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?What is life? 2013
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vert bar The Central Dogma

In the years after their momentous insight, Crick, Watson and others had discovered many of the principles of how the genetic information was decoded. In a 1958 paper Francis Crick coined the term ‘the central dogma’ to describe what would become a long-held understanding of how DNA operated.

The dogma can be stated 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 → RNA → protein". These three processes are known as transcription (DNA → RNA), translation (RNA → protein), and replication ((DNA → DNA). New knowledge also revealed a fourth step, that of RNA processing, known as splicing.

Transcription is the process by which the information contained in a section of DNA is converted into a molecule known as messenger RNA (mRNA). This is achieved by the action of RNA polymerase and various transcription factors.

In eukaryotic cells the initial molecule of mRNA is processed or ‘edited’. 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.

Translation The action of the ribosome is 
not unlike a knitting machine - the
messenger RNA provides the pattern, and
transfer RNAs bring in the correct amino
acid at each step, with the peptide chain
emerging from the top of the ribosome as 
the finished protein product
Eventually the 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 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 in sets of three bases at a time (triplets or codons). Each codon corresponds to a specific amino acid. The ribosome brings each amino acid attached to a transfer RNA (tRNA) 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 other proteins, called chaperone proteins. Occasionally proteins themselves can be further spliced, when this happens the inside "discarded" section is known as an intein.

The last step in the Central Dogma refers to the way the genetic information is transmitted between parents and progeny. The DNA must be replicated faithfully, just as Watson and Crick had foreseen in their original model of the molecule. Replication is carried out by a complex group of proteins that unwind the superhelix, unwind the double-stranded DNA helix, and, using DNA polymerase and its associated proteins, copy or replicate the master template itself.

Exceptions to the central dogma
The central dogma is not really a dogma in the traditional sense of the word, like all scientific theories it is modified as we learn more details of the processes. The biggest revolution in the central dogma was the discovery of retroviruses, which transcribe RNA into DNA through the use of a special enzyme called reverse transcriptase. Other viruses only use RNA, and not DNA. With the discovery of prions, a further exception to the central dogma has been discovered whereby Protein → Protein. That is proteins directly replicate themselves by making conformational changes in other proteins. Although retroviruses, certain primitive viruses, and prions may violate the central dogma, they are technically not considered "alive", and thus the rule that "all cellular life follows the central dogma" still holds true... for now.