Large-Scale Analysis of Yeast and Human DNA 
Maynard V. Olson, Ph.D. — Investigator 
Dr. Olson is also Professor of Genetics at the Washington University School of Medicine. He was trained as 
a chemist, receiving his B.S. degree from the California Institute of Technology and his Ph.D. degree from 
Stanford University. After five years on the faculty at Dartmouth College, he moved to the University of 
Washington and changed fields from chemistry to genetics. He has served on the National Research 
Council Committee on the Mapping and Sequencing of the Human Genome and presently serves on the 
NIH Program Advisory Committee of the National Center for Human Genome Research. 
MOST human cells contain 6 billion base 
pairs of DNA. Embedded therein are an un- 
known number of genes, perhaps 100,000, that 
direct the biochemical events in the cells. At pres- 
ent, well over 95 percent of this DNA is 
unexplored. 
Geneticists have developed powerful methods 
with which to study DNA in small packets. Gene- 
splicing techniques, DNA sequencing, and meth- 
ods of reintroducing altered DNA molecules into 
cells allow the detailed structural and functional 
analysis of DNA molecules containing up to tens 
of thousands of base pairs. Our laboratory seeks 
to extend these approaches to encompass mole- 
cules ranging up to millions of base pairs in size. 
In the short run, this research should allow the 
analysis of larger functional units of DNA — for 
example, large human genes, clusters of coregu- 
lated genes, and such structures as centromeres 
and telomeres, which govern the behavior of hu- 
man chromosomes during the cell division cycle. 
In the long run these methods should allow the 
systematic analysis of the whole human genome 
— the entire complement of DNA sequences — 
thereby creating tools, such as detailed maps, that 
would be of permanent value in biology. 
Our approach has been to build on progress in 
the genetic analysis of microorganisms, particu- 
larly yeast. The common laboratory yeast Sac- 
charomyces cerevisiae, familiar from its use in 
baking and wine making, is an ideal model for 
studies of human cells, because its genetic organi- 
zation and biochemical pathways are similar to 
those in higher organisms. It is a powerful tool as 
well, because human DNA can be altered by 
gene-splicing techniques into a form that is stably 
propagated in yeast. 
We are just completing a long-term project to 
map the 1 4 million base pairs of DNA present on 
the 16 natural yeast chromosomes. The yeast 
chromosomes have been mapped at a resolution 
(i.e., the average spacing between landmarks) of 
3,000 base pairs, and nearly 200 genes have been 
localized on the map. New genes can now be 
placed on this map in only a few hours. This pro- 
cess, which replaces mapping techniques that re- 
quired weeks of effort, is now in use in more than 
100 laboratories. 
Despite this success, it has long been apparent 
that no straightforward extension of these meth- 
ods would be successful on the greatly expanded 
scale of the human genome. For this reason, we 
developed a method to import manageable seg- 
ments of the human chromosomes into yeast, 
where they can be propagated as yeast artificial 
chromosome (YAC) clones. One advantage of 
YACs over previous cloning systems is that there 
is no absolute upper size limit to the clones. We 
can now prepare large collections of YAC clones, 
each containing a different segment of human 
DNA, averaging hundreds of thousands of base 
pairs in size, a 10-fold improvement over the ca- 
pacities of previous systems. Another advantage is 
that the methods of packaging, maintaining, and 
replicating DNA are more similar in yeast than in 
bacteria to the analogous processes in human 
cells. Consequently, a higher fraction of the hu- 
man genome can be successfully propagated in 
yeast than in the more-conventional bacterial 
hosts. 
Methods of recovering any desired segment of 
human DNA as a YAC have become standard dur- 
ing the past three years. YAC clones played a cen- 
tral role in two recent major successes in human 
genetics: discovery of the molecular basis of the 
fragile X syndrome, carried out in part in the labo- 
ratories of Thomas Caskey (HHMl, Baylor College 
of Medicine) and Stephen Warren (HHMI, Emory 
University) ; and the discovery of the gene that is 
mutated in familial adenomatous polyposis, 
carried out in part in the laboratory of Raymond 
White (HHMI, University of Utah). The fragile X 
syndrome is a common heritable cause of mental 
retardation; familial adenomatous polyposis is a 
genetic predisposition to colon cancer. 
Now that YAC cloning is a proven method for 
recovering large blocks of human DNA, our atten- 
tion has turned to the analysis of YACs. A need for 
efficiency arises from the sheer scale of human 
chromosomes: approximately 10,000 YACs 
would be required to recover the DNA present in 
all the human chromosomes. To meet this chal- 
lenge, it will be necessary to develop a new area 
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