Genome size is the total amount of DNA contained within one copy of a single genome. It is typically measured in terms of mass in picograms (trillionths (10^-12) of a gram, abbreviated pg) or less frequently in Daltons or as the total number of nucleotide base pairs typically in megabases (millions of base pairs, abbreviated Mb or Mbp). One picogram equals 978 megabases. In diploid organisms, genome size is used interchangeably with the term C-value. An organism's complexity is not directly proportional to its genome size; some single cell organisms have much more DNA than humans (see Junk DNA and C-value enigma).