I have come to understand that the mitochondria…the engines of our cells…contain DNA. Does this Dna contribute to our genome?
The genome is defined as the entire complement of genes of an organism. This includes genes from organelles such as the mitochondria and, in plants, chloroplasts. DNA from organelles is usually treated separately from nuclear DNA, so you’ll sometimes hear terms such as “mitochondrial genome” and “nuclear genome”.
You can get an overview of the mitochondrial genome of humans from these web pages:
http://www.ncbi.nlm.nih.gov/cgi-bin/Entrez/framik?db=Genome&gi=12188
http://www.gen.emory.edu/mitomap.html
Note that the mitochondrial genome is degenerate. Many of its functions have been taken over by nuclear DNA. Mitochondrial DNA still encodes for its own ribosomes, tRNA, and certain proteins involved in the respiratory pathway. Here’s a page about mitochondrial biology:
http://cellbio.utmb.edu/cellbio/mitoch2.htm
Genes found in the mitochondrial genome are responsible for some rare diseases like Pearson marrow-pancreas syndrome. More information on mitochondrial diseases can be found at the web page of the United Mitochondrial Disease Foundation:
I didn’t check out all the above Web-sites, but they probably give you the facts. I would just add that it’s the mito DNA that is used in such determinations as ‘did Jefferson father children by Sally Hemmings’.
The egg and sperm merge to form a zygote–a diploid cell that everybody arises from. This cell needs mitochondria for energy production. The mitochondria is passed down by the mother’s eggs; a sperm cell is basically a DNA delivery system. So basically, the sperm provides half the DNA and the egg provides everything else. Since sperm dosen’t contain organelles (and the DNA in mitos) and eggs do, this maternal DNA can’t be used to trace definite paternal heritage. The caveat is that you can’t determine what male the DNA came from.
To me this is why I find the case that Thomas Jefferson was the father of some of Sally Hemmings children unconvincing. Maybe he was, but the DNA evidence doesn’t prove it, no matter what one reads in the general media. All it proves is that it was some Jefferson male was–because all their mitochondrial DNA is passed down by the females in their family. Jefferson mitochondrial DNA is not unique to a particular male, AFAIK.
It is my understanding, and I may of course be wrong, that the sperm does indeed contain mitochondrial DNA, but that it is contained in the midpiece of the sperm cell (the section between the head and the flagellum (tail) of the sperm). Since the midpiece and tail “break off” and only the head enters the egg during fertilization, the sperm does not contribute any mitochondrial DNA to the zygote.
troub–yeah, the flagella biomotor needs mitochondria to power itself–sperm are energentic little creatures (and these mitochondria would contain DNA). But, baically you’re right, these mitochondria only power the sperm; they don’t contribute to the zygote.
troub is correct that the sperm contains a single mitochondrion in its midpiece and therefore does have mitochondrial DNA. Indeed, it is this mitochondrion that powers the tail, which allows the sperm to swim. The midpiece and the tail are left behind during fertilization, so only the mother contributes her mitochondria to the offspring.
647 is also correct that the DNA tests show that a Jefferson - but not necessarily the Thomas Jefferson - fathered Eston Hemmings, Sally Hemmings’s son. However, the test didn’t involve looking at mitochondrial DNA but rather the Y-chromosome. The Y-chromosome is one of the sex chromosomes and is passed only from father to son. Thomas Jefferson therefore had the same Y-chromosome as his brothers (if any), his father, his uncles, his nephews (if any), his paternal male cousins, etc.
Good link Terminus–I answered off the top of my head and I was pretty sure T. Jefferson didn’t have a surviving male line.
Thanks.
One thing to add, yo.
Certain diseases (especially ones involving energy-sucking tissues like muscle and brain) are based in mutations on the mitochondrial genome. These are inherited directly from mother to child. A condition known as heteroplasmy determines how severe the condition is – the population of mitochondria in the oocyte is mixed, and some biased selection may make for a more or less diseased child.
Also, there are no mitochondria in the acrosome of the sperm (the bit that merges with the egg), and the inclusion of paternal mitochondria is quite a controversial point in the field (at least the last time I looked…)
647 et al
You should note that the conclusion reached in re the Hemmings case did not depend solely on the genetic research but rather on the entire body of evidence. The documentary evidence pointed rather strongly in the direction of Jefferson, the genetics simply bore out that direction by excluding the historically advanced “responsible parties” (i.e. the men advanced historically and by historians until recently) for Sallie’s children, notablly the Carr brothers.
One needs to look, then, at the entire record. I’m sure you can follow up for yourselves.
I think the most interesting aspect of mito DNA is the theory that mitochondria were independent organisms that joined in a symbiotic relationship with early cells.
If this is true, then every single cell of our bodies is a symbiosis of two creatures! I find that neat.
A perusal of the links that Terminus so graciously provided states that a very small percentage of sperm mitochondrial does enter into the union with the oocyte, but it is so small that it plays no part in the eventual development.
Altho the MtDNA is technically, by definition, part of the genome, it has not been included in the latest hoopla concerning the mapping of the entire human genome, the reason being that it was mapped entirely years ago.
The way I was taught in biology class, oh so long ago, was that sperm mitochondria never enter the egg. More recent evidence shows that sperm mitochondria do indeed enter the egg, but they compose an insignificant fraction of the total mitochondrial population in the zygote. Even more recent evidence suggests that the egg actually destroys the sperm mitochondria, so that you end up with purely maternal mitochondria. On the other hand, there is a report that paternal mitochondria could survive this process, persisting in descendant populations.