“This is a big step forward in a puzzle that biologists have been chipping away at for over 150 years,” said Ben E. Black, PhD, assistant professor of Biochemistry and Biophysics at the University of Pennsylvania’s School of Medicine.
Each nucleus of each cell in your living body is an almost absurdly crowded place, stuffed with DNA; proteins packing up that DNA into a more correct space; proteins patching up DNA where it breaks; proteins opening up DNA to transcribe it. The nucleosome is the combination of DNA wound around a histone protein core — the DNA thread wrapped around the histone spool. This is a maze of molecular nanomachines whose structure and dynamics we are finally coming to understand. Quantitatively: “we might think of how anything can be accurately located in 2840 miles of linguini in a 150 foot sphere” [“The cell nucleus and its DNA on a human scale –I”.
Before a human cell divides into two it first duplicates its genetic material, the DNA tightly packed into its chromosomes. Each human cell’s DNA totals about 3 meters (6 feet) in length. Just to squeeze that into a typical human cell’s nucleus, about 10 microns (10 one millionths of a meter) in diameter, requires coiling and bending and compressing the DNA, with the help of proteins called histones, into something 1/100,000 of its unfolded length. The two new sets of chromosomes then have to be teased apart from one another and correctly distributed to the resulting “daughter” cells, so that both daughter cells are genetically identical to the original, or “parent,” cell. [“Link Between Control Of Chromosome Duplication And Segregation Discovered”]
The nanomachine that somehow does this trick — 100,000 times more amazing than a stage magician shoving a rabbit into a hat — is the cellular organ called the centrosome. During cell division a centrosome, and a copy of the centrosome, position themselves at opposite ends of the dividing cell. Each centrosome anchors a spindle. The spindle is a complex structure of filamentary tubules that radiate from each centrosome, connecting with special sites on the chromosomes called centromeres. Pulling on the chromosomes, spindles separate them into two sets. It is crucial that cells duplicate their centrosome only once during each division cycle because extra copies could result in the incorrect distribution of chromosomes, leading to deadly genomic instability.
A year and a half ago, Professor Bruce Stillman, Ph.D., and his lab team at CSHL (Cold Spring Harbor Laboratory) identified a protein molecule that controls the copying of the centrosome in human cells and prevents it from being erroneously re-duplicated. Stillman commented: “I also think that this discovery suggests an ancient link between the processes that duplicate DNA and the processes that separate the DNA in cells before cell division.”
Now we knew that a single molecule was the piece at the center of the jigsaw puzzle. The big picture that puzzle revealed was exactly how human DNA is duplicated then maneuvered correctly and equally into two daughter cells, producing two exact copies of the mother cell.
Science Daily summarizes the big picture [“At the Crossroads of Chromosomes: Study Reveals Structure of Cell Division’s Key Molecule”: “On average, one hundred billion cells in the human body divide over the course of a day. Most of the time the body gets it right but sometimes, problems in cell replication can lead to abnormalities in chromosomes resulting in many types of disorders, from cancer to Down Syndrome.”
Scientists have known for decades that a key part of the chromosome, called the centromere, is a narrow region that specialized molecules, called spindle fibers, attach themselves to in order to chemo-mechanically pull daughter cells apart during cell division. For a few years, it has been known that some mysterious structure of the CENP-A molecule is the basis of the centromere’s action. But what structure, working in what way?
“Our work gives us the first high-resolution view of the molecules that control genetic inheritance at cell division,” says Black. He speaks for his team, which includes Nikolina Sekulic, PhD, a postdoctoral fellow, as well as graduate student Emily A. Bassett, and research specialist Danielle J. Rogers. The Black group’s work was funded by the National Institute for General Medical Sciences, the Burroughs Wellcome Fund, the Rita Allen Foundation, the American Cancer Society, and the American Heart Association.
Researchers have known for about 15 years that aspects of cell division are controlled by epigenetic processes. Epigenetics is something that Mendel and Darwin knew nothing about. It is so recently part of our understanding of heredity that scientists are confused about the definition. These processes are changes in a chromosome without alterations in the DNA sequence. The actions involved operate by affecting the protein spools around which DNA is tightly wound, the same spools that are also active in packing the 6 feet of DNA into a microscopic structure. The epigenetics are in this interaction of DNA and protein, rather than encoded in the DNA sequence itself. The spools for winding DNA tightly are built of histone proteins, and various chemical changes to these spool proteins, such as methylation, result in either loosening or tightening their geometry with DNA. Epigenetics alters the reading out of genetic code, which can, for example, ramp a gene’s expression up, or damp it down. In the case of the centromere, epigenetics marks the site where spindle fibers attach independently of the underlying DNA sequence. CENP-A has beensuspected of being the key epigenetic marker protein.
What nobody knew until now is how CENP-A epigenetically marks the centromere to direct inheritance. The Black team found the structural features that confer upon CENP-A the ability to mark centromere location on each chromosome. This is important because without CENP-A, or the centromere mark it creates, the entire chromosome — and all of the genes it houses — are lost at cell division.
In the study, just published in Nature, the Black group solved CENP-A’s structure thereby determining how it specifically marks the centromere on each chromosome. From that they hypothesize how the epigenetic mark is copied correctly in each cell division. They found that CENP-A changes the shape of the nucleosome of which it is a part, also making it more rigid than those nucleosomes without CENP-A. The CENP-A nucleosome is copied several times to create a unique epigenetic area, distinct from the remainder of the chromosome. CENP-A replaces the histone protein called H3 in the nucleosomes located at the centromere.
This CENP-A centromere identifier attracts other proteins, and in cell division builds a massive motor-like structure, the kinetochore, for pulling the duplicated chromosomes apart during cell division.
This is a major advance in understanding the structure and function of the molecules driving human inheritance. Where might this research lead? The tantalizing possibility is that the key epigenetic components now in hand could be reverse engineered and used in synthetic biology to engineer clinically useful artificial chromosomes. Imagine if that could be inherited alongside our own natural chromosomes and with the same high fidelity, says Ben E. Black. He won’t say so for the record, but this could give birth to new technologies for controlling everything from cancer to Down Syndrome. Consider the implications for Life Extension, or for carrying artificial information within natural cells. The implications are only limited by what Sir Arthur C. Clarke calls “failure of imagination.” For scientists and artists with enough imagination, one can easily come up with utopian or dystopian scenarios of how control of cell division could bring immortality or terrible nightmares, depending on how wisely it is used. Are we protected by, or endangered by what Sir Arthur C. Clarke calls “failure of nerve”?
Orc1 controls centriole and centrosome copy number in human cells. Science, February 6, 2009