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Sex Mechanics

Without sex, Earth might be little more than a slimy rock hurtling through space. We can thank a biological wonder called meiosis for saving the planet from such a boring fate.

In short, meiosis introduced sexual reproduction to the globe. It is one of nature's most elegant feats of engineering, a precise choreography of proteins and DNA. First discovered by biologists working with sea urchin eggs in the 1870s, meiosis eventually became recognized as the driving force behind cell division in all organisms that depend on sex to reproduce (note: not all do).

But the ingenious meiosis machinery can break down, and when it does, it can produce birth defects and sterility. Biologists have never fully understood how meiosis works, much less how it doesn't.

Working with yeast cells (a common, powerful tool in cell biology research), Hong-Guo Yu , assistant professor of biological science is adding to the modern understanding of meiosis. Last fall, the Journal of Cell Biology, (September issue) published some of his latest findings.

Yu's lab suspected that a single protein, called pds5, was a key part of the meiosis machinery, but they didn't know exactly what its role was in the process. Yu's group engineered yeast cells so that as they began to divide, the pds5 protein would disappear. The result was dramatic—the protein's absence completely derailed nature's machinery.

Normally during meiosis, Yu explained, a cell begins dividing by duplicating the set of chromosomes it inherited from its parents. A twin set of chromosomes is formed, a set for each new cell being created. This pair of new cells splits yet again to form a total of four new cells, each carrying the number of chromosomes as the original cell. In humans, some of this dividing-up process results in egg cells, and some results in sperm cells.

Yu found that when pds5 is absent, the original duplicate sets of chromosomes literally get stuck (to other cellular components) in the dividing-up process, and they become abnormally short and dense. Ultimately, all these errors lead to yeast spores that are sterile. In humans, errors during meiosis also can lead to genetic disorders, such as Down syndrome and other developmental conditions.—C.S.