There was a time when only scientists talked about DNA (Deoxyribonucleic acid) now it has entered everyday conversation. DNA plays a vital and complex role in living organisms, but what is it?
DNA is a double stranded nucleic acid. It is present in most living organisms and determines their hereditary characteristics.
The structure of DNA resembles a ladder twisted into a spiral shape; this structure is described as a double helix. The two backbone strands consist of long polymers of nucleotides composed of sugar (deoxyribose) and phosphorus. The two strands are linked by hydrogen bonds between 4 types of complementary bases - adenine (A), guarine (G) cytosine (C) and thymine (T).
In a human there are approximately 3 billion DNA base pairs but only a small proportion is coding DNA. The linear sequencing or grouping of the four bases provides a code that controls protein synthesis in cells, thus providing a blueprint for building and controlling cells. A gene is a length of encoded DNA which provides instruction to produce a certain characteristic eg hair colour, eye colour.
Within a single animal or organism, the DNA is present and the same in the nucleus of its individual cells, even if the cells perform different functions eg muscle cells, skin cells etc. Asexual reproducing organisms have one set of chromosomes. The body cells of sexual reproducing organisms have matched pairs of chromosomes, one from the mother and one from the father.
Chromosomes are essential to control cell division. In a process called mitosis one cell splits into two daughter cells, all three cells are genetically identical. However reproductive cells must have only half the usual number of chromosomes, so a process called meiosis divides one cell into 4 daughter cells, which contain half the usual number of chromosomes. Due to the crossing over of chromosomes in this process, the cells differ genetically from each other and the parent cell.
Most DNA is found in the nucleus of an organism's cell, however DNA is also found in the mitochondria and this is called mitochondrial DNA (mtDNA). Mitochondria are organs within the cell which metabolise food into useable energy. While nuclear DNA is inherited from both the mother and father, mtDNA is inherited from the mother only. So male and female offspring from the same mother will have identical mtDNA, the females will pass on their mtDNA but the males cannot. Because the mtDNA is passed from mother to female offspring without recombination and the only changes occur through random mutation, it is possible to trace maternal lineage far back in time.
Many species share a core set of DNA, it is estimated that humans and chimpanzees share 96-98% DNA. Some segments of uncoded DNA (so called junk DNA) are identical in vertebrates and appear to be completely unchanged by evolution.