Copy the information.
Before pressing Play, predict how chromosome count and DNA quantity will change. Watch S phase, then compare both daughter cells.
FIELD NOTE 20 / CELL DIVISION & CHROMOSOMES
Life makes
another beginning.
Inside a cell, an extraordinary handover is taking place.
Follow the DNA as it is copied, exchanged and divided. Look through the cell’s layers—and discover what each new cell receives.
Follow the science ↓0:00 / 1:04 illustration time
WHAT IS HAPPENING?
The nucleus contains four unreplicated chromosomes: two homologous pairs. Their DNA is extended chromatin.
Motion, stage duration, organelle placement and relative sizes are illustrative. This is a chromosome-accounting model, not a physical simulation of a living cell.
COUNT THE INFORMATION
Copies are not chromosomes.
Per current cell
C is the DNA amount in one unreplicated haploid genome. Here, 1C contains one chromosome A and one chromosome B.
A LIVING INTERIOR
Look beneath the surface.
01
Nucleus
The nuclear envelope surrounds most of the cell’s DNA. It opens during the illustrated division, then reforms around the separated chromosome sets.
FOLLOW ONE CHROMOSOME
The same thread. A new story.
A1–A8 and B1–B6 name illustrative DNA regions. They are not real genes or human chromosome numbers. Parent-origin colours stay with each region after an exchange.
THE HANDOVER
What did each cell receive?
Complete the division to reveal each daughter cell’s chromosome complement.
Playing. View and playback controls work live; biological setup changes are applied with Restart.
FOLLOW THE SCIENCE
Life copies information.
Division decides its destination.
A cell must do more than grow and split. It must copy its genetic information, organise that material and distribute it. The machinery is microscopic; the consequences reach across generations.
This lab uses a deliberately small genome: two homologous pairs, A and B. One member of each pair came from each parent. Human body cells typically contain 23 pairs, but a four-chromosome model lets you follow every copy without losing the story.
MAKE A PREDICTION. THEN LOOK CLOSER.
Four investigations inside a cell.
Make a new combination.
Follow chromosome A from Parent 1 through a crossover. Compare the two sister chromatids and their final destinations. Then try the other orientation of pair B.
Choose the wrong pole.
Start with a meiosis-I separation error. Inspect all four products. Change the setup to a meiosis-II error and restart: which products now have a complete set?
Count more than chromosomes.
Start with a reciprocal rearrangement and adjacent-1 segregation. All four products still have two chromosomes. Does each contain one copy of every DNA region?
An X is already a copy.
The familiar X represents a condensed chromosome with two sister chromatids joined at a centromere region. Each chromatid carries one DNA molecule. Before DNA replication, a chromosome has one chromatid; during interphase, its DNA is much less condensed.
Copying DNA doubles its quantity without doubling the chromosome count. Once the sisters separate, each becomes a chromosome. That is why an undivided cell briefly has twice as many chromosomes in mitotic anaphase.
Preserve a set.
Or reduce it.
Mitosis normally gives each daughter the same chromosome complement as the starting cell. Meiosis has one round of DNA copying followed by two divisions: homologous chromosomes separate first, sister chromatids second.
There is no DNA replication between meiosis I and II. Our symmetric illustration produces four meiotic products. The development of eggs and sperm differs; human oogenesis does not make four equivalent eggs.
Same regions.
New combinations.
Crossing-over exchanges corresponding DNA between non-sister chromatids of a homologous pair during prophase I. One crossover in this model changes two of the four chromatids, while the other two remain unchanged.
Independent assortment adds another source of variation. With two pairs and no crossing-over, four whole-chromosome combinations are possible across meioses. A single run usually gives two reciprocal combinations, each twice.
A count cannot tell
the whole story.
Nondisjunction sends chromosomes that should separate to the same side. An error in meiosis I affects the complement of all four products here; an error in meiosis II in one cell affects two.
A reciprocal translocation exchanges material between different chromosome types. A balanced starting cell retains all its regions, but its meiotic products need not. A product can have two chromosomes and still have too much of one region and none of another.
