I’m really enjoying running on-line tuts with Schol Bio students, because the questions & discussions are so interesting. (So, hopefully the students enjoy them too!) Last week we got onto talking about the enzyme lactase and the fact that in some populations many individuals continue to produce it into adulthood (thus making them lactose-tolerant, & able to consume dairy products without unfortunate intestinal reactions to the milk sugar lactose).
While I’ve written about lactose, lactase, & the evolution of lactose tolerance before (see here and here), I thought it would be useful to update that, not least because there are some excellent on-line resources available for teachers & students: for example, this video from Biointeractive about the evolution of lactase persistence.
Lactose is a sugar that’s found in milk, and all mammals are capable of digesting it in infancy due to the presence of an enzyme called lactase. This is because one facet of mammalian evolution has been a pattern of gene expression in the small intestine that varies with developmental age: infant mammals produce a large amount of lactase, but the expression of the gene concerned is turned off when they’re weaned (perhaps by methylation of the DNA that blocks an activator from binding to it). This is true for many humans too, (in which case we’d say they’ve become lactose-intolerant; see also here) – perhaps by methylation of the DNA , blocking any further transcription. (This would suggest, in answer to another question from the class, that it can’t be “turned on” again after weaning, but I’d welcome clarification on that!)
The students asked about the specifics of how the lactase was regulated, & in looking for a more detailed answer than I could give at the time, I found this excellent video on the Biointeractive website. While it appears that we still don’t know all the specifics of how the gene that codes for lactase is regulated, we do know that the mutations that confer lactase persistence sit in a region of DNA that acts as an enhancer for the gene. (There’s a nice explanation of the term enhancer on SciTable, and at this link there’s a video about the control of gene expression in both prokaryote and eukaryote cells.)
I think that captures everything we talked about – hopefully our remaining sessions this year will be just as stimulating.