Within its genome, an organism must encode all of the genes necessary for maintaining life and for multiplying. As organisms increase in complexity, the size of their genomes and the total number of genes tend to increase. While there are exceptions to this trend, organism complexity can still be useful for the estimation of genome size.

Genome is the total collection of genes present in an organism. This genome actually controls the characters of an organism. The bigger the organism, the bigger is the number of characters. So, more genes will be needed to control the genetic features of the organisms. So this will lead to more amount of genes, and larger genome.

Among the organisms present here, the smallest is the bacteria E coli. Then comes the yeast Saccharomyces. Then the Nematode which is smaller than Drosophila, the fruit fly. Then comes the mouse, then the rice plant and the Mousear plant. The biggest among them are humans.