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Human Chromosomes Are Only Half DNA, And That Changes Everything

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For a long time, scientists thought that DNA was the only thing inside your chromosomes. A new study turns that assumption on its head.

Why It's Important

Inside every one of your cells is a sort of control hub called the nucleus. Inside every nucleus are a bunch of thread-like structures called chromosomes. Chromosomes are what contain your DNA, that familiar double-helix structure that contains every one of your 25,000 or so genes, which determine everything from what color eyes you have to your likelihood of developing various diseases.

DNA was first identified nearly 150 years ago in 1869 by Swiss chemist Friedrich Miescher, though at the time he called it "nuclein" (which became "nucleic acid," which then became "deoxyribonucleic acid," or "DNA"). Other scientists, including Phoebus Levene, Erwin Chargaff, and famously, Rosalind Franklin, made their own important discoveries about the way DNA works before James Watson and Francis Crick published their seminal paper about the structure of DNA. One of those scientists was Walther Flemming, who in 1879 coined the term "chromatin" for the substance within the nucleus, which was later found to be the complex of DNA and proteins that makes up chromosomes. For the more than 100 years that followed Flemming's discovery, scientists had naturally assumed that chromosomes contained nothing but DNA-containing chromatin.

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Why People Are Talking About It

In 2016, a study published in the journal Molecular Cell found something startling: just a little over half of every chromosome is DNA. The rest—30 to 47 percent—is made up of an unidentified sheath-like structure. They were able to make such an earth-shattering discovery because of a super-precise imaging technique called 3D-CLEM, which combines light and electron microscopy with computer modeling to create detailed 3D images of all 46 human chromosomes.

The researchers aren't sure what this sheath-like structure does, but they think it might keep chromosomes separated during cell division. That might protect DNA from the kinds of coding errors that cause birth defects and other genetic issues. Either way, this forces scientists to change the way they think about everything from the structure of chromosomes to the process of cell division.

Editors' Picks: Our Favorite Videos About DNA

A Crash Course On DNA

Find out what's special about that double-helix structure.

James Watson In Conversation

Hear about DNA from the man credited with the discovery of its structure.

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Great Minds: Rosalind Franklin

Discover how she contributed to what we know about genetics.

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