Eyeing the ingrained origins of DNA.

Eyeing the ingrained origins of DNA 

Using extremely sensitive microscopy, two scientists have identified the specific cellular sites at which DNA replicates itself. The work might someday help researchers develop cures for cancer and other diseases caused by malfunctions in cell proliferation, says biologist Ronald Berezney of the State University of New York at Buffalo, who coauthored the study.

About 6 feet of DNA is coiled within the nucleaus of a typical mammalian cell. To duplicate all that material before cell division takes place, different DNA portions must replicate simultaneously at many different sites. Although scientists have attempted to localize these cellular replication sites, the lower-resolution techniques used in previous studies weren't up to the task, Berezney says.

He and Hiroshi Nakayasu were able to locate hundreds of granular sites within the nucleus by using high-resolution fluorescent microscopy. This is the first time the technique has been used to reveal where DNA replicates in mammalian cells, they report in the January JOURNAL OF CELL BIOLOGY.

Berezney and Nakayasu allowed kangaroo kidney cells to take up labeled DNA building blocks and used their technique to see where new DNA was being made. The sites appeared as granules distributed throughout most of the nucleus. The granules were all about the same size and often appeared linked in chains or rings. The scientists then isolated the fibrous molecular matrix inside the nucleu, and "were amazed" to see granules of similar size and number and in a similar spatial distribution as had appeared in the intact cells, Berezney says. In mouse cells, they found that the granules were distributed differently at different times during replication.

The scientists hypothesize that each granule consists of a cluster of DNA segments undergoing replication and that each cluster is attached, along with various enzymes needed for replication, to the protein matrix webbed throughout the interior of the nucleus. "It will be very interesting to look in cancer cells and see if there's the same type of arrangement [of DNA replication sites]," says Berezney, who is now developing electron microscopy techniques to visualize the three-dimensional structure of the granules.