Friday, November 11, 2011

Oops Redux

The other day I wrote a short response to a foolish bit of creationist fluff published on-line by the Johannesburg, SA Times live. That piece by creationist Joseph Ulicki was trivial enough; Mr. Ulicki quoted a bit if Geneses, and then he wrote
"And Cutting-Edge Science tells us:

"The specific complexity of genetic information in the genome does not increase spontaneously. Therefore, there is no natural process whereby reptiles can turn into birds, land mammals into whales, or chimpanzees (or any other supposed common ancestor) into human beings".

Biblical creationism and Cutting-Edge Science are in agreement:

And I wondered what "Cutting-Edge" bullshit was this man smoking?

I found this particular bit of creatocrap was from the American Catholic website "The Kolbe Center for the Study of Creation." So, I decided to write a short comment about genetic addition of "specified complexity" by various mechanisms. I called it Cutting edge or bleeding Idiot?. Submissions are by email only, and a day or two later the piece appeared. And, I saw that I had written, "One of the more obvious is simple duplication of a gene during mitosis."

Oh shite!

I wrote "mitosis" instead of "meiosis." There was no way to correct it. Then I thought, Oh shite, Oh shite! because I had actually been thinking of bacterial fission instead of eukaryote cellular division anyway. So, here is a short video on the difference between mitosis, and meiosis.

Play the video.

So, in meiosis there is just 1/2 of the genes of the parent organism in the resulting four haploid cells (ignoring for the moment sex differences). Then these cells can combine their genes with a 1/2 gene complement from another haploid cell to form a full complement, or diploid cell. In Mitosis, the entire gene complement is copied, and two diploid cells are produced, each with the entire set of genes. In both of these kinds of cellular reproduction, proteins called polar fibers, or "spindles" attach to the duplicated chromosomes and pull them to opposite ends of the cell prior to division. There is a fantastic resource that gives excellent definitions of all these terms, and more the Talking Glossary of Genetic Terms created by the National Human Genome Research Institute (NHGRI).

Bacterial cell division is a very different process, as we understand it today. First, bacteria do not have their genes arranged into chromosomes in the same way as ours- the bacterial chromosome forms a circle of DNA. DNA duplication is followed with each chromosome attaching to the cell membrane, and the daughter cell "pinches off" from the mother cell, a process called "Cytokinesis."

This all is high school biology today (noted that people received Noble Prizes for what our high school students are expected to learn today- a blog for a future day). So how did I manage such a dumb mistake? Working back through my mistake this morning, I realized it was in a weird way rather sophisticated. ("polishing a turd" ain't it)?

What I was thinking was "Where/When does gene duplication actually happen?" My top of the head answer was wrong, but less now than I first thought.

Where does eukaryote gene duplication happen? In the adult gonad, there are both stem cells which multiply symmetrically- mitosis- growing the gonad matrix, and asymmetrically- meiosis yielding germ cells. The symmetrically reproducing cells form a cap surrounding the stem cells dividing by meiosis. So, what we see happening is that copying errors during meiosis make one chromosome with both copies of a gene, and one that lacked it. The resulting cell missing the gene will most likely be sterile. The one with the duplicated cell now has extra evolutionary resources. And this happens at the interface between stem cells that duplicate by mitosis, or meiosis. Since a sterile egg, or sperm cell is essentially free to the parent organism, the conferred evolutionary advantage is overwhelming. (Otherwise we human boys would never survive puberty). There is a long term cost when organisms get very elaborate (like us), and that is that asymmetric cell division at the wrong time, in the wrong organ will cause cancer. But that is the typical evolutionary solution; there is never any anticipation for future events or consequences. For an excellent current introduction to this, I recommend the review article;

Sean J. Morrison, Judith Kimble
2006 “Asymmetric and symmetric stem-cell divisions in development and cancer”
Nature 441(7097): 1068-1074.

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