Tuesday, January 4, 2011

You are your mother's mother's mother.

After a halting start, this blog seems to have fallen by the wayside.  So my resolution for 2011 is to do one paper a week.  We'll see how that goes, but in the meantime let's talk about a little round piece of extranuclear genetic material in all of our cells: mitochondrial DNA, or mtDNA.

The mitochondria are the little energy factories within our cells, taking sugar (well, sugar derivitaves really) mixing in oxygen, and making lots of ATP, carbon dioxide, and water.  It's hypothesized they started as a symbiotic bacteria in an ancestor cell looooong ago which got cozy and took up permanent residence.  The mitochondria precursor had it's own circular DNA which liked its new home so much, it  kicked out lots of the genes it no longer needed.  Nature is lazy, so duplicate genes in the chromosomes were deleted from the mtDNA.  Mitochondrial DNA contains some genes essential for making copies of itself (replication), and for a few things that only happen in the mitochondria, just 37 in total.  It's a lean, mean replicatin' machine.

There's lots about mtDNA we can talk about, but let's focus on inheritance.  You inherited 99.999999999% of  your mtDNA from your mother.  And she inherited hers from her mother.  And so on. So much so, a really neat book on the subject was written that traces all of humanity back to seven women, based on mtDNA studies.  Why do we all carry our mother's mtDNA?  When you were just two halves of a cell, the big pillowy egg cell from your mom carried all the good gooey innards whereas the teeny sperm cell from your dad only carried tightly packaged chromosomal DNA - no room for mitochondria, no room for mtDNA.

A big question is why trace lineages from mtDNA?  It could also be done through X or Y chromosomes.   However, the Y chromosome is not a good candidate because, first, only half the population will have a Y chromosome.  Second, the Y chromosome is highly palindromic, meaning lots of the sequence mirrors itself.  Thus, it's a technical challenge to be able to accurately sequence most regions. Perhaps this warrants further exploration another day. The X chromosome would be a great candidate, but it's big.  Much, much bigger than most chromosomes.  Containing about 1098 genes and containing 150,396,262 base pairs (give or take), the X-chromosome is quite a beast.  Getting all researchers to agree to track subtle changes in small regions of such a large hunk of genetic material would be fraught with lots of ego battles, for sure. Not that there's any ego in any of this, right?

So, why mtDNA to trace lineages?  The answer is that humans, like nature, are pretty lazy too.  MtDNA is pretty small - only 16,500 base paris.  And there are just a few regions that can be mutated with no consequence to the genes.  Since these areas mutate at a predictable frequency, about 1 in every 33 generations, (much higher than in chromosomal DNA) these regions can serve as pretty neat molecular clocks.  The mutations give clues to how long it's been since these mutations arose, and when one person's mtDNA is compared another's, or to ancestors, researchers can make guesses as to who is related to who.

Neat?  You bet.  Practical?  Of course.  Stay tuned!


  1. Hooray for gooey cell parts! Glad you're back.

  2. oh yay! i've missed your science posts! this one was well timed. my hubby is learning about yeast to brew his own beer. and we were just talking about the mom/mitochondria connection right before i logged onto the computer. love the big world of science ;)

  3. Very interesting... can't wait for more! :)

  4. K, funny you say this. I've been thinking (somewhat) seriously about setting up my own little garage yeast lab specifically to engineer better beer yeast strains. Good luck with the home brewing! If you need a taster, let me know.