You are roughly thirty trillion cells. Every one of them traces back to a single fertilised egg, copied and split, copied and split. Right now, somewhere in your body, a cell is doing it again.
As a cell gets bigger, its volume grows faster than its surface area. The membrane it relies on to move materials in and out can no longer keep up with the demands of the interior.
So an organism doesn't grow by inflating its cells. It grows by making more of them. Repair works the same way, a wound closes because new cells fill the gap.
And every one of those new cells needs a complete copy of the organism's DNA. That requirement shapes everything that follows.
Stretched out, the DNA in one cell would run about two metres. You cannot drag a loose thread that long into two neat piles without it tangling.
So before division, the DNA supercoils, wound tightly around proteins into compact chromosomes. Condensed like this, it can be sorted cleanly.
It has also already been copied. Each chromosome is now two identical sister chromatids, joined at a single point called the centromere.
When one cell divides into two, how much DNA does each daughter cell end up with?
Hold that thought. Watch what happens next, and check yourself.
The nuclear membrane breaks down. Spindle fibres reach in from opposite poles and attach to each chromosome at its centromere.
Each chromosome is pulled to the middle and held there, tension balanced between the two poles. Nothing moves until every one is attached.
The centromeres split. One chromatid from each pair travels to each pole. Both ends of the cell now hold an identical, complete set.
In one sentence, what makes the two daughter cells genetically identical to each other?
Between divisions the cell spends most of its life in interphase, growing, copying organelles, replicating DNA. A family of proteins called cyclins decides when it may advance. When the right cyclin reaches a high enough level, it triggers the next stage.
Raise the cyclin level and drive the cell through its cycle.
Each cyclin rises and falls at its own moment, handing control to the next. Drag across the cycle, or tap a phase, to see which one is in charge and what it triggers.
Cyclin D builds through G1, driving growth and pushing the cell toward DNA synthesis.
A mutagen damages a gene that normally regulates the cycle. The cell can no longer read its own checkpoints, the signals that would normally say wait, or stop and repair, or self-destruct.
With the checkpoints gone, what does the cell do?
It divides over and over. A healthy cell with unfixable damage would normally trigger apoptosis, a built-in self-destruct. A cancer cell has lost that brake, so it keeps cycling, building a tumour. Cells that should have died instead make more of themselves.
This is why cancer is, at its core, a disease of the cell cycle, not of growth itself, but of the controls on growth.
Why would a tissue with an unusually high mitotic index suggest cancer?
The mitotic index is the fraction of cells caught mid-division. Connect it to what you just decided about lost checkpoints.
Fill in the blanks. Stuck? Tap Reveal answers.
Before division, DNA condenses into , each already copied into two sister . During these are pulled to opposite poles, giving two genetically cells. The whole cycle is controlled by proteins called , and when that control fails, the result can be .
Write your answer first, then reveal the mark scheme and award yourself a point for each idea you covered.
Explain how the cell cycle is controlled. A complete answer names the controlling molecules, describes how their activity changes, and links that change to progression through the cycle.
Examiner's note · four distinct ideas score full marks. Naming cyclins but not explaining the rise-and-fall, or the CDK link, is the most common way to drop a mark.
A cell copies its DNA, sorts it into two identical sets, and splits. Doing it on schedule, and only when permitted, is what keeps a body working.
Cells don't divide at random. They move through interphase and mitosis in a fixed, repeating order, copying everything before they split.
Cyclins rise and fall to license each stage. Checkpoints can pause the cycle, demand repairs, or trigger apoptosis.
When that control is lost, division runs unchecked. The mitotic index climbs, and a tumour grows. Cancer is the cell cycle without its brakes.