Different River

”You can never step in the same river twice.” –Heraclitus

February 14, 2005

Mitochondrial DNA #2

Filed under: — Different River @ 1:40 pm

I’ve updated the post on mitochondrial DNA to reflect a reader e-mail, which answers the question I posed, and then some. Another reader also left a comment to that post linking to a web page with a slightly (at least) different answer.

Sperm cells certainly have mitochondria; they have a good number relative to the size of the cell. They are needed to produce the burst of energy-on-demand that the sperm requires. At some point (before fertilization) the sperm’s mitochondria are marked with ubiquitin which is a protein that marks other proteins and organelles for destruction. Occasionally this process fails and paternal mitochondria do survive. This only happens very, very rarely (without human intervention).

And now to your question:

Your intuition is right — when the mitochondrial DNA and nuclear DNA do not match, undesireable behavior is the result. Cellular processes need to “recognize” the mitochondria, probably most importantly to govern the various signals the encode for their replication.

To combat this, when the embryo that became Dolly was created, a technique known as egg cytoplasmic donation was used. This technique is sometimes used in humans (though it is pretty far down on the list of fertilization treatments to try). It involves injecting the cytoplasm (everything outside the nuclear membrane) of an oocyte (the female’s cell that goes through meiosis and ends up as an unfertilized egg) with the cytoplasm from the nuclear donor. Thus the new nuclear DNA has some mitochondria it can work with. Presumably the mitochondria donated from the original egg are either recognized, perhaps as they undergo fission, or they are actively destroyed, or they eventually die on their own.

So, the mitochondiral DNA problem probably has nothing to do with Dolly’s early demise. That problem is actually (probably) from the chromosomal DNA.

Oh, and the current best hope for understanding Dolly’s apparently premature aging is telomere cropping. Telomeres are long, non-coding segments of DNA that appear at the end of chromosomes. These caps shrink with age and apparently restrict the number of times a cell can viably undergo mitosis without its DNA falling apart. Since Dolly’s original DNA came from an older cell (ie with already-shortened telomere sequences) the thought is that her cells operated as older cells might, i.e. undergoing mitosis fewer times and with more time between divisions.

In order to keep the discussion in one place, I’ve disabled comments on this post; please leave comments on the original post here.

Carnival of the Capitalists

Filed under: — Different River @ 1:18 pm

This week’s Carnival of the Capitalists, the weekly round-up of economics blogs, is up here at WeekendPundit.

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