Sequencing, Mapping, and Chips!

I came across two press releases yesterday, entitled:

Entire Yeast Genome Sequenced

and

University of Toronto scientists map entire yeast genome

Upon reading the first, I thought, hasn't the entire genome of Saccharomyces cerevisiae already been sequenced? And haven't other yeast genomes been sequenced as well? What in the world could they be referring to? Did yet another yeast species gets its genome sequence? Does that really warrant a press release (sorry Jason)? And the second made me think that the person in charge of titling the press release decided that genome sequencing is synonymous with mapping (it's not).

It turns out that both press releases contain the exact same text, despite their unique titles. They're hyping this article that presents a genome-wide map of nucleosomes in S. cerevisiae. This is important because it represents an important step in our understanding of how DNA is packaged in a cell. The authors incorporate experimental and computational tools to determine which segments of the genome are wrapped into 147 base pair bundles and which make up the space in between those bundles. This wrapping marks the first step of packaging the genome up into the tiny nucleus.

The article does not report the sequence of the genome, as the first press release claims. And while they do present a map, it's not a map in the classical genetics sense of the term. However, a map is just the selective representation of a particular set of features in a certain area. So the second press release has an accurate, albeit incomplete, title.

The title of this entry refers to not only the fact that one press release calling this "sequencing" and the other "mapping", but also to one of the techniques used by the authors of the paper in mapping the nucleosomes. They hybridized genomic DNA to microarrays (also known as "gene chips" or "chips") to determine which segments are wrapped in nucleosomes and which are not.

Lee et al. 2007. A high-resolution atlas of nucleosome occupancy in yeast. Nat. Genet. 39:1235-1244. doi:10.1038/ng2117