Genome Size

The fruit fly has four chromosomes and the DNA sequence of these, the genome sequence, was made public in the year 2000. The fly genome contains most all the information for building a fly--a genome is often referred to as a "blueprint" for an organism--written out in a four-letter code.

When we count up the countable units in the fruit fly genome, we find that the four pairs of chromosomes comprise approximately 180 Mega base-pairs (or Mb) of DNA, meaning that 180 million As, Cs, Gs, and Ts that are the individual units of DNA (like the individual snap-beads on a plastic necklace) line up in a particular order to form the four chromosomes of the fly.

For comparison, the 180 Mb Drosophila genome is about sixty times the average size of the genome of a bacterium (about 3 Mb); roughly the same size as the sea squirt genome (120 Mb); around half the size of the puffer fish genome (390 Mb); and about one sixteenth the size of the human genome, which clocks in at 3,000 Mb.

So it's small critter, small genome; large critter, large genome. Right? Not so fast. There is no simple correlation between genome size and the size of an organism.

For example, the European brown grasshopper--a critter about the size of your thumb--has a genome that's one hundred times the size of the fruit fly genome and six times the size of the human genome at 18,000 Mb. Many plants also have large genomes, including rice, 480 Mb; corn, 2400 Mb; and wheat, 16,000 Mb. Some of the factors that influence the size of a genome are the level of redundancy in the information and the amount of padding of DNA base-pairs that separates regions with meaningful information.