Mysteries aggravate scientists in general, but for several decades one particular mystery has vexed biologists studying the transmission of human DNA. It seems that every time a child is conceived, a certain chunk of genetic information from the father goes missing.
Call it the Curious Case of the Disappearing DNA. Crystal Dilworth and Trace Dominguez sleuth out the situation in today's DNews dispatch.
First, a little background: As we all know, when sperm and egg make their formal introductions, DNA from mother and father combine in the egg to complete the set of 46 chromosomes needed to make a human. But in addition to that chromosomal DNA, a tiny bit of additional genetic information is encoded in something called mitochondrial DNA (mtDNA).
Mitochondria are kind of like little organic batteries -- they provide the energy that cells need to live and move about. They also contain mtDNA, which contains a few bonus instructions on how to make certain specialized proteins. Both the mother and father carry this genetic information, but at some point during the fertilization process, dad's mtDNA disappears.
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Chromosomal DNA gets through just fine, but mitochondrial DNA goes MIA. That means you can only inherit mtDNA from mom, which is very weird, biologically speaking.
It gets weirder: According to new research published in the prestigious journal Science, it appears that paternal mitochondria actually self-destructs during fertilization. Using powerful electron microscopes, scientists observed that the male mitochondria in sperm break themselves down by way of an enzyme that eats both the mtDNA and its protective membrane.
Previously, it was speculated that cleaner-upper lysosomes in the egg attacked the paternal DNA for some biologically advantageous reason. And indeed they do. But the new research suggests that paternal mtDNA essentially offs itself first, after which lyosomes from the egg move in to make sure everything is dead and disposed of.
It seems that Mother Nature, for some reason, really does not want that particular bit of male DNA to become part of the embryo DNA. It's still not clear why. But it means that all of the mitochondria in all humans today can be traced back to one common female ancestor -- scientists call her Mitochondrial Eve -- who lived about 200,000 years ago in Africa. Really.
NCBI: Ubiquitinated Sperm Mitochondria, Selective Proteolysis, And The Regulation Of Mitochondrial Inheritance In Mammalian Embryos.
U.S. National Library Of Medicine: Mitochondrial DNA
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