Chromosomes are long DNA molecules that contain the genetic material of an organism. These structures were discovered in the late 1800s, after the optical microscope was invented.
Scientists led by Professor Daniel Panne, of the University of Leicester, and Dr Benjamin Rowland, of the Netherlands Cancer Institute, now believe that the answer may be a protein called shugoshin (also known as SGO1), which helps to “lock” the chromosomes in the X shape.
Chromosomes become X-shaped during cell division, but researchers wanted to know more about how this happens.
When a cell divides in two to make new cells, it copies the DNA and divides it equally between the two new cells.
The DNA in each cell is about two meters long, so cells use a process to fold DNA into compact packages to fit.
As part of the process, the cells leave the two copies attached in the middle until they divide.
Under the microscope, such a package looks like an X.
Just before the cell divides, the X is released in the middle and each arm of the X goes to a separate cell.
If this goes wrong, new cells can end up receiving either more or less DNA than normal, which can lead to cancer or genetic conditions such as Down syndrome.
The team studied a key component of a ring-shaped protein complex called cohesin, which is known to play a key role in holding chromosomes together during cell division.
Dr. Rowland stated: “A chromosome actually consists of two identical long strands of DNA that are initially connected along their entire length. A series of ring-shaped cohesin molecules hold the two strands together. When a cell is about to divide, the cohesin rings open and the DNA arms separate.”
The researchers discovered that the protein, SGO1, uses a “molecular key” that fits into a “keyhole” in cohesin, locking the cohesin rings and giving the chromosomes their X shape.
When cells begin to divide, the “molecular scissors” cut the cohesin rings, eventually separating the DNA.
Professor Panne stated: “It is exciting to finally understand, at the molecular and atomic level, how the iconic X shape of chromosomes is generated during cell division. This has not only intrigued generations of scientists, but is also important to our understanding of how this process can go wrong in disease.”
The research is published in the journal Nature Structural & Molecular Biology.
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