What I taught yesterday: master genes and maps

On Wednesdays, I try to break away from the lecture format and prompt the students to talk about the science of development. We're working our way through Sean Carroll's Endless Forms Most Beautiful, and yesterday we talked about chapters 3 and 4.

Chapter 3 has an overview of basic molecular biology — transcription and translation, that sort of thing — and since these are junior and senior students who've already heard that a few times, we skipped right over it and they explained to me what master genes are, with specific examples of homeobox-containing genes like the Hox genes and Pax6. They caught on fast that what we call master genes are actually just transcription factors located high up in a regulatory hierarchy.

I think we also got across a less-than-naive idea of the evolution of Hox genes. There is a recognizable, conserved motif in each of these genes, but the proteins are far more than just their homeodomains, and can exhibit considerable variation — necessary functional variation, because the expression of different Hox genes are going to have distinct morphological consequences.

Chapter 4 has a general theme of maps and geography — what does it mean for a cell to be in a particular position and to have a particular fate? We also get into details. This is a very fly-centric chapter, and we get a picture of early development in the fly and the specific patterning and positional organization in the early embryo of that organism, with an introduction to many genes we'll be hearing much more about during the course of the term. We also got enough information on vertebrate development that I could ask them to play the compare and contrast game: what's different and what's the same in fly and mouse development? I'm trying hard to be the Reese's Peanut Butter Cup of development in this class: it's so easy to say, "they're the same!" and focus on common molecular mechanisms, or to say "they're different!" and talk about the numerous quite radical innovations between them (especially in the fly, which is a weird, highly fine-tuned machine for rapid robust development). I'm trying to get across that both statements are absolutely true, and they really taste great together.

Friday is their first exam. Next Monday, class will be an overview of nematode development, to prime them for the lab exercises for the next two weeks which will be all about photomicrography of worm development and behavior, and also more details about early fly embryology to get them prepared for a couple of weeks of nothin' but flies. I also warned them that next Wednesday we'll be discussing chapter 5 in Carroll, just chapter 5, because I've found in the past that that's usually the brain-clogger chapter, with all its talk of boolean logic and gates and circuits.