Virtually all proteins are very complex and consist of very many amino acids joined together in a very precise sequence. For example, human hemoglobin, the essential constituent of blood, is a complex molecule consisting of two chains of 141 amino acids each joined to two other chains of 146 amino acids each - or a total of 574 amino acids.

DNA makes protein
The manufacture of chemicals needed by a cell begins at the DNA. To make hemoglobin, for example, a group of chemicals come to a particular location on the very complicated blueprint that is the DNA molecules. They split apart the strands of the DNA at the right location, revealing the information at that location and enabling the relevant portion of the message to be copied.

After the message is copied chemically, other chemicals have to "translate" the message into a functional protein molecule by joining together the precise amino acids in the precise sequence called for by the blueprint. Because there are only 20 amino acids that can be used as the building blocks of all proteins, the DNA contains 20 sets of "words," one for each amino acid, and one other set of "words" which instructs the chemical factory to stop production. These twenty one sets of words are written by the four "letters" of the DNA language. Each word consists of three letters, i.e. three "letter" molecules next to each other along the length of the DNA.

Perfect copying is important
A single error in the DNA can sometimes lead to death as it can cause the substitution of one amino acid for another or a change in their sequence. A disease that afflicts over 50,000 blacks in this country, sickle cell anemia, and is often fatal, has been traced to a single 'error' in the DNA which causes the substitution of one amino acid for another on one of the hemoglobin chains. Because accuracy in copying is so important, in the cells of higher life forms there exist special molecules whose function is to safeguard that as few errors as possible find their way into the DNA.