Investigation 7: Cell Division, Mitosis, and Meiosis
Investigation 7 uses onion root tip slides to estimate how long each phase of mitosis lasts, and Sordaria ascospore patterns to measure how far genes are from their centromeres.
An onion root tip slide freezes hundreds of cells mid-cycle, each caught in whatever phase it happened to be in when the slide was made. Since cells spend time in each phase roughly in proportion to how long that phase actually lasts, the fraction of cells you see in, say, metaphase estimates the fraction of the whole cell cycle spent in metaphase. Count enough cells, multiply the fraction by total cycle length, and you get a real time estimate without ever filming a single cell dividing.
Telling phases apart means reading chromosome behavior. Prophase shows chromosomes condensing into visible threads, still scattered in the cell. Metaphase lines chromosomes up in a single file down the cell's middle. Anaphase pulls sister chromatids apart toward opposite poles, so you see two separating clusters. Interphase looks uneventful, but that is misleading: the cell is growing, running its normal metabolism, replicating its entire genome, and passing checkpoints before it ever risks dividing. Nothing about that is idle.
The second half of this investigation shifts from mitosis to meiosis, using the fungus Sordaria to study crossing over. Sordaria ascospores sit in a row inside a sac, and a crossover between a gene and its centromere during meiosis shows up as a visible shift in spore color pattern. Counting the fraction of asci with that crossover pattern lets you estimate map distance, connecting a visual count directly to genetic recombination frequency, the same logic as the root tip count but applied to meiosis instead of mitosis.
Key terms
- Mitotic index
- The fraction of cells in a sample that are actively dividing, used to estimate time spent in each phase.
- Interphase
- The phase between divisions where a cell grows, replicates its DNA, and passes checkpoints.
- Crossing over
- The exchange of DNA segments between homologous chromosomes during meiosis.
- Ascospore
- A spore produced inside a sac in fungi like Sordaria, whose arrangement reveals crossover events.
- Map distance
- An estimate of how far a gene sits from its centromere, calculated from crossover frequency.
Watch it explained
- How it worksHow do you tell prophase from metaphase from anaphase under a microscope?
- Worked exampleFind how many minutes of a 24 hour cycle are metaphase if 8 of 200 cells are in it.
- How it worksWalk me through modeling crossover distance with sordaria ascospore arrangements.
- Common mistakeWhy isn't a cell that looks like it is resting actually doing nothing?
Check yourself
End of topic.