The central dogma of molecular biology defines the uni directional flow of genetic information from DNA through RNA to protein. RNA is made as a copy of a DNA template through a process termed transcription. Translation then follows where the RNA is "read" through triplet codons and polypeptide synthesis occurs by additional of specific amino acids determined by the template RNA. The process of converting DNA into protein involves three distinct types of RNA, transfer (tRNA), messenger (mRNA), and ribosomal (rRNA).
Transcription is the first stage of gene expression in which an mRNA complementary copy of the template DNA strand is formed in the nucleus, initiated when the enzyme RNA polymerase (DNA dependent RNA polymerase), binds to the promoter. The mRNA strand is synthesized continually as the RNA polymerase moves along the DNA until reaching the transcriptional stop signal, when the RNA polymerase and newly synthesized mRNA are disengaged from the DNA. This strand of mRNA is termed the primary transcript. The primary mRNA transcript is then processed to form mature mRNA. A methylated cap is added to the 5' end of the transcript which functions to assist ribosome binding and mRNA stability. At the 3' end a poly (A) tail is added, a stretch of adenosine nucleotides usually 150 to 200 residues in length. Splicing then follows in which introns are removed from the mRNA transcript bringing the coding regions, exons, together to form mature mRNA. This processed mRNA molecule passes into the cyto plasm triggering a ribosome, composed of rRNA and ribosomal proteins, 10 assemble upon it at the AUG start codon. Transfer RNAs associated with specific atmino acid residues, termed aminoacyl tRNAs, are drawn into the ribosome in an order determined by the nucleotide sequence of the mRNA. The amino acid residue is inserted into the fortning polypeptide chain through the codon being read on the mRNA and the complementary anti-codon found on the tRNA. The ribosome moves along the mRNA in a 5' to 3' direction until a stop codon is reached which initiates the release of the complete protein.
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