Highly repetitive genome regions are notoriously difficult to assign to a particular chromosome, let alone sequence, because assembly programs can overlap them in many ways and places. So it is for the telomeres, the tips of chromosomes that in humans and other vertebrates consist of repeats of the sequence TTAGGG.2 "Using shotgun cloning, most of the human genome has been sequenced. However, this approach has missed most of the regions near telomeres, because repetitive sequences do not clone well. The subtelomeric regions will be one of the last aspects of the human genome to be completed," says Jerry Shay, a professor of cell biology at the University of Texas Southwestern Medical Center in Dallas.
The exclusion of the telomeres and much of their environs from the draft sequence is largely because the bacterial artificial chromosomes (BACs) used to sequence most of the rest of the genome do not include restriction ...
