‹ Biology simulationsBiology · AS Level 9700 · 5 The mitotic cell cycleSeven simulations covering Topic 5: chromosome structure (5.1) and mitosis (5.2). Unpack a chromosome in 3D, run the cell cycle clock, watch mitosis from every angle, count cells in a root tip squash, trim telomeres, repair a wound with stem cells and see how mutations let a cell escape control.
About 2 metres of DNA fits into a nucleus a few micrometres across. It is wound round histone proteins to make nucleosomes, the nucleosomes coil into a fibre, and before a cell divides the fibre loops and folds into a compact chromosome made of two identical sister chromatids joined at the centromere. Step up through the levels of packing.
The cell cycle is the sequence of events from one division to the next. Most of it is interphase: the cell grows in G1, copies its DNA in S and prepares to divide in G2. Mitosis divides the nucleus and cytokinesis divides the cytoplasm. Checkpoints stop the cycle if something is wrong, and specialised cells leave it for G0.
A cell with four chromosomes (two homologous pairs, red from one parent and blue from the other) divides by mitosis. Scrub through the stages or press Play. Watch the chromosomes condense, line up on the equator, split into chromatids that are pulled to opposite poles by the spindle, and end up as two nuclei, each with the same four chromosomes.
A stained, squashed onion root tip under the microscope. Click a cell to magnify it, then name its stage. The mitotic index is the proportion of cells that are dividing. Because a cell is more likely to be caught in a long stage, the share of cells in each stage estimates how long that stage lasts.
DNA polymerase cannot copy the very end of a linear chromosome, so every replication leaves the new DNA a little shorter. Telomeres are long runs of a repeated, non-coding sequence (TTAGGG in humans) at the ends. They get shorter instead of the genes, until the cell stops dividing. Stem cells and cancer cells use the enzyme telomerase to rebuild them.
Your skin sheds tens of thousands of cells a minute. Stem cells in the bottom layer of the epidermis keep dividing by mitosis: each division gives one cell that stays a stem cell and one that moves up, differentiates and is finally shed. Cut the skin and the stem cells speed up to repair it. Destroy them too and the gap cannot refill.
In healthy tissue a cell divides only when it is needed, for example to fill the space left by a dead cell, and damaged cells kill themselves. Mutations in the genes that control the cycle can switch those brakes off. A cell that divides without control forms a tumour; if its cells break through the basement membrane into the blood, it is malignant and can spread.
Seven interactive 3D and 2D simulations for Cambridge AS Level Biology 9700 topic 5 The mitotic cell cycle: DNA packed into nucleosomes, chromatin and chromosomes in 3D, the cell cycle with interphase, checkpoints and G0, mitosis in 3D for animal and plant cells, a root tip squash with the mitotic index, telomeres and the end-replication problem, stem cells repairing the skin, and uncontrolled cell division leading to cancer.
Seven simulations covering Topic 5: chromosome structure (5.1) and mitosis (5.2). Unpack a chromosome in 3D, run the cell cycle clock, watch mitosis from every angle, count cells in a root tip squash, trim telomeres, repair a wound with stem cells and see how mutations let a cell escape control.
The mitotic cell cycle is a free, browser-based simulation for Cambridge International AS & A Level Biology 9700 (topic 5 The mitotic cell cycle). The course is assessed in Paper 1 (Multiple choice), Paper 2 (AS structured), Paper 3 (Advanced practical), Paper 4 (A Level structured), Paper 5 (Planning, analysis, evaluation). Syllabus for examination in 2025-2027. There are 7 tabs, each a self-contained experiment with its own controls and live readings, and nothing to install: it runs on a laptop, tablet or classroom projector.
Full syllabus map: AS Level Biology 9700 topic 5.
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