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The central dogma of molecular biology (sometimes Crick's central dogma after Francis Crick who coined the term and discovered some of the principles) states that the flow of genetic information is "DNA to RNA to protein". With a few notable exceptions, all biological cells conform to this rule.
It 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". 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.
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 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 triplate codons, usually beginning with an AUG, or initiator methonine codon downstream of the ribosome binding site. Complexes of initiation factors and elongation factors bring amino acylated 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 polypepeptide chain requires addtional 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.
Replication Finally, as the final step in the Central Dogma, to transmit the genetic information between parents and progeny, the DNA must be replicated faithfully. 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 so the cycle can repeat DNA → RNA → protein in a new generation of cells or organisms.
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 has resulted in an exception to the central dogma; RNA → DNA → RNA → protein. Also, some virus species are so primitive that they use only RNA → proteins, having not developed DNA. With the discovery of prions, a new exception to the central dogma has been discovered, Protein → Protein. That is, proteins directly replicating 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.
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