DNA of all organisms above the level of bacteria and blue-green algae is retained as "archive material" within the nucleus of the cell, whereas protein synthesis takes place outside the nucleus in the cytoplasm, or cell fluid. Information is carried from DNA to the site of protein synthesis by appropriately named messenger RNA (mRNA). Messenger RNA contains a copy of the base sequence from one strand of DNA, with the minor change of a substitution of thymine for uracil. The copying of genetic information from DNA to RNA is called transcription, and the subsequent use of messenger RNA base sequences to synthesize specific protein chains is called translation. The transcription and translation processes are diagrammed HERE..

During transcription, a local unwinding of the double-helical DNA occurs, thereby giving access to the strand to be copied. An RNA polymerase enzyme travels along the DNA strand, adding complementary nucleoside triphosphates to build a messenger RNA strand that is the complement of the original DNA. The completed mRNA strand falls away from the DNA and diffuses out of the nucleus to ribosomes, where translation into a polypeptide chain takes place. The nucleus resembles a rare book room of a library, in which the books themselves cannot be checked out, but photocopies of selected parts may be made for use and eventual discard outside the library.

Ribosomes are RNA-protein complexes, 200Å in diameter with an overall molecular weight of 3,600,000. Their role is to read the codon information on mRNA and use it to make the corresponding polypeptide chain. With an electron microscope we can see several ribosomes spaced down the same length of mRNA like locomotives down a track, puffing their protein chain behind them. Ribosomes have one problem that the RNA polymerase enzyme does not: translating from one language (nucleic acid sequence) into another (amino acid sequence), with the symbols in the two languages in a 3 to 1 ratio. The translating units are small molecules of transfer RNA (tRNA). Each amino acid has one or more kinds of tRNA. On one end of the tRNA molecule is an anticodon of three bases that is complementary to the codon for an amino acid, and at the other end is a binding site for that particular amino acid. The tRNA molecule is therefore a coupler, making sure that the right amino acid is matched with the right triplet codon. Each tRNA molecule has its own "charging enzyme" that mates tRNA and amino acid before the charged complex migrates to the ribosome and is fed into the growing chain.