Signals from the polarized cell cortex (in green) in mouse oocytes regulate microtubule tyrosination (in white) to generate an asymmetric spindle in the first part of meiosis, the cell division process that creates an egg in females and sperm in males. This asymmetry can be exploited by selfish genetic elements to bias their transmission to the egg.
Each of your cells contains two copies of 23 chromosomes, one inherited from your father and one from your mother. Theoretically, when you create a gamete - a sperm or an egg - each copy has a 50-50 shot at being passed on. But the reality isn't so clearcut. Scientists have observed that chromosomes can "cheat," biasing the chance that they will make it into a sex cell. Now, a team from the University of Pennsylvania has shown how this bias arises in female cells. With careful observation and experiments with mouse oocytes, the precursors of eggs, they've detected molecular signals that create an asymmetry in the machinery that drives meiosis, the cell-division process that gives rise to gametes. Certain chromosomes, the researchers found, exploit this asymmetry to move themselves over to the "right" side of a cell during division and wind up in the egg.
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