Calico cats have distinctive tortoiseshell coats because of a "silencing" of the X chromosome.
Researchers worked to get to the bottom of how one X chromosome could be completely inactive, a Biophysical Society news release reported.
Female mammals contain two copies of the X-chromosome, but only need one; this causes the other chromosome to be "turned off." Calico cats have orange fur genes in one X-chromosome and black in the other, the "silencing" of one X chromosome in each cell is what causes the patches of color on the coat.
Researchers are not sure what causes these certain chromosomes to be silenced by cells. To find out the team found out how to look at the X-chromosome inside the cell.
"A cell's nucleus contains the genetic code, its DNA. But while the structure of the DNA was determined more than 50 years ago, and we're rapidly determining the position of specific genes on chromosomes, no one had visualized the DNA within an intact nucleus -- an unfixed, hydrated whole cell," Elizabeth Smith a postdoctoral fellow working in Carolyn Larabell's lab in the Anatomy Department at UCSF said in the news release. "We decided to try."
The finding could help researchers learn how to switch off just one type of gene without completely changing the DNA sequence.
"The inactivation of one out of two X chromosomes in females is an enormously important epigenetic process," Smith said. "Uncovering how only one X chromosome is inactivated will help explain the whole process of epigenetic control, meaning the way changes in gene activity can be inherited without changing the DNA code. It can help answer other questions such as if and how traits like obesity can be passed down through generations."
The team used soft x-ray tomography to visualize the within an intact nucleus.
"We obtained high-resolution, 3-dimensional views of the intact nucleus and, by using a prototype cryo fluorescence microscope along with the x-ray microscope, we were able to identify one specific chromosome, the inactive X chromosome of female cells," Smith said.
When the team performed the visualization they were surprised to see the broad variation of structural organization within the chromosome.
"We were able to show a remarkable substructural organization of this chromosome, which consists of three distinct domains of differing amounts of chromatin," Smith said.
"With new fluorescent probes, we can start identifying the position of specific genes in context -- inside the tangled network of DNA within the intact nucleus," she said.
