These exons are shuffled, seemingly at random, into a section of chromosomal DNA as many as a million letters long.
A typical human or mammalian gene can have 10 exons or more.
Recently, scientists reported a gene written in 54 separate exons. Another, linked to breast cancer, has 27.
"This situation is comparable to a magazine article that begins on page one, continues on page 13, then takes up again on pages 43, 51, 53, 59, 70, 74, 80 and 91, with pages of advertising and other articles appearing in between," Pevzner said. "We don't understand why these jumps occur or what purpose they serve. Thankfully, like a maga-zine, the exons stay in order. They don't jump backward. You always read in the same direction."
The jumps are inconsistent from species to species. An "article" in an insect edition of the genetic magazine will be printed differently than the same article appearing in a worm edition. "The pagination will be completely different," Pevzner explained, "and it will not be con-sistent:
The information that appears on a single page in the human edition may be broken up into two in the wheat version, or vice versa."
Pevzner noted yet another complication. "The genes themselves, while related, are quite different. The mouse-edition gene is written in mouse language; the human-edition gene in human language. It's a little like German and English, which are related languages: many words are identical or similar, but many others are not. Nevertheless, to find the analogous genes, we must be able to recognize these differently spelled words written on different pages as the same message."
Even there, the complications
do not end. If it were just a matter
of picking out a known "magazine
story," whether in mouse-DNA language
or human-DNA language,
from intervening material that
'"/>
Contact: Eric Mankin
mankin@usc.edu
213-740-9344
University of Southern California
20-Aug-1996