Central dogma of molecular biology

The central dogma of molecular biology was first enunciated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:
 * The central dogma of molecular biology deals with the detailed residue-by-residue transfer of sequential information. It states that such information cannot be transferred from protein to either protein or nucleic acid.

In other words, 'once information gets into protein, it can't flow back to nucleic acid.'

The central dogma is often misunderstood. It is frequently confused with the standard pathway of information flow from "DNA to RNA to protein". There are notable exceptions to the normal pathway of information flow and these are often mistakenly referred to as exceptions to the central dogma.

The standard information flow pathway can be summarized in a very short and oversimplified manner as "DNA makes RNA makes proteins, which in turn facilitate the previous two steps as well as the replication of DNA", or simply "DNA &rarr; RNA &rarr; protein". This process is therefore broken down into three steps: transcription, translation, and replication. By new knowledge of the RNA processing, a fourth step must be included: splicing.



Transcription
Transcription is the process by which the information contained in a section of DNA is transferred to a newly assembled piece of messenger RNA (mRNA). It is facilitated by RNA polymerase and transcription factors.

Splicing
In eukaryote cells the primary transcript (pre-mRNA) is processed. One or more sequences (introns) are cut out. The mechanism of alternative splicing makes it possible to produce different mature mRNA molecules, depending on what sequences are treated as introns and what remain as exons. However, not all living cells have mRNA that undergoes splicing; splicing is absent in prokaryotes.

Translation
Eventually, this mature mRNA finds its way to a ribosome, where it is translated. In prokaryotic cells, which have no nuclear compartment, the process of transcription and translation may be linked together. In eukaryotic cells, the site of transcription (the cell nucleus) is usually separated from the site of translation (the cytoplasm), so the mRNA must be transported out of the nucleus into the cytoplasm, where it can be bound by ribosomes. The mRNA is read by the ribosome as triplet codons, usually beginning with an AUG, or initiator methonine codon downstream of the ribosome binding site. Complexes of initiation factors and elongation factors bring aminoacylated transfer RNAs (tRNAs) into the ribosome-mRNA complex, matching the codon in the mRNA to the anti-codon in the tRNA, thereby adding the correct amino acid in the sequence encoding the gene. As the amino acids are linked into the growing peptide chain, they begin folding into the correct conformation. This folding continues until the nascent polypeptide chains are released from the ribosome as a mature protein. In some cases the new polypeptide chain requires additional processing to make a mature protein. The correct folding process is quite complex and may require