THE ORDINARY DIVISION · NO ERRORS
Track two different quantities.
| Stage | Chromosomes | DNA molecules | DNA content |
|---|---|---|---|
| Before replication | 4 | 4 | 2C |
| After replication | 4 | 8 | 4C |
| Each mitotic daughter | 4 | 4 | 2C |
| Each meiosis-I daughter | 2 | 4 | 2C |
| Each meiosis-II product | 2 | 2 | 1C |
n counts a chromosome set; C measures DNA quantity. These are related, but they are not interchangeable. During anaphase, separated chromosomes are counted inside the still-undivided cell until cytokinesis is complete.
The model, assumptions & references
Exact DNA accounting inside an illustrative animation
Chromosome A has eight equal-length model regions and B has six. Each carries a parent-origin tag and a stable identity. One unreplicated haploid genome therefore contains 14 region units, defined as 1C. These regions are abstract bookkeeping units, not named genes or physical distances.
DNA content in C = total region units / 14
DNA replication: 2C → 4C
Ordinary mitosis: 4C → 2C + 2C
Ordinary meiosis: 4C → 2C + 2C → 1C + 1C + 1C + 1C
DNA replication: 2C → 4C
Ordinary mitosis: 4C → 2C + 2C
Ordinary meiosis: 4C → 2C + 2C → 1C + 1C + 1C + 1C
Replication duplicates every source region exactly once. Separation redistributes complete chromatids without creating or deleting DNA. The total region inventory across all final products equals the inventory after replication, including in error cases. Fractional C values in unbalanced products are calculated from region content, not rounded chromosome counts.
Crossovers and independent orientation
The optional single crossover exchanges the distal portions of A/Parent 1 chromatid 2 and A/Parent 2 chromatid 1. The boundary can follow region A3 through A7, beyond the model centromere. Corresponding region identities and global copy counts are conserved. Pair A’s orientation is fixed as a reference; flipping B yields the two distinct complementary whole-chromosome allocations. This model does not include multiple crossovers, crossover interference or realistic crossover frequencies.
Errors selected for comparison
The mitotic error sends both sisters of A/Parent 1 to the first pole, giving final counts of 5 and 3. The meiosis-I error sends both A homologues to one daughter, giving 3, 3, 1, 1. The meiosis-II error affects A in only the first daughter, giving 3, 1, 2, 2. Every chromatid still has exactly one destination. These outcomes are selected demonstrations, not sampled biological probabilities.
Reciprocal translocation
Before replication, the distal A6–A8 region of A/Parent 1 is exchanged with B5–B6 of B/Parent 1. Each derivative retains its original centromere. The carrier still has every DNA region twice. Matching regions pair as a quadrivalent during meiosis I.
Alternate segregation sends the two derivatives together and the two original chromosomes together. All products have a complete regional complement. Adjacent-1 sends one derivative and the other chromosome’s normal homologue to each side. The resulting products retain two chromosomes, but have complementary missing and duplicated distal regions. Those two selectable modes are not exhaustive or equally likely. Crossing-over and nondisjunction are disabled for this structural comparison.
A balanced rearrangement is balanced in DNA quantity. A breakpoint can still disrupt a gene or its regulation. This lab models the location and dosage of abstract regions; it cannot infer health effects, fertility, embryo viability or personal reproductive risk.
What the illustration represents
The cell membrane, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, ribosomes, nucleus and spindle provide anatomical context. Organelle numbers and trajectories, membrane motion, nuclear reassembly, chromosome paths, relative sizes and stage durations are choreographed for visibility. Spindle attachment is represented at centromere/kinetochore regions. This is an animal-cell illustration with open nuclear-envelope breakdown and cleavage-furrow cytokinesis, not a model of every organism or cell type.
Mitochondria have their own DNA, which is not included in this nuclear chromosome model. The lab omits molecular checkpoints, replication chemistry, ATP consumption, motor-protein forces, three-dimensional cell mechanics and detailed gamete development. It shows successful completion of the selected redistribution, rather than simulating biological responses to errors.
Controls and reproducibility
Play/Pause, speed, timeline, organelle visibility, labels, spindle display and selection preserve the active setup. All animation follows the same illustration clock and freezes when paused or hidden. Reduced motion starts paused. Setup controls prepare a new division without changing the active one; Apply setup & restart and the labelled investigation buttons explicitly begin again. Results are deterministic. A kept comparison is a snapshot of the earlier completed run and lasts until the page is reloaded or cleared.