The two polymers are linked to each other by hydrogen bonds to form a double helix . These are the AT and CG base pairs . Generally speaking, the only thing that makes DNA from a human different from the DNA from a dog, an oak tree, a fungus or a bacterium is the order of the four nucleotides. In this model carbon is gray, nitrogen is blue, oxygen is red, and phosphorus is orange. In this tutorial you will mostly see lists of letters.
Genes are regions of DNA that serve some sort of function usually via the proteins they encode.
Genes were initially defined by their function - a gene could be identified when a mutation in a given gene caused an observable phenotype. Now we can locate genes on chromosomes simply by their sequence. Deciphering the function of a gene identified in this way involves comparing it's sequence to other known genes and observing the phenotype of the organism in which that gene has been disrupted. Also, unlike DNA, RNA usually exists as a single stranded molecule. After the mRNA for a particular gene is made it is used as a template with which ribosomes synthesize the protein in a process called translation .