DNA is the hereditary material in all living organisms and is found in every single living cell (except some viruses that contain the related RNA).
This DNA is usually packaged into structures called chromosomes, which were first observed using a special dye; hence the name Gr. chroma, colour; soma, body).
DNA can be a very long molecule made up of a variable sequence of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These pair up with each other (A with T and C with G) to form units called base pairs which together are arranged in two long strands forming a spiral called a double helix.
DNA can replicate - each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. Therefore, when cells divide each new cell has an exact copy of the DNA.
Human cells contain around 3 billion of the four different bases forming DNA. These are arranged in a specific, yet unique, order to each of us. E. coli cells contains 4,6 million base pairs, the fruit fly has 160 million base pairs and the dog 2.4 billion. However, the size of the genome often bears little relation to the complexity of the organism - cells of the onion contain five times as much DNA as a human cell!
Human DNA contains an estimated 22,000 genes. A gene is the region of DNA sequence that can code for a protein; this information stored is transferred to a similar molecule called RNA which carries the information out of the nucleus and into the cell where it is used to assemble amino acids to form a protein.
How do many organisms, seemingly less “advanced” than ourselves, have more genes than we do? While sharing over 99.9% identical gene sequences among ourselves as a species, we also share about 96% of our genes with chimpanzees, 80% with mice, 75% with dogs, 50% with the fruit fly (Drosophila) and 30% with a simple yeast!
One explanation for how this might be is that many genes are capable of making more than one protein, allowing human cells to make perhaps 80,000-100,000 proteins from our genes.
But the important take-home message is that what actually defines us genetically is not how much DNA an organism contains, or even how many genes are present; it is the complexity of how our genes are used and how these genes are controlled to interact with one another to carry out the various functions that give us our unique characteristics as humans.