Scientists at Penn State University have achieved a major milestone in the attempt to assemble, in a test tube, entire chromosomes from their component parts. The achievement reveals the process a cell uses to package the basic building blocks of an organism’s entire genetic code — its genome. The evidence provided by early research with the new procedure overturns three previous theories of the genome-packaging process and opens the door to a new era of genome-wide biochemistry research.
The research was accomplished with the help of a new laboratory procedure developed by the team of scientists led by B. Franklin Pugh, the Willaman Chair in Molecular Biology at Penn State. The procedure allows scientists, for the first time, to do highly controlled biochemical experiments with all the components of an organism’s genome.
The team’s research is designed to reveal the construction process for the chromosome — the super-compressed marvel of molecular packaging that contains all an organism’s DNA and associated proteins. “Our procedure starts with an entire genome of DNA from yeast cells that we propagate through bacteria, then purify, “Pugh said. “Next, we add equal parts of pure histones, the protein building blocks of chromosomes. Then we allow the assembly process to begin.”
The result was that short sections of the lanky string of gene-containing DNA became wound around a series of histone proteins, forming a line of knots called nucleosomes separated by unknotted sections of DNA. Although earlier studies in other labs had shown that histones and DNA alone could construct a series of nucleosome knots along the DNA string, the overall structure of this construction was not nearly as organized as it needed to be in order to look like chromatin inside of a cell — the material that the cell remodels to form chromosomes. Pugh’s team sought out the recipe that would produce the actual, highly organized structure of chromatin.