In teaching cell division, I leave out the terminology about the different phases (I mean, really, leptotene and zygotene?) and focus almost entirely on the behavior of chromosomes. The main emphasis in class is on meiosis, to set the stage for Mendelian genetics. Students need to know that most of the action occurs in the first division:
- homologous chromosomes pair up and align in prophase I
- crossing over occurs between homologous chromosomes in prophase I
- homologous chromosomes separate to daughter cells, and sister chromatids do not separate
- the first division is when daughter cells become functionally or genetically haploid
The last point appears to be the most difficult for students to grasp. I illustrate this point using the X and Y chromosomes. They pair in prophase I, and then separate in the first division. The daughter cells have either an X or a Y; they don’t have both. Each cell now has only one sex chromosome, like a haploid cell.
Several years ago I designed an interactive flash tutorial, with programming assistance from Pearson. It can be used as a classroom demonstration, or a homework exercise for students. It uses human chromosome 7, with wild type and cystic fibrosis alleles for CFTR, to track segregation through meiosis, with and without crossing over.