- Written by Nadia O'Carroll
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. The strands of DNA in the nucleus of an average human cell if stretched would measure about 3 metres. The DNA can fit into a cell’s nucleus because it is packed very tightly and compactly into bundles called chromosomes. 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. The number of chromosomes in a cell’s nucleus varies considerably from species to species eg humans have 46, dogs 78, kingfishers 132, goldfish 100-104.
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. This promotes genetic diversity and explains why the same parents do not produce offspring who are identical to each other.
The investigation of DNA and genetics continues to reveal information about the links between species. Many species share a core set of DNA, it is estimated that humans and chimpanzees share 96-98% DNA. Other findings are more unexpected for instance the DNA of coral is more similar to human DNA than it is to worms or fruit flies, and some segments of uncoded DNA (so called junk DNA) are identical in vertebrates and appear to be completely unchanged by evolution.