Sunday, January 06, 2008

Why is it so hard to clearly explain what 'chromosome' means?

I'm polishing up some of the material I'll present in classes #4 and #5 of my intro biology course. I think class #4 is OK. It introduces DNA, chromosomes and genes, although 'introduces' is hardly the right term for something the students will have been learning about about since grade school. In this class chromosomes are just DNA molecules big enough that they have lots of genes, and different chromosomes have different sequences and different genes. But it gets a lot more complicated in class #5.

Class #5 is about how DNA and genes and chromosomes vary. It first introduces the evolutionary concept of homology - defined as similarity because of descent from a common ancestor. Then we take the previous class's introduction to chromosomes etc. and consider the relationship between our paternal and maternal sets of chromosomes, and why we refer to them as 'homologous'. I push the idea of these being different versions of the same chromosome, but students often find the whole business confusing, which puts them in deep trouble when we move on to meiosis and genetic analysis.

Part of the confusion arises because chromosomes are physical things but they are also conceptual categories of things. Two particular DNA molecules in a particular cell in your body (e.g. in the skin cell closest to the tip of your left index finger) are chromosome 13s, but we can also refer to 'your maternal chromosome 13' (of which there are as many as there are cells in your body, about 10^10) and to 'human chromosome 13' (2 x ~10^10 x ~7x10^9). And these are far from identical.

Students need to think about the differences between the versions of human chromosome 13, as well as what unites them, and this isn't easy. This point in the class will be a good place (one of many) to emphasize the importance of variation in biology. For first-year students, having to think about variation and diversity will be new, and it's one of the big things that separates biology from the physical sciences (see Why biology is harder than physics).

Maybe I can give them a sketch that moves out from the single cell to the human population. Let's see what I can pull together from Google Images.

1 comment:

Fred Ross said...

Something that might be useful in introducing chromosomes...

In section 1.2 of John Gillespie's, 'Population Genetics : A Concise Guide', he has a short, lucid discussion of identity of genes (by state, by origin, by descent). "A human chromosome 13" is a shorthand for a more precise correspondence. He's talking about hunks of DNA on chromosomes, but it will apply to whole chromosomes just as well.