Deoxyribonucleic acid (DNA) was first observed by a German biochemist named Frederich Miescher in 1869. However for many years, researchers did not realize the importance of this molecule. It was not until 1953 that James Watson, Francis Crick, Maurice Wilkins and Rosalind Franklin figured out the structure of DNA – a double helix – which they realized could carry biological information.
DNA is a molecule that contains the instructions an organism needs to develop, live, and reproduce. These instructions are found inside every cell and are passed down from parents to their children.
DNA is a two-stranded molecule that appears twisted, giving it a unique shape referred to as the double helix. Each of the two strands is a long sequence of nucleotides or individual units made of a phosphate molecule, a sugar molecule called deoxyribose which contains five carbons, and a nitrogen-containing region.
There are four types of nitrogen-containing regions called bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The order of these four bases forms the genetic code which carries information to the cell telling the cell how to behave.
The bases of the two strands of DNA are stuck together to create a ladder-like shape. Within the ladder the bases are in pairs – A always sticks to T, and G always sticks to C. These base pairs create the “rungs.” Human DNA has 3 billion base pairs. The length of the ladder is formed by the sugar and phosphate groups.
Similar to the way the order of letters in the alphabet can be used to form a word, the order of nitrogen bases in a DNA sequence forms genes which tells cells how to make proteins in the language of the cell.
Strands of DNA are looped, coiled, and wrapped around proteins called histones. In this coiled state, it is called chromatin. Chromatin is further condensed through a process called supercoiling and it is then packaged into structures called chromosomes. Each chromosome contains one DNA molecule. Humans have 23 pairs of chromosomes or 46 chromosomes in total. Half the chromosomes are inherited from one parent and half from the other. As humans we therefore have 23 chromosomes from each parent. This explains why organisms can share characteristics from both parents. A child might have brown hair like her father and short height like her mother.
Each cellular organism has DNA, although it differs in size, content, and complexity. Scientists have determined that in many higher organisms, only about 1 to 3 percent of the DNA molecule is made up of genes. A gene is a specific sequence of bases that function as a unit, carrying particular instructions for the body (or mind). Our genes can determine a myriad of “traits” including the height of our body, the colour of our eyes and hair.
An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.