During egg and sperm production, the two copies of a duplicated chromosome must be bound together until it is time for their separation, A protein that protects this chromosomal glue has now been discovered. It is estimated that about 20% of human eggs have an abnormal number of chromosomes, and it is well known that the incidence of fetuses with three copies of some chromosomes ―rather than the usual two ― increases markedly with the age of the mother. The fact that human eggs can arrest in the early stages of their creation for up to 45 years is clearly a factor. For this entire period, to prevent abnormal chromosome numbers, the two copies of a duplicated chromosome ('sister chroma-tids') need to remain tethered to each other, and, the chromosome needs to be connected to its opposite number, until they are told to separate. On page 510 of this issue, Kitajima et al. reveal how the molecular tether between sister chromatids is kept in place.
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