GARY CARLSON / SCIENCE PHOTO LIBRARY GARY CARLSON / SCIENCE PHOTO LIBRARY
Meiosis. Artwork of the first prophase stage of meiosis. In meiosis, a cell in the testes or ovaries containing 46 chromosomes (red) divides to form four germ cells (spermatozoa or eggs) each with 23 chromosomes. During first prophase, the centrioles (yellow) move apart and spindle micro- tubules (white) form between them. Only part of the fragmenting nuclear membrane (lower right) remains. Meiosis involves two divisions, the first producing two and the second, four daughter cells. During the first division genetic material is exchanged between the paired chromosomes so each of the final cells contains a unique mix of the parental genes.
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