Regulation of Cell Cycle
The cell cycle does not run on a clock — it runs on a chemistry that builds itself up and tears itself down. The engine is a family of enzymes called cyclin-dependent kinases (CDKs), and on their own they do nothing. A CDK only switches on when it binds its partner protein, a cyclin. The active cyclin-CDK complex is what actually drives a cell forward — phosphorylating the targets that commit it to copying its DNA and then to dividing. No active complex, no progression.
The key is that cyclin levels are not constant. A given cyclin is synthesized gradually through one phase, so its concentration rises; when the complex has done its job, that cyclin is abruptly destroyed and its level falls, shutting the kinase back off. This rise-and-fall is what makes the cycle a cycle rather than a runaway reaction. Layered on top are checkpoints — the G1, G2, and M checkpoints — that hold a cell in place until conditions are right: enough growth, undamaged DNA, correctly attached chromosomes. A checkpoint is a gate, and the cyclin-CDK activity is what the gate is deciding whether to let through.
Interactive · Cyclin Control
Build cyclin up and watch its CDK partner switch on to push the cell through a checkpoint — then destroy the cyclin and see the kinase go quiet. Cyclin levels rise and fall; the complex is only active while the cyclin is present.
Cyclin Control · Open the full sandbox →The mistakes here all confuse the engine with a switch that never moves. One is imagining cyclin as a constant presence — a protein that is simply "there" keeping the cycle going — when in fact its whole job depends on being made and then destroyed, so that CDK activity turns on and then off. The other is reading cancer as cells that are merely dividing fast, when cancer is really a loss of checkpoint control: the gates that should halt a cell with damaged DNA or improper growth signals fail, so it divides when it never should have been cleared to. Every scenario in this topic asks you to reason from what the cyclin level is doing and whether the checkpoints still hold to what happens to the cell.
The work
3 ways in · any order
Lesson
Regulation of Cell Cycle
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The cell cycle is driven by cyclin-CDK complexes whose activity rises and falls as cyclin is built up and destroyed, and it is gated by checkpoints. The lesson walks the ways students misread that: picturing cyclin as a constant presence rather than a rising-and-falling signal, and reading cancer as fast division rather than a loss of checkpoint control. It closes with a ten-scenario applet that asks you to reason from cyclin levels and checkpoint state to what a cell actually does.
Diagnostic
10-item topic check
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Ten items on regulation of the cell cycle — that cyclin-CDK activity rises and falls as cyclin is synthesized and destroyed, so cyclin is not a constant presence (U4-BIO14); and that cancer is a loss of checkpoint control, not merely cells that divide fast (U4-BIO13). Take it cold to surface which of these are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.