Size of the Eukaryotic Genome

DNA complexity/size - The DNA of higher eukaryotes must code for all of the specialized proteins found in different tissues, so such organisms should have considerably more DNA than is found in a typical prokaryote such as E. coli. What is surprising, however, is how much more DNA there actually is. Most eukaryotic cells contain 100 to 10,0000 times as much DNA as E. coli does (Figure 28.1).

The only exceptions are found among the lower fungi, such as yeast and Neurospora, which do not exhibit the complex tissue differentiation found in higher plants and animals. The profound differences between the genomic needs of multicellular and unicellular organisms is reflected in the genome sizes. The entire yeast genome contains 12.07 x 106 bp, only 2.5 times the size of the E. coli genome. It takes at least 108 bp to describe an insect, on the other hand, and about 109 bp to describe a mammal.

No apparent correlation to organismal complexity - There is no simple correlation between the quantity of DNA in a plant or an animal and the complexity of the organism. An amphibian is certainly no more complex a being than a human, yet some amphibia have genomes over 50 times larger than the human genome. The fact that the human genome must code for many more proteins than a bacterial cell (probably about 20 times as many) cannot alone explain its 1000-fold greater size. Therefore, much DNA is present in eukaryotes that does not code for protein.

See also: Repetitive Sequences, Introns, Telomeres

INTERNET LINKS:

> 1. Yeast Genome Sequence

> 2. E. coli Genome Center

> 3. Human Genome Project

> 4. Genome Organization