Nature of Genetic Coding

Have you ever played with "secret decoder rings" that you sometimes get in cereal boxes? Or perhaps you have played games where the object of the game is to figure out a code - to discover what the various symbols or letters represent. In order to discover the genetic code in humans, scientists first had to learn about the structures that contain the genetic code. These structures are called chromosomes, and they occur in the nucleus of cells.

Chromosome Types in Humans

Images found at The Genetics of Cancer at http://www.cancergenetics.org/chrm-abe.htm

Genes

The coding for inheritance originates in the sequence of molecules in the DNA of chromosomes. "DNA" stands for deoxyribonucleic acid. This name comes from the fact that the compound is acidic and found in the cell nucleus ("nucleic acid") and is built up of a 5-carbon sugar, called ribose, that lost an oxygen atom when it was generated ("deoxyribose"). A "gene" is that length of DNA that is needed to code for a complete protein. There are only four coding molecules in DNA: 

• adenine (A, for short) 
• thymine (T)
• cytosine (C) 
• guanine (G)

They occur in bonded pairs, either A-T or C-G, called base pairs. The base pairs are held together like a ladder by hydrogen bonds (dashed lines in diagram above). A "gene map" shows the sequence of pairs, for example in the diagram to the right:: C-G, A-T, G-C, T-A.

We have said that genes provide the code for making proteins. A gene is a section of the chain of A-T and C-G sequences that codes for a particular protein. Proteins are made as a string of compounds called amino acids (see the module on proteins). It takes three base pairs to code for a given amino acid. Thus, the first three pairs code for one amino acid, the next three pairs code for another amino acid, and so on, until the complete sequence of a protein is accounted for.