Key Concepts of Section 8.2

• In the genomes of prokaryotes and most lower eukaryotes, which contain few nonfunctional sequences, coding regions are densely arrayed along the genomic DNA.

• In contrast, genomes of multicellular animals and plants contain many sequences that do not apparently code for functional RNAs or have any regulatory function. Much of this nonfunctional DNA is composed of repeated sequences. In humans, only about 2.9 percent of total DNA (the exons) actually encodes proteins or functional RNAs.

• Variation in the amount of nonfunctional DNA in the genomes of different species is largely responsible for the lack of a consistent relationship between the amount of DNA in the haploid chromosomes of an animal or plant and its phylogenetic complexity.

• Simple-sequence DNA, consisting of short sequences repeated in long tandem arrays, is preferentially located in centromeres and telomeres.

• The length of a particular simple-sequence tandem array is quite variable between individuals in a species, probably because of unequal crossing over during meiosis. Differences in the lengths of some simple-sequence tandem arrays form the basis for DNA fingerprinting (see Figure 8-7).