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The X chromosome is not, in fact, X-shaped

A paper published in Nature this week describes the first 3D model of the X chromosome - and it's not X-shaped.

By Contributor / September 25, 2013

The X chromosome is more shapeless than X-shaped.

Peter Fraser/Babraham Institute

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Humans, we know, have 23 pairs of chromosomes in each cell, including one pair of sex chromosomes: females have two X chromosomes, and males have an X and a Y chromosome.

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What is less well known is that the X chromosome does not look like an “X.”

Despite its moniker, the X chromosome’s shape has been a missing puzzle piece in genomics – as has, in fact, the shape of all chromosomes, the spools of protein that contain our DNA. Though scientists have had a general sense of the X chromosome’s structure (at least, scientists have long known that it wasn’t X-shaped), the exact placement of its flips and folds, its coils and curlicues, had remained unknown.

But a paper published in Nature this week describing the first 3D-model of the X chromosome changes that. The model is an enthralling portrait of just seven micrometers – smaller than a red blood cell but larger than a E. coli bacterium – that could help scientists’ effort to plot which genomic regions are involved in aging and disease, and how so.

That the X chromosome is not shaped like an “X” upends popular renderings of the chromosome, but it is not a revelation to scientists. The tale of how the X chromosome came to be pictured as an “X” is a long one, unfolding around 1890 when scientists were first piecing through the foreign language of our bodies and happened on an unusual chromosome. It was called “X,” a placeholder for “unknown.” Later, its brother chromosome was called “Y,” after the next letter in the alphabet.

Then, it turned out that chromosomes – all of them – are shaped like an “X,” but for just a moment. This X-shaped structure is a pit stop right before an organism’s cells divide, in what is known as mitosis. The X-shape is also not one chromosome: it’s two. One of the two angled columns that make that “X” is a new, identical copy of the other, made so that when the cell splits, one chromosome goes to one cell, and the other goes to the other cell.

All of a cell’s 46 chromosomes must manufacture copies when a cell splits, which means that there are 92 chromosomes in the cell during mitosis, all nipped and tucked into X-shaped pairs.

The mitosis shape has been the preferred means of representing chromosomes, since this is the point at which each chromosome (or, really, pair of chromosomes) is distinguishable from another pair, and, no less, has been neatly packaged into a familiar shape.

“What you normally see are these X-shaped X chromosomes, like blobby cylinders joined at the middle,” says Peter Fraser, a researcher at the Babraham Institute, in the UK, and an author on the paper. “It’s an easy way to show people what an X chromosome looks like.”

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