When a woman becomes pregnant, the success of that pregnancy depends on more than her health during adulthood. It can trace back to events that occurred while she was still in her mother’s womb. During this time, her egg cells were already forming, and their quality depends on whether chromosomes were copied, cut, and rejoined properly. In men, a similar process creates sperm in the testes after puberty.
If this process goes wrong, the eggs or sperm may end up with the wrong number of chromosomes. According to Neil Hunter, professor of microbiology and molecular genetics at the University of California, Davis, this can lead to infertility, miscarriage, or genetic conditions such as Down syndrome.
A new discovery in chromosome protection
In a study published in Nature, Hunter’s team reports how certain proteins safeguard the process of chromosome “crossovers.” These crossovers happen when matching chromosomes pair up and exchange DNA during the development of eggs and sperm. This step is vital because it keeps chromosome pairs connected until they are divided correctly into reproductive cells.
To make these discoveries, the researchers used budding yeast—a simple organism that shares many genetic processes with humans. By tracking molecular changes in yeast cells, they mapped out how proteins protect chromosome connections from being disrupted too early. Remarkably, every protein studied in yeast has a direct counterpart in humans, making the results highly relevant to fertility research.
Why crossovers matter
Humans have 23 pairs of chromosomes, one from each parent. Before egg or sperm cells are fully formed, these chromosomes pair up and swap segments of DNA in a process known as recombination. These crossovers serve two important roles:
- They mix parental genes, ensuring genetic diversity in children.
- They hold chromosome pairs together, so that they are distributed accurately when cells divide.
In women, this process is especially complex. Egg cells form before birth, pause in development for years or even decades, and only resume dividing when ovulation occurs. During that long pause, the crossover connections must remain intact. If they break down, the chromosomes can separate incorrectly, producing eggs with too many or too few chromosomes. This raises the risk of miscarriage, infertility, or conditions like Down syndrome.
The role of protective proteins
Hunter and his colleagues focused on structures called “double Holliday junctions,” where DNA strands cross and exchange segments. These junctions must remain stable long enough for crossovers to form. The team found that proteins such as cohesin prevent other enzymes, like the STR complex (also known as the Bloom complex in humans), from dismantling these structures too soon. By protecting the junctions, these proteins help ensure proper chromosome separation and safeguard fertility.
Implications for human fertility
Although the experiments were done in yeast, the findings apply broadly to humans because the crossover process has changed very little through evolution. Understanding how these proteins work could lead to better ways of diagnosing or even preventing certain fertility problems.
The research also highlights the power of studying simple organisms to answer fundamental questions about human health. With the help of advanced imaging and genetic tools, scientists are beginning to reveal the molecular choreography that underlies reproduction—and what happens when it goes wrong.
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