence. Crick defined genetic information as "the determination of a sequence of units".
The sequence hypothesis. He argued that the most likely source of a protein's folded shape is simply the order of its amino acids.
The central dogma. In a private note from October 1956, he wrote: "Once information has got into a protein it can't get out again."
Cobb lays out Crick's scheme. Four transfers clearly happened: DNA to DNA, DNA to RNA, RNA to protein, and RNA to RNA in RNA viruses. Two had no evidence but were possible: DNA directly to protein, and RNA to DNA. Three were ruled out: protein to protein, protein to RNA and protein to DNA. Those three bans are the central dogma.
Did reverse transcriptase break the rule?
In June 1970, Howard Temin and Satoshi Mizutani reported an RNA-dependent DNA polymerase in Rous sarcoma virus, and David Baltimore reported the same kind of enzyme in RNA tumour viruses. These enzymes copy RNA into DNA. A Nature editorial called it "Central dogma reversed".
Crick answered in Central dogma of molecular biology (1970). As Cobb summarises, he repeated what he had said in 1957 and pointed out that he had never claimed RNA to DNA was impossible. Crick also admitted the word "dogma" was a poor choice. A better description, he said, would have been "basic assumption". Baltimore and Temin shared the 1975 Nobel Prize in Physiology or Medicine with Renato Dulbecco.
Why does the simple arrow confuse students?
A study by Wright, Fisk and Newman (2014) asked undergraduates what the arrow in "DNA to RNA" means. 36% of 128 students described transcription as DNA being chemically converted into RNA, or suggested the RNA existed before transcription began. The arrow means information is copied. The DNA template is unchanged.
Worked reasoning: does it break the central dogma?
Use Crick's rule: ask whether sequence information is leaving a protein.
A retrovirus copies its RNA genome into DNA. RNA to DNA. Crick listed this as possible. Not a violation.
An RNA virus copies its RNA genome. RNA to RNA, one of Crick's four known transfers. Not a violation.
A ribosome reads mRNA to make a protein. RNA to protein, the normal direction. Not a violation.
A prion protein makes normal copies of the protein misfold. Cobb notes that this involves a change in shape, not the making of a new sequence. Not a violation.
A cell reads a protein's amino acid order and builds a matching mRNA. Protein to RNA. This would break the dogma, and no such mechanism is known.
Common mistakes:
Calling reverse transcription a violation. It is an exception to the NCERT flow diagram, not to Crick's ban.
Treating "dogma" as unquestionable. Crick meant a working assumption.
Confusing information with matter. DNA is not turned into RNA. Its sequence is copied.
For the exam, write NCERT's version and mention reverse transcription when the question asks about exceptions. The NCERT Class 12 chapter itself asks you to name the reverse flow in some viruses.
What else did the lecture predict?
Cobb points out two forecasts. Crick and Sydney Brenner proposed small "adaptor" molecules that would carry amino acids to the template and pair with it, which turned out to be tRNA. Crick also predicted a field he called "protein taxonomy", comparing protein sequences between species to read evolutionary history. Our posts on the genetic code and translation and DNA replication and transcription cover the chemistry he could only guess at.
What can a student take from his way of working?
Define your terms. Crick's precise idea of "information" is what made the dogma testable.
State what would prove you wrong. His three banned transfers are clear predictions.
Admit loose wording. He later said "basic assumption" would have been a better phrase.
Checking your understanding
If the difference between transcription, translation and reverse transcription still blurs, study each as a separate idea. The Fermi Concept Library has every concept in biology with its own lesson and its place in the map, so you can see how the pieces connect.
Try it now: draw Crick's nine possible transfers between DNA, RNA and protein, and mark which three his rule forbids.
