The nucleus of a sperm or an egg prior to fertilization. Sperm and egg cells carry half the number of chromosomes of other nonreproductive cells. When the pronucleus of a sperm fuses with the pronucleus of an egg, their chromosomes combine and become part of a single nucleus in the resulting embryo, containing a full set of chromosomes.

Dr. Anthony Wynshaw-Boris, of the National Human Genome Research Institute's Laboratory of Genetic Disease Research, defines pronucleus.

Related terms: chromosome, deoxyribonucleic acid (DNA), nucleus, transgenic.