The exclusively female origin of all the mitochondria in an embryo has an important consequence. Mitochondria, recall, carry a small, independent genome that resides within the mitochondrion itself-not in the cell’s nucleus, where the twenty-three pairs of chromosomes (and the 21,000-odd human genes) can be found. These mitochondria are the energy-producing factories of the cell they are so anatomically discrete and so specialized in their function that cell biologists call them “organelles”-i.e., mini-organs resident within cells. Aside from proteins, ribosomes, nutrients, and membranes, the egg also supplies the embryo with specialized structures called mitochondria. But the cellular material of the embryo comes exclusively from the egg the sperm is no more than a glorified delivery vehicle for male DNA-a genome equipped with a hyperactive tail. The genetic material of this embryo comes from two sources: paternal genes (from sperm) and maternal genes (from eggs). “Consider the genesis of a single-celled embryo produced by the fertilization of an egg by a sperm.
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