Cracking the Code: Have You Ever Wondered Why Our DNA Has An X Shape?

A groundbreaking discovery at the Netherlands Cancer Institute has finally revealed the reason behind DNA’s X shape, potentially offering far-reaching implications for our understanding of cell behavior. Researchers believe they have unveiled a universal mechanism that determines the structure of DNA in cells.

Benjamin Rowland, one of the key researchers involved in the study, shared his initial disbelief and curiosity about the finding. During a phone conversation with a colleague from England, he recalls, “We saw a suspicious similarity between two molecules in the nucleus of our cells. They seemed to have exactly the same building blocks in one place, which could explain a lot of what happens within cells. So of course we had to investigate that!”

DNA’s X-Shape: A Fundamental Concept in Biology Textbooks

Within our bodies, cells are constantly dividing to create new cells. This process involves copying a cell’s DNA and dividing it equally among the two resulting cells. However, this task is more complex than it seems. Our DNA, spanning several meters in length, is intricately packed like strands of spaghetti into the minuscule nucleus of a cell. Distributing this equally is no easy feat.

To address this challenge, cells employ an ingenious strategy. They replicate their DNA and convert it into compact bundles. During this process, the two copies remain connected at the center until the cell divides. When viewed under a microscope, these bundles appear as an X-shape – a familiar sight in biology textbooks. This iconic X-shape is a fundamental representation of how cells manage the complex task of distributing DNA fairly during cellular division.

The Exotic Factor: Shugoshin’s Crucial Role in Cell Division

Just before a cell divides, the connection at the center of the X-shaped DNA bundle is released, allowing the arms of the X to separate and move to different cells. When this process falters, new cells may end up with too much or too little DNA, potentially causing cellular dysfunction. This is commonly observed in cancer cells, which often contain abnormal amounts of DNA.

Researcher Benjamin Rowland explains, “A chromosome actually consists of two identical long DNA threads that at first are connected along their entire length.” A collection of ring-shaped cohesin molecules holds these strands together.

Rowland continues, “When a cell is about to divide, the cohesin rings open, and the arms of the DNA come apart. The rings in the middle of the DNA remain tightly closed.”

This vital step in the process is orchestrated by a protein with the exotic name shugoshin – SGO1.

Unlocking the Secret: How Shugoshin Locks Cohesin Rings

While the X-shape of chromosomes has been a well-known fact since the late 19th century, the underlying mechanism remained elusive until recently. Alberto García-Nieto, a PhD student working with researcher Benjamin Rowland, has discovered that shugoshin utilizes a molecular key that fits precisely into a specific keyhole within the cohesin molecules. By doing so, it locks the cohesin rings in place.

Since shugoshin operates at the center of chromosomes, it only locks the rings in that region, resulting in the characteristic X shape of the chromosomes. Once a cell has prepared itself for division and aligned everything correctly, it releases the remaining locked rings using molecular “scissors.” This separation of the DNA strands allows the cell to complete the division process successfully.

A Universal Key: The Shared Molecular Mechanism that Shapes DNA

An unexpected similarity led researchers to a groundbreaking discovery: a small fragment of shugoshin is identical to a segment of another protein they had previously studied – CTCF. Interestingly, CTCF possesses the same molecular key that fits into the cohesin keyhole. Although it also locks cohesin rings, CTCF does so in a different context. Cohesin, in fact, serves another function altogether – compacting chromosomes by creating DNA loops. Despite the difference in context, the mechanism of locking remains the same.

“We seem to have found a universal mechanism by which cells determine the shape of DNA,” says Rowland.

What’s even more fascinating is that CTCF and shugoshin may not be the only proteins that utilize these building blocks. Rowland and his UK colleagues have found evidence suggesting that various essential proteins regulating our DNA employ the same molecular key to control cohesin.

“By locking cohesin at exactly the right time, as well as at the right place on the DNA, you can determine the shape of our chromosomes.”

It’s crucial to remember that DNA is the code of life – its structure influences its function and, consequently, the behavior of our cells. Uncovering the factors that determine DNA structure could have significant implications for our understanding of life and cellular processes.

Source: 10.1038/s41594-023-00968-y

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