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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.

Image

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.

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.

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