Two are one too many – this is the motto used by cells of a female organism: These contain two X chromosomes, one of which always becomes inactivated. How does the cell recognize that it contains two of these sex chromosomes and how does it choose which one to turn off? Scientists of the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ), working together with French colleagues, have now been able to elucidate an early step in this complex process.
Forty-five years ago, British scientist Mary Lyon already described this chromosome inactivation typical of female cells. Lyon proposed a hypothesis: With two copies of the X chromosome, all X-linked genes are present in two copies. However, in a male organism, which is equipped with a set of one X and one Y chromosome, the X genes are present in only one copy in each cell. To restore genetic balance, a female cell inactivates one of its two X chromosomes.
During development of a female embryo, inactivation of either of the X chromosomes, the one inherited from the father or the one inherited from the mother, occurs at random. To coordinate inactivation, the cell first needs to determine whether it contains more than one X chromosome and then make a choice which of the two to switch off. Since the mid-1980s it has been known that a specific region of the X chromosome termed X inactivation center (Xic) is crucial for a correct inactivation process.
Julia Rautenstrauch | alfa
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