| The idea that each organism possesses a unique | | | | on RNA for transport. Significantly, it focuses on |
| genetic code which is passed to future generations | | | | protein as the product of gene expression, an idea |
| was hypothesised long before the actual | | | | that is now well understood due to successful |
| mechanisms, or indeed the source, of such inheritable | | | | mapping of the genome. |
| information was agreed. After Mendel (1857) | | | | Modern discoveries have highlighted that the flow of |
| demonstrated that phenotypic characteristics could | | | | genetic information is much more dynamic. For |
| be transferred from parent to offspring, Fred Griffith | | | | example, some RNA does not code for protein and |
| set out in 1928 to find evidence that the molecule | | | | instead is destined to remain as an RNA nucleotide. |
| responsible for preserving and transmitting this | | | | This type of RNA is known as functional or ncRNA - |
| information was DNA. This was later confirmed | | | | i.e. non-coding - such as tRNA and rRNA. Additionally, |
| conclusively by experiments conducted by Avery and | | | | ribozymes can act as catalysts, performing their own |
| (later) by Hershey & Chase, establishing that | | | | 'protein' functions without ever completed the |
| DNA was indeed the arsenal of heritable information. | | | | sequential route all the way to protein. Another |
| From this affirmation and the knowledge that RNA | | | | modern extension of the dogma is that RNA can also |
| was the immediate precursor for protein, Francis | | | | act as a template for DNA synthesis. This process is |
| Crick proposed in a 1954 paper - and reasserted in | | | | known as reverse transcription which uses the |
| 1970 - formulated a flow diagram which has become | | | | enzyme reverse transcriptase and has been |
| known as the central dogma of molecular biology: | | | | demonstrated to exist in retroviruses. These findings |
| DNA to RNA to Protein. | | | | are important for the development of the theory as |
| The central dogma is essentially a framework that | | | | it confirms Crick's postulations that RNA could go |
| outlines the transfer of sequential information from | | | | back to DNA and further extends our understanding |
| storage as DNA to the expression of that | | | | of the central concept. |
| information as a functional entity as protein. Most | | | | The importance of the central dogma as a concept is |
| importantly, it dictates that information can only flow | | | | perhaps best illustrated, somewhat paradoxically, by |
| from nucleic acid to protein, and not from protein to | | | | the one discovery which directly challenges it. In his |
| nucleic acid i.e. that "once (sequential) information has | | | | original assertions, Crick explicitly stated that 'transfer |
| passed into protein it cannot get out again (F.H.C. | | | | from protein to protein' was impossible. It is now |
| Crick, 1958). At the time of print, all evidence | | | | widely accepted that infectious proteins known as |
| suggested that this transfer or flow of information | | | | prions, previously thought to be viral in nature, are |
| occurred linearly, however modern advances in | | | | built directly from protein by triggering abnormal |
| molecular biology and genetics have shown that this | | | | synthesis from its native form. This initially sparked a |
| idea was too simple. | | | | myriad of papers postulating counter theories, based |
| The interrelationship between these three important | | | | on the assumption that protein self replication |
| molecules may be more complex than once thought, | | | | violated the central dogma. It is clear from such |
| however the essential concept still holds true. All | | | | actions that the central dogma has core significance |
| organisms (with the exception of some viruses, | | | | that has developed way beyond the actual |
| which use RNA) use DNA as a storage facility for | | | | statements in Crick's original paper. Scientific |
| their genetic information. This information, which in | | | | discovery in every field is fueled by the desire to find |
| reality is triplet base pair codons, is then used a | | | | simple, underlying theories that can explain the many |
| template to be faithfully transcribed to an | | | | complexities of their endeavors in an uncomplicated |
| intermediate RNA. Once transcription is complete, it | | | | theoretical framework. We know that DNA is the |
| can then be translated in the ribosome to a | | | | root of our heritable information and we know that |
| corresponding amino acid sequence that codes for | | | | the functional units that make life possible are |
| the assembly of a functional protein. The dogma | | | | proteins. Despite requiring some adjustment to the |
| infers that it is DNA that directs the development of | | | | finer detail Crick's version of the central dogma |
| the organism and that protein formation ultimately | | | | certainly laid such a foundation, giving biologists a |
| relies on the DNA sequence. Additionally, the concept | | | | central concept to both orientate from and build |
| is also important in that it stresses that the | | | | around. |
| information contained within DNA must first depend | | | | |