In "Hamlet," Rosencrantz and Guildenstern deliver a letter to the rulers of England that carries the ill-fated duo's own death sentence. Perhaps Shakespeare knew a bit about reproductive biology.
Scientists have now found that during a sperm's creation, its mitochondria energy-producing units that power all cells acquire molecular tags that mark them for destruction once the
sperm fertilizes an egg. This death sentence, a protein called ubiquitin, may explain why mammals inherit the DNA within mitochondria only from their mothers, a biological curiosity geneticists have used to trace human evolution (SN: 2/6/99, p. 88: http://www.sciencenews.org/sn_arc99/2_6_99/bob1.htm). The finding may also have implications for th