The size of the genome (which can be said to be the amount of DNA in an organism's genes) varies enormously from one species to another. The smallest size, for viruses, varies from 2 thousand base-pairs to over a million.
Humans have about 3 billion (forming around 19,000 genes) - but some plants have more than 10 times as much. The lack of correlation between the 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.
Further reading Wikipedia
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.
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.
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