information transfer that cells use. HIV does not. The very chemical it uses as its genetic material is different from that used by all cells and most viruses.

CENTRAL DOGMA

The genes of most of the world's creatures are stored in the form of a large molecule called deoxyribonucleic acid, or DNA. An individual's (or a cell's) DNA contains all of the information that the cell will require to produce proteins, the real stuff of which the cells are made, and the molecules that accomplish all of the cellular work. Proteins are made of smaller molecules called amino acids. Twenty different amino acids go into the construction of most human proteins. DNA is often called a blueprint, and it tells the cell the correct order in which to string the amino acid building blocks of a particular protein.

In a way, DNA can be considered an archive, a catalog of all the proteins a cell will ever need to carry out its replication, daily maintenance, and the specialized functions it performs. For that reason preservation of a cell's DNA is paramount. Within the cells of all plants and animals the DNA is sequestered in a special compartment called the nucleus, the way a precious book is kept under glass in the library.

Of course, the DNA must be consulted at times when particular proteins must be manufactured. A system has evolved in which the archival DNA is maintained in the nucleus but where the necessary information can be exported outside the nucleus into the large cellular compartment known as the cytoplasm where the proteins are made. Since it would be too risky for the cell to allow its DNA to be transported into the cytoplasm, where it could possibly be damaged, copies of the relevant portions of the DNA are made as the proteins they code for are needed by the cell. The process again is analogous to the library where photocopies of particular pages of the precious book can be made as they need to be read. In the context of the cell, the photocopy is another molecule, chemically very similar to DNA, called ribonucleic acid, or RNA.

When a cell needs to manufacture a particular protein, an RNA duplicate of the DNA coding for that protein is copied, or transcribed, in the nucleus. This RNA copy leaves the nucleus and enters the cytoplasm, where it is "translated" into the proper sequence of amino acids required for the final protein product. When it is no longer necessary to produce the protein, the RNA copy in the cytoplasm is destroyed. Thus, the RNA transcript is as temporary as the DNA is permanent. The DNA



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement