Physical Mapping 



Identification and cloning of tfie human telomere to define the ends of 

 the human genetic and physical maps. Telomeres are defined as the ends 

 of cfiromosomes. Tfiese specialized structures are involved in the replication 

 and stability of linear DNA molecules. Investigators at Los Alamos National 

 Laboratory (LANL) have identified and cloned the human telomere [Moyzis et 

 al., Proc. Natl. Acad. Sci. USA85. 6622-6626 (1988)]. Fluorescence in situ 

 hybridization has been used, in addition to other biochemical and biophysical 

 techniques, to localize this unusual sequence. (TTAGGG)„, to human 

 telomeres. Seen in the inset photograph as fluorescent yellow spots on red- 

 stained human chromosomes, this sequence is present at the telomeres of all 

 vertebrate species and. hence, must have arisen over 400 million years ago 

 [Meyne et al.. Proc. Natl. Acad. Sci. USA 86. 7049-7053 (1989)]. 



The ultimate proof that this repeating DNA sequence TTAGGG is the human 

 telomere is to show that the sequence functions as a unit in an artificial 

 chromosome. In collaboration with the staff of Fvlaynard Olson's laboratory at 

 Washington University, the LANL staff was able to construct yeast artificial 

 chromosomes (YACs) in which the natural human (TTAGGG)„ sequence 

 functioned as a telomere in yeast cells [Riethman et al., Proc. Natl. Acad. Sci. 

 86, 6240-6244 (1989)]. These results indicate that the yeast telomere 

 replication machinery can indeed recognize the human telomere, even though 

 the common ancestor of yeast and humans lived over one billion years ago. 



In addition to demonstrating that the (TTAGGG)^ sequence functions as a 

 telomere, these YACs allowed large (100,000-200,000) nucleotide fragments 

 to be isolated from the ends of human chromosomes. Seen in the large 

 photograph is in situ hybridization of DNA from one of these YAC clones that 

 originated from the telomere of human chromosome arm 7q. DNA from such 

 YACs can be used to define the ends of the human genome genetic and 

 physical maps. 



LANLs discovery of the human telomere is a significant milestone in efforts 

 to map the human genome. Prior to identifying the telomeric sequence, 

 investigators were without a reference point from which to orient DNA 

 mapping studies for identification of DNA markers that would be useful for the 

 analysis of disease genes. [The inset photograph was first published in Proc. 

 Natl. Acad. Sci. USA 85, 6622-6626 (1988). The large photograph was first 

 published in Proc. Natl. Acad. Sa. USA 86, 6240-6244 (1989). Photographs 

 provided by Robert Moyzis, Los Alamos National Laboratory.] 



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