If egg and sperm form first cell of baby, then why does mtDNA comes from mother only?
In: Biology
The sperm only carries the father’s DNA, it doesn’t contribute any of the organelles that are used to express that DNA and run the cell. The egg is what contains all the necessary apparatus for the running of the initial cell that splits into the embryo, as well as containing the mother’s DNA. The mitochondria are one of these organelles, and they actually contain their own specific DNA just for making more mitochondria, and that DNA is kept separate from the DNA that is in the nucleus of the cell. So the reason mitochondrial DNA only comes from the mother is because only the mother provides mitochondria in the egg cell.
Just like any other cell, the egg contains mitochondria–they’re required for it to make use of energy and divide, which are both quite important functions for an egg! Since the mitochondria are already present in the egg before the sperm gets anywhere near it, they contain only the mother’s DNA. The mitochondria in the sperm, which are far less numerous than the ones in the egg anyway, are destroyed as part of the fertilisation process, nobody is entirely sure why.
The mitochondria are not present in the “package” of genetic information which enters the egg cell so the only mitochondria present are those present in the egg cell of the mother. https://youtu.be/eu64-ltm30k
I vaguely remember reading about this in one of Dawkin’s books. As he put it – the answer is in the exception. There is one species where mtDNA *initially* comes from both parents^1 – the two mtDNA strains fight it out until one strain has complete possession. This is of course a waste of resources, so most species choose the egg’s mtDNA by convention. It may not entirely be convention though – mtDNA would slow down sperm so there would be selection pressure *against* them carrying it.
^1 This also happens very rarely in species (such as humans) where it *isn’t* the rule.
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