I’m just starting the Cell Cycle/Cell Divison topic with Year 12.
I usually kick off with a question along the lines of what they were like when they first started life’s journey. Yes, that’s right, a zygote. A fertilised egg.
I draw a little cell on the board.
So what did that cell have to do in order to become you?
Again, correct; it had to divide.
I draw two little cells on the board.
And divide again.
I draw four little cells on the board.
What’s it become now?
No longer a zygote, but not yet a fetus, you’re an embryo. And so on. Until there’s roughly 50 trillion of them. Depending on the group, I might get them to calculate how many division this wold take.
OK, if that’s all that happened, what would you look like now?
A nice comedy moment this as they visualise the ever increasing 50 trillion cell blob that cell division alone would make them.
So what do cells need to know???
This is a really good discussion point as they figure out the necessary skill set of any cell. We eventually agree on the following:
- it must know when to start dividing
- it must know when to stop dividing
- it must know where it is
- it must know what’s it’s going to be
Right! Now we’ve got something to work with. Let’s start with the division process.
I’m sure you already have an impressive armoury of amazing replication factoids to impress your students with.
Here are some of my favourites.
- The DNA in your cells could stretch from here to the sun and back 600 times. That’s 68,000,000 x 2 x 600 miles.
- From zygote to fully formed, differentiated, multi-tissued, multi-organed functioning organism, takes a mouse 19 days. 19 days! That’s just incredible.
- Copying takes place in a cell at 2000 base pairs per second.
So how long would it take to replicate the entire genome???
If they work their calculators correctly, they should figure that the 3,000,000,000 base pairs are copied in about 8 hours.
- Polymerase has an error rate of between 1 in 1000 to 1 in 100,000.
- Yet the overall mutation rate is 1 in 100,000,000 per cell cycle.
These mutations are the basis of ageing. But how to explain this discrepancy between error rate and mutation rate?
It’s a good way of introducing the idea of check points, controlled by an army of checking and correcting enzymes that oversee the process. You might even want to flirt with the concept of nucleotide excision repair.
But the key point is that these check points are REALLY important because you want your genome to look like this…
not like this…
which is what the karyotype of a human breast cancer cell line, MDA 231, looks like.
There’s enough questions and disuccsion and intriuge here to last us another half hour or more. But when we’re done, we’ll launch into the cell cycle, followed by Mitosis, or possibly watch the FOP video by way of looking at the catastrophic consequences of cells not knowing what they’re meant to be…
An update on Year 9 microbiology projects to come soon!