The size of the genome (the amount of DNA in an organism's genes) varies enormously from one species to another. The smallest sizes, for viruses, ranges from around 2 thousand base-pairs to over a million.
Humans have about 3 billion (forming around 19,000 genes *) - but some plants have more than 50 times as many. The New Caledonian fork fern (Tmesipteris oblanceolate), for example, has 160 billion. (ref.)
The lack of correlation between the apparent complexity of an organism and its genome size is called the G-value paradox
There is currently no explanation for the wild variations in genome size.
These variations have not yet been successfully explained, but may have a relation to the age of the species - i.e. older species tend to have more."
Source : Wikipedia.
" Note: Many organisms have large percentages of DNA (in some cases up to 98%) which appears to be redundant - and has been termed 'Junk DNA'. Its function (or non-function as the name suggests) is currently unclear. See Wikipedia :
Also see: DNA / RNAplugin-autotooltip__plain plugin-autotooltip_bigDNA / RNA
The vast majority of human DNA lies outside of the genes within the cells. Of this, recent work has discovered that 85% of these stretches of DNA appear to make RNA - which, for the most part, have yet-to-be-determined functions.
A study published in the online journal