The Central Dogma (DNA -> RNA -> Protein)
DNA segments in a chromosome encode genes that are transcribed into messenger RNA and pushed out of the nucleus, where they encounter ribosomes and are translated into proteins. This is a fundamental process of molecular biology that produces a vast array of proteins, and the diversity of life.
The informational relationships among DNA, RNA, and protein are intertwined: DNA directs the synthesis and sequence of RNA, RNA directs the synthesis and sequence of polypeptides, and specific
proteins are involved in the synthesis and metabolism of DNA and RNA. This flow of information is referred to as the “central dogma” of molecular biology.
DNA contains the complete genetic information that defines the structure and function of an organism. Proteins are formed using the genetic code of the DNA. Three different processes are responsible for the inheritance of genetic information and for its conversion from one form to another.
1. Replication : a double stranded nucleic acid is duplicated to give identical copies. This process perpetuates the genetic information.
2. Transcription : a DNA segment that constitutes a gene is read and transcribed into a Vierstraete Andy (version 1.01) 1/02/2000 -Page 2 – single stranded sequence of RNA. The RNA moves from the nucleus into the cytoplasm.
3. Translation : the RNA sequence is translated into a sequence of amino acids as the protein is formed. During translation, the ribosome reads three bases (a codon) at a time from the RNA and translates them into one amino acid.
In eucariotic cells, the second step (transcription) is necessary because the genetic material in the nucleus is physically separated from the site of protein synthesis in the cytoplasm in the cell. Therefore, it is not possible to translate DNA directly into protein, but an intermediary must be made to carry the information from one compartment to an other.
Genetic information is stored in DNA by means of a code (the genetic code, discussed later) in which the sequence of adjacent bases ultimately determines the sequence of amino acids in the encoded polypeptide. First, RNA is synthesized from the DNA template through a process known as transcription. The RNA, carrying the coded information in a form called messenger RNA (mRNA), is then transported from the nucleus to the cytoplasm, where the RNA sequence is decoded, or translated, to determine the sequence of amino acids in the protein being synthesized. The process of translation occurs on ribosomes, which are cytoplasmic organelles with binding sites for all of the interacting molecules, including the mRNA, involved in protein synthesis. Ribosomes are themselves made up of many different structural proteins in association with a specialized type of RNA known as ribosomal RNA (rRNA). Translation involves yet a third type of RNA, transfer RNA (tRNA), which provides the molecular link between the coded base sequence of the mRNA and the amino acid sequence of the protein.