DNA isn't the only thing in Human Chromosomes

Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.

The term chromosome comes from the Greek words for color (chroma) and body (soma). Scientists gave this name to chromosomes because they are cell structures, or bodies, that are strongly stained by some colorful dyes used in research.

Chromosomes vary in number and shape among living things. Most bacteria have one or two circular chromosomes. Humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell.

The only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome. When two reproductive cells unite, they become a single cell that contains two copies of each chromosome. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells.

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

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.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.