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Two females can't fertilize a human egg right
Q: I just read the fascinating USA Today story about the monkey embryo created by parthenogenetic
reproduction (where an unfertilized egg develops into a new individual: common among insects). Is it
possible to take the chromosomes from one female egg and inject them into another female egg so they
attach and actually form a fertilized egg? --Stan C., Naples, Florida
 A: It probably is not possible with humans but we have done it with mice. We have moved the entire cell
nucleus from one mouse egg to another mouse egg and joined them to form an embryo. The resulting embryo
had no chromosomes from a father but it was a fertilized egg.
[© Michael W. Davidson, Florida State U, National High Magnetic Lab] The cell pulling chromosomes pairs
apart
It kind of works in the mouse, except chromosomes have different properties depending on whether they come from the male (sperm) or
female (egg/oocyte), says Michael Tucker of Georgia Reproductive Specialists.
That's the rub. Reproduction in mice and men evolved using both male and female genes in conjunction, complementing each other.
When male chromosomes are missing, things don't always work well and sometimes catastrophically. The fetus dies.
Let's wander into a cell and see where and why things go wrong. A cell's nucleus contains a tangled structure called the chromatin.
When the cell divides, the chromatin condenses into 23 (for humans) distinct entities called chromosomes. The microscopic chromosomes
occur in matched pairs (one from each parent) and each chromosome contains many genes that also occur in matched pairs. Offspring
inherit one-half of their genes from each parent and then mix the two sets of genes together to form an unique individual.
Before the fertilized egg divides, it duplicates each chromosome. A tiny DNA filament connects each duplicate chromosome pair in the
middle. See figure.
When the cell divides, the centrisome (a part of the sperm outside the nucleus) organizes the division so it happens right. The centrisome
splits into two poles that move to opposite ends of the cell. It also establishes a long rod at each pole. The chromosomes align along each
rod and point to the opposite pole: like spread-apart fingers of two hands, barely touching at the fingertips. The cell pulls apart each
chromosome duplicate pair, like separating hands. See figure. The cell divides along a line about half-way between the two poles and
each new cell has a full, proper system of chromosomes.
Without this organizing system from the male, the chromosomes will scatter throughout the cell. Consequently, when the cell divides, the
resulting daughter cells will not have a complete and proper chromosome set. Subsequent cell divisions will compound the mess and not
form a viable embryo.
That's why fertilization that results in a fertilized egg containing only-female chromosomes will probably not work for humans.
(Answered Aug. 16, 2002)
Further Surfing:
USATODAY.com: Created monkey embryo, no fertilization
Department of Energy: Genetics primer
Florida State U, National high magnetic lab: Mitosis
U of Washington: chromosomes
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