Visitors to the Genome exhibition are frequently intrigued by the Genome Ball, a three-dimensional model of the human genome that represents a creative synthesis of scientific knowledge and technical innovation. For students and adults raised on clinically produced karyotypes – those artificially arranged pairs of X-shaped chromosomes photographed during cell division – the Genome Ball will challenge all their previous (mis)conceptions and show the human genome in a new light.
How did we first begin to grasp the structure of the nucleus? It all began in 1682 when Anton van Leeuwenhoek, a fabric merchant in the Dutch city of Delft, examined blood cells of fish. Leeuwenhoek used a microscope with lenses he’d ground himself, and reported his observations in a letter to the Royal Society:
I came to observe the blood of a cod and of a salmon, which I also found to consist of hardly anything but oval figures … it seemed to me that some of them enclosed in a small space a little round body or globule …
If you’ve looked at human blood under a microscope, that description may sound odd: Mature red blood cells (RBCs) don’t contain nuclei – do they? You’re right! However, the RBCs of fish (and amphibians and reptiles) do indeed have nuclei, and Leeuwenhoek was the first to describe them. Nevertheless, the Royal Society wasn’t blown away by his letter (after all, how much could a business man with “little fortune and no formal education” know about science?). The “little round body or globule” remained nameless for the next 150 years.
Then, in 1831, the Scottish botanist Robert Brown was studying plant fertilization when he noticed that pollen moved in and out of “ovals” in the plant cells. He called each oval a “nucleus,” a Latin word meaning “nut” or “kernel” – a bit like a black walnut surrounded by its thick green hull. Not only did Brown’s name stick, but his 1833 paper even suggested that the nucleus was probably involved in fertilization and the development of embryos.
The next step in our nuclear narrative was taken by Friedrich Miescher, a Swiss physician who extracted and isolated a previously unknown substance from pus-soaked bandages at the hospital where he worked. White blood cells, a major constituent of pus, have very large nuclei, and Miescher correctly concluded that the substance came from those nuclei. He called it “nuclein.” Today we call it DNA.
How many chromosomes in a human cell?
Although the fine points of cell division were still unexplained, scientists in the early 1900s were eager to learn the number of chromosomes in human cells. However, counting the number of human chromosomes during cell division turned out to be quite a challenge. Even when chromosomes were lined up on the “midline” of a cell, scientists’ counts ranged from 16 to 36.
Evidently, Hans von Winiwarter got tired of these wide-ranging approximations. Using the best microscopes available to him in 1912, he produced early karyotypes by capturing and fixing human cells at the moment of cell division. Despite his best efforts, Winiwarter’s counts ranged from 46 to 49; and while noting correctly that women have two X chromosomes, he mistakenly concluded that males had only one X and no Y. For the next 40+ years, students were generally taught that human cells contained 48 chromosomes.
Finally, in 1956, the correct value of “46” was confirmed – 22 pairs of autosomes and 1 pair of sex chromosomes in human cells other than eggs or sperm. It’s surprising to learn that Watson & Crick had published their model of DNA’s structure, opening the world of modern genetics, several years before the number of human chromosomes was firmly established!
Good things come in small packages
By the end of the 20th century, knowledge of DNA structure and the mechanisms of cell division had advanced dramatically. Yet, based on their school textbooks, most people still tended to picture chromosomes as the condensed “X-shaped” bodies seen in karyotypes. DNA was known to uncoil between cell divisions, but it was hard to imagine how such long straggling threads (more th