SCI:BIO: raw genome length not a good measure of organism complexity

Eugene Leitl (
Mon, 13 Jan 1997 10:40:09 +0100 (MET)

Molecular Biology of the Cell, 3rd ed., Bruce Alberts et al., Garland
Publishing (1994), pp. 339-340

Most Chromosomal DNA does not code for Proteins or RNA

The genomes of higher organisms seem to contain a large excess of DNA.
Long before it was possible to examine the nucleotide sequences of
chromosomal DNA in the haploid genome of an organism, it was evident that
the amount of DNA in the haploid genome of an organism has no systematic
relationship to the complexity of the organism. Human cells, for example,
contain about 700 times more DNA than the bacterium E. coli, but some
amphibian and plant cells contain 30 times more DNA than human cells
(Figure 8-6). Moreover, the genomes of different species of amphibians
can vary 100-fold in their DNA content. [...]

The most important implication of this estimate is that although the
mammalian genome contains enough DNA, in principle, to code for nearly 3
million average-sized proteins (3x10^9 nucleotides), the limited fidelity
with which DNA sequences can be maintained means that no mammal (or any
other organism) is likely to be constructed from more than perhaps 60000
essential proteins.