The X chromosome is one of the two human sex chromosomes, along with the Y. Females typically have two X chromosomes in each cell and males one X and one Y. When a cell divides and its chromosomes are packed most tightly, the X is about 4–5 micrometers long. A micrometer is a millionth of a meter, or a thousandth of a millimeter.

Size

A chromosome is made of a single long molecule of DNA, together with proteins. Most of the time it is spread out inside the cell's nucleus. Before a cell divides, it copies each chromosome, and the two copies, called sister chromatids, stay joined at a pinched point called the centromere. During division the chromosomes condense and become visible through a microscope.

A chromosome's length therefore depends on when it is measured, because its arms shorten as division goes on. A 2024 electron-microscope study of dividing human cells found about 33 million base pairs, the building blocks of DNA, in each micrometer of a chromatid at metaphase. That is the stage when the chromosomes line up in the middle of the cell.

The X holds about 155 million base pairs, so at that stage it would be about 4.7 micrometers long. The explorer's 4 micrometers is close to that estimate. A metaphase chromosome, both chromatids together, is about 1.2 micrometers thick.

Size comparisons

The X chromosome's DNA, stretched out, would be about 5 centimeters (2 inches) long, about 10,000 times the length of the chromosome it is packed into.
If an X chromosome were enlarged to the height of a person 1.7 meters (5 feet 7 inches) tall, its DNA would stretch about 17 kilometers (10 miles).
The X holds about two and a half times as much DNA as the Y chromosome, whose first complete sequence, published in 2023, has about 62 million base pairs.
An X chromosome at metaphase is about three-fifths to two-thirds as long as a red blood cell is wide, which is 7–8 micrometers across.
About 17–21 X chromosomes, laid end to end, would span the width of a human hair, which is 80–100 micrometers wide.

One active copy

The X chromosome holds about 5 percent of the DNA in a cell and likely carries 900–1,400 genes that give instructions for making proteins. The Y is thought to carry only 70–200.

A cell with two X chromosomes switches one of them off early in the development of an embryo. This is called X-inactivation. Which X is switched off is usually random, so in some cells the X from the mother is active and in others the X from the father. Some genes escape it, and many of these lie near the ends of the chromosome, in regions that the X shares with the Y.

How the X got its name

In 1891, the biologist Hermann Henking was studying how cells divide in the testes of the firebug, Pyrrhocoris apterus. One chromosome behaved differently from all the others, and he called it "Element x". It later became known as the X chromosome.

The name comes from Henking's label, not from the chromosome's shape. Copied chromosomes are often drawn as an X because the two chromatids are joined at the centromere, and that is true of chromosomes in general, not only the X.

Sources

X chromosome. MedlinePlus Genetics, National Library of Medicine, 2026.
Y chromosome. MedlinePlus Genetics, National Library of Medicine, 2022.
The complete sequence of a human Y chromosome. Rhie et al., Nature, 2023.
Near millimolar concentration of nucleosomes in mitotic chromosomes from late prometaphase into anaphase. Cisneros-Soberanis et al., Journal of Cell Biology, 2024.
3D-CLEM Reveals that a Major Portion of Mitotic Chromosomes Is Not Chromatin. Booth et al., Molecular Cell, 2016.
How big are genomes?. In Cell Biology by the Numbers, 1st edition. Garland Science, 2015.
Chromatid. National Human Genome Research Institute.
Centromere. National Human Genome Research Institute.
Chromosome. National Human Genome Research Institute.
Metaphase. National Human Genome Research Institute.
The X chromosome still has a lot to reveal – revisiting Hermann Henking's work on firebugs. Paliulis et al., Journal of Cell Science, 2023.
The International System of Units (SI), 9th edition. Bureau International des Poids et Mesures, 2026.
MH 033hr Blood Smear. Histology Guide, University of Minnesota.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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