SPECIAL

HOW CHANGES IN DNA CAN CAUSE UNUSUAL ABNORMALITIES, BUT ALSO SHAPE THE EVOLUTION OF A SPECIES

WORDS SCOTT DUTFIELD

What makes you who you are? It’s a molecule called deoxyribonucleic acid, better known as DNA. This omnipotent molecule is made up of twisted strands of sugars and phosphates, creating its double-helix shape. Connecting the twisting strands are four bases known as nucleotides: adenine, cytosine, guanine and thymine. The sequence in which these bases find themselves along each strand is what ultimately determines the development and function of an organism.

The way DNA works is similar to the way a computer reads binary code. A molecule of DNA is divided by varying lengths of base sequences called genes. Some genes contain hundreds of bases, others millions. Each gene acts as a set of instructions to program an organism, controlling what it looks like, how its body functions and even how it behaves. The complete set of genetic information in an organism is known as its genome. The human genome is held in 23 pairs of chromosomes, long bundles of DNA which sit inside the nucleus of a cell. The size of an organism’s genome varies across the spectrum of life – for example, a fruit fly has only 4 chromosome pairs, while a dog has 39.

There are two important distinctions within a DNA molecule: genes that are referred to as coding genes and non-coding genes. Around one per cent of DNA is made up of coding genes, which actively supply the information for the production of proteins. Proteins are vital molecules that act as the raw ingredients to grow and develop an organism. The remaining 99 per cent of DNA is made of non-coding genes; these don’t provide the blueprints for protein production but can influence the way in which proteins are made and even prevent production.

Like lines of computer code, when nucleotide sequences within a gene are changed, deleted or swapped, an organism’s biological program is altered. This is known as a mutation. Mutations can be split into one of three categories: silent, missense and nonsense. Silent mutations are, as the name suggests, an alteration of nucleotides that have no protein production. Missense mutations, on the other hand, will alter the function of a protein. Finally, nonsense mutations cause proteins to be nonfunctional. Mutations occur during the time the cells of an organism divide and grow. During cell division, all the DNA information within the nucleus is unzipped and exactly copied to create a perfectly functioning replica. However, on some occasions the genetic duplication isn’t exact – some information is left behind or replaced, and a mutation occurs.