The size of the genome (which can be said to be the amount of DNA) varies a great deal from one species to another. The smallest size, for viruses, varies from 2 thousand base-pairs to over a million.
Humans have around 3 billion - but some plants have more than 10 times as much.
There is no explanation for this wild variation in genome size.
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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 that much of lifeform's DNA has been termed 'Junk DNA' - in some cases over 98%. The function of which (or non-function as the name suggests) is currently controversial.
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