The central dogma of molecular biology describes the one-way flow of genetic information inside cells: DNA is copied into RNA, and RNA is used to build proteins. It’s the foundational rule for how your body reads and uses the instructions stored in your genes. Francis Crick first proposed the idea in 1957, and it remains the organizing principle of modern genetics.
What Crick Actually Meant
The term “central dogma” sounds grand, but Crick’s original statement was surprisingly narrow. In his 1957 paper, he wrote: “Once information has got into a protein it can’t get out again.” By “information,” he meant the specific sequence of building blocks that make up a protein. The dogma wasn’t really about the direction DNA to RNA to protein. It was about what can’t happen: proteins cannot pass their sequence information back to DNA or RNA.
This distinction matters because it’s been widely misunderstood. When scientists discovered in 1970 that certain viruses could copy RNA back into DNA, the journal Nature ran an editorial titled “Central Dogma Reversed.” Crick responded with a somewhat irritated correction, pointing out that he had never claimed RNA-to-DNA transfer was impossible. What he had claimed, and what still holds, is that proteins never reverse-dictate information back into nucleic acids.
The Three General Transfers
In a 1970 paper in Nature, Crick laid out three categories of information flow. The “general transfers” are the ones that happen in essentially all cells:
- DNA to DNA (replication): a cell copies its entire genome before dividing.
- DNA to RNA (transcription): a gene’s information is copied into a messenger molecule.
- RNA to protein (translation): that messenger molecule is read to assemble a chain of amino acids into a protein.
These three steps are the backbone of gene expression, and every living cell on Earth relies on them.
How Replication Works
Before a cell divides, it needs a complete copy of its DNA. An enzyme called DNA polymerase walks along each strand of the double helix and builds a matching partner strand, one building block at a time. It always works in the same direction along the strand (5′ to 3′, in biochemistry shorthand), and no enzyme has ever been found that works the other way. This one-directional constraint means that one of the two new strands has to be built in short segments that are stitched together afterward, a process sometimes called “backstitching.”
DNA polymerase also proofreads its own work. If it adds the wrong building block, it can reverse a step, remove the mistake, and try again. This self-correction is a big reason why DNA replication is remarkably accurate.
How Transcription Works
When a cell needs to use a gene, it doesn’t read the DNA directly. Instead, an enzyme called RNA polymerase reads one strand of the DNA and builds a single-stranded RNA copy of that gene. This RNA copy, called messenger RNA (mRNA), is then processed and sent out of the nucleus into the main body of the cell.
Transcription happens in a repeating cycle: the enzyme binds the next nucleotide building block, forms a chemical bond to attach it to the growing RNA chain, releases a byproduct, and shifts forward one position along the DNA. This cycle repeats thousands of times per gene. Human cells have multiple types of RNA polymerase, each responsible for producing different kinds of RNA.
How Translation Works
Translation is where the cell’s protein-building machinery reads the mRNA and assembles a protein. The key players are ribosomes (molecular machines that hold everything in place) and transfer RNAs (small adapter molecules that act as translators between the RNA code and amino acids).
Each transfer RNA has two important regions. One end carries a specific amino acid. The other end has a three-letter code that matches a corresponding three-letter code on the mRNA. The ribosome moves along the mRNA, and at each step a transfer RNA with the matching code slots into position. The ribosome then links the amino acid to the growing protein chain and advances to the next code. This continues until the ribosome hits a “stop” signal, at which point the finished protein is released.
Not All RNA Becomes Protein
One surprising wrinkle in the central dogma’s simple framework: around 98% of all the RNA produced by human cells is never translated into protein. Only about 2% of the genome’s output codes for proteins. The rest, called non-coding RNA, performs a wide range of jobs on its own, from regulating which genes get turned on to helping build the ribosome itself. The discovery that the vast majority of genetic output doesn’t follow the classic DNA-to-RNA-to-protein pathway has pushed biologists to think of the central dogma as a core rule with a lot of nuance surrounding it.
Exceptions Crick Predicted
Crick categorized certain information transfers as “special,” meaning they happen in unusual circumstances rather than in every cell. Two of these have been confirmed.
The first is reverse transcription, where RNA is copied back into DNA. Retroviruses like HIV depend on this. When HIV infects a cell, it uses an enzyme called reverse transcriptase to convert its RNA genome into DNA, which then inserts itself into the host cell’s own genome. This is how the virus establishes a persistent infection and why it’s so difficult to eliminate. The discovery of reverse transcriptase in 1970 by Howard Temin and David Baltimore initially shocked the scientific community, though it never actually violated Crick’s original rule. Information was still flowing between nucleic acids, not backward out of a protein.
The second is RNA-to-RNA copying, which some RNA viruses use to replicate their genomes without ever converting to DNA.
Prions: The Real Challenge
The closest thing to a genuine violation of the central dogma comes from prions. Prions are misfolded proteins that can force normal copies of the same protein to adopt their abnormal shape. They cause fatal brain diseases, including mad cow disease and Creutzfeldt-Jakob disease. The misfolded prion acts as a template: it binds to a normal protein and somehow dictates that the incoming molecule refolds into the same pathogenic shape. This is protein-to-protein information transfer, one of the three flows that Crick said should never occur.
Prions carry no DNA or RNA. They replicate purely by reshaping other proteins, and they can amplify themselves more than ten-billion-fold in an infected individual. The specific geometry of the misfolded seed dictates the shape each new molecule adopts. Whether this truly counts as “sequence information” in Crick’s sense is still debated, since prions don’t change the amino acid sequence of their targets, only the three-dimensional folding. But it’s the strongest challenge to the central dogma’s boundaries that biology has produced.
The Central Dogma in Modern Medicine
The framework Crick described isn’t just a textbook concept. mRNA vaccines, including the COVID-19 vaccines developed by Pfizer-BioNTech and Moderna, are a direct application of the central dogma’s logic. These vaccines deliver a piece of synthetic mRNA into your cells. Your ribosomes read that mRNA and produce a viral protein (in this case, the coronavirus spike protein). Your immune system then learns to recognize that protein and mount a defense.
The mRNA never enters the nucleus and never alters your DNA. It simply participates in the RNA-to-protein step of the central dogma, then gets broken down. The entire technology depends on the predictable, one-directional flow of information that Crick described more than 60 years ago.