Sunday, June 30, 2013

Textbook awesome

Dear B.,

Today I read a truly awesome sentence in a genetics textbook.

I don't know how much you remember about cellular meiosis, but to refresh your memory: in cellular mitosis, a cell splits into two cells, each with its same chromosome count. In meiosis, a cell splits into four cells, each of which has the normal chromosome count, so that it can be combined with another cell during fertilization to produce a new organism. The reason this works is that before mitosis or meiosis, each chromosome builds up an extra chromotid--a copy of itself, still attached to the original chromosome through a centromere--which splits during meiosis to produce two chromosomes.

The splitting takes place in two phases. In the first part, the two versions of each chromosome (e.g. the X chromosome you got from your mom and the one you got from your dad) pair up into what are called "bivalents": like two little X's attached to each other. Then one chromotid of each swaps parts of itself with a chromotid from the other, so you wind up with four chromatids (one identical to the mother's, one identical to the father's, and two that are part of each). Then the cell splits, and one chromosome goes to each new cell. In the second phase, the chromosome itself splits the chromatids apart, and the cell splits again, with each cell getting one chromatid. (This second part is just like mitosis, only the cell already only has half the right number of chromosomes.)

Okay, so the awesome sentence went something like this: "During zygotene [a subphase of meiosis], each chromosome somehow seeks out its homologous pairing in order to recombine." Key word there: "somehow." The textbook went on to acknowledge that the mechanism is not well-understood. It's kind of mind-boggling when you think about it: if a chromosome is just a string of genes, how does your chromosome #11 from your mom know that it's chromosome #11 and is supposed to pair up with your other chromosome #11 from your dad? Rather, the chromosome itself MUST have some extra information that says, "Hey, I'm a #11" The reason I think this is awesome is that here is a fascinating scientific question that most textbooks would probably just gloss over because the answer isn't known, but THIS textbook specifically calls out the fact that the answer isn't known--and questions are the essence of science. Knowing how to ask questions, and what to do to find real answers, and tell them from false answers.

I wish this kind of thing were more common. I wish the average 10th-grade high school student finished the school year knowing more than just a bunch of facts and accepted theories about biology--I want them to know what we don't know as well. Do we know where new phenotypes originate? (If a giraffe evolves a longer neck, where did the gene for longer necks come from in the first place?) That evolutionary theory of punctuated equilibrium that you just taught us--do we know if it's true based on paleontological evidence, or is it just a theory still looking for evidence? Do we really understand how neurons pass information to each other? How do homologous chromosomes find each other? (Maybe there are answers to some of these questions. It's hard for me to tell the difference between 'the details are so complicated that I just won't mention them here' and 'nobody knows how it works but I don't want to admit that in a textbook.' It seems like you need a lot of expertise in a subject to be able to confidently assess which questions are unsolved--you can't easily learn this on your own even with the Internet, which is another reason it should be in textbooks.)

"Somehow." This is my new favorite science word.


Deck thyself now with majesty and excellency; and array thyself with glory and beauty.
Cast abroad the rage of thy wrath: and behold every one that is proud, and abase him.
Look on every one that is proud, and bring him low; and tread down the wicked in their place.
Hide them in the dust together; and bind their faces in secret.
Then will I also confess unto thee that thine own right hand can save thee.

I could not love thee, dear, so much,
Loved I not Honor more.

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