The total amount of DNA within a single set of chromosomes in a cell is called genome size or C-value. Commonly, genome size is measured in picograms or the number of nucleotide base pairs (one picogram of DNA is equal to 978 megabases of DNA). The common methods for determining genome size are: flow cytometry, computerized image analysis, densitometric measurement after Feulgen staining. Recently, due to rapid advancement in sequencing technology and the reduction in cost of sequencing, whole genome sequencing method is becoming popular for the genome size determination tasks. Knowing the size of the genome of interest is important to planning genetic studies of specific species. Also, if someone is interested in sequencing the whole genome of a particular species, knowing the genome size can help to estimate the time and costs of such project. The information about the genome size can provide insights into the evolutionary history of the species.
So how big is the genome? Over the past 50 years, genome size for thousands of Eukaryotes species have been determined and those data are available online in separate databases for fungi, plants, and animals. So far, the largest genome size known for any organism is from a rare Japanese flower, Paris japonica with 149 Gbp, and the smallest genome was found in a bacterium, Hodgkinia cicadicola with 0.14 Mbp. The genome size data for over 15,000 species of Eukaryotes are available. The difference between the smallest and the largest genome sizes in Eukaryotes is over 64,000 folds. Among Eukaryotes, plants are the most studied groups with genome size data available for over 12,000 species, and differences in genome size for plants is about 2,400 folds. In animals, the difference in genome size is over 3,000 folds. The size of the human genome is 3.23 Gbp. As compared to the Eukoryotes and fungi, Prokaryotes have much smaller genome sizes ranging from 0.14 Mbp (Hodgkinia cicadicola) to 14.78 Mbp (Sorangium cellulosum). Most of the Prokaryote genomes consist of a single DNA molecule.
In Prokaryotes, the number of genes identified in the genome is positively correlated with the size of the genome, whereas in Eukaryotes, there is no correlation between the size of genome and the number of genes present. This inconsistency could be due to the presence of large amount of non-coding regions of DNA in Eukaryotes. Also, in some Eukaryotes, scientists have found that many of the genes occur in duplicates giving false impression about the number of genes.