The building blocks of genes

DNA contains the genetic instructions for all living organisms

Our bodies are made up of some five trillion cells with a multitude of functions. Within the nucleus of almost every one of these cells are long molecules called DNA (deoxyribonucleic acid). DNA is like an organic blueprint or book of recipes. Your DNA is a genetic code that contains all the instructions needed to make an organism like you, to renew your cells and to keep your body functioning properly.

The language of life

DNA molecules are found within the nuclei of your cells and are really long chains of building blocks called nucleotides. These chains form the iconic twisting ladder structure (the double-helix) that was discovered by the Nobel laureates Francis Crick and James Watson in 1953. Nucleotides are the ‘rungs’ in the twisting ladder of DNA molecules. The long chains of DNA form the ‘words’ and ‘sentences’ of your genetic code, in which nucleotides are the ‘letters’. Nucleotides come in four different versions, adenine, thymine, cytosine and guanine, usually represented by the letters A, T, C and G, respectively. The human genetic code consists of about 3 billion of these four kinds of nucleotides. Each of your cells contains two copies of this genetic code, one inherited from your mother and the other from your father.

Genes are the most important part of our genetic code

Genes are especially important segments of DNA that directly influence one or more traits. They are relatively small segments of chromosomes, where the sequence of DNA nucleotides encodes a recipe for making a protein. Small differences in the sequence of DNA nucleotides of a particular gene can lead to differences in the structure and behavior of the proteins they encode. It is these differences, in turn, that account for the variable characteristics of the people around you. The human genome has approximately 20,000 – 25,000 genes. Surprisingly, these genes cover only about 5% of our chromosomes. The remaining 95% of our DNA contains either repeated sequences of nucleotides or sequences that have not been revealed as having any function.

This content was last reviewed on February 18, 2010.