Biology  5/e   Raven/Johnson
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The nucleus of a cell is somewhat analogous to the office of an architect. In the form of DNA, the master plans for the molecules that are to be made by the cell, are housed and protected in the nucleus. A disposable working copy of the plan, transcripted as a coded sequence complementary to the master plan in a molecule of messenger RNA, is sent to the construction site. The site is the ribosome, which has its own instruction set in the molecules of ribosomal RNA. Polypeptides that may be either end products or enzymes needed to make the end product, are formed by the ribosome from amino acids delivered by molecules of transfer RNA. The process is not quite as straight forward in eukaryotes as it is in prokaryotes. However, the system of checks and balances that make DNA replication work is remarkable.

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