DNA, genes and chromosomes sim

What the sim shows

DNA, genes and chromosomes: teaching models on a lab bench, and four zoom levels from a body cell to the DNA double helix. The nucleus of a human body cell holds 46 chromosomes in 23 pairs; a gamete has 23, one from each pair. Each chromosome (before it is copied) is one long DNA molecule coiled up with proteins, and once copied before cell division it is two identical sister chromatids joined at the centromere. A gene is a short section of DNA that codes for a protein. DNA is a double helix: two strands with sugar-phosphate backbones, joined by pairs of bases held by hydrogen bonds, A with T and C with G, the strands running in opposite directions (5′ to 3′ and 3′ to 5′). So in double-stranded DNA A = T and C = G. Three bases in a row code for one amino acid. Simplified on purpose: not to scale from one zoom level to the next: each level has its own rough size (≈) and scale bar; the sizes are rough guides, because real cells, nuclei and chromosomes vary; the chromosomes are drawn condensed and copied (X-shaped), as they look early in cell division, just before the nuclear membrane breaks down; most of the time they are long thin threads too fine to see; the nuclear membrane is still drawn round the condensed chromosomes; in the nucleus the chromosomes are drawn about 13 times shorter than their real length (the highlighted one against the 5 µm chromosome at level 3) and spread out, so all 46 can be seen and counted; their lengths are only a rough guide; the gene is drawn as a wide band; a real gene is a tiny part of a chromosome; a short made-up gene sequence of 12 base pairs, so a few base pairs stand for the thousands in a real gene; the helix is drawn simplified and flat, with its two backbones evenly spaced (no major or minor groove), and it shows about 36 base pairs; the ladder beside it is a short stretch of the same two strands, unwound; the bases are drawn as tiles, and the reason they pair is kept to GCSE level: base size and the number of hydrogen bonds; the proteins that DNA is coiled round are not drawn; the bottom rung is given as an example; the sorted pairs show a female (XX); a male has XY; the bench models are teaching models, not real objects at any one scale.

• Glow: the model being zoomed into, and your row on the zoom map
• Close-up: the zoom level, each drawn to its own rough scale (≈)
• Dashed slot: an empty place for a base on strand 2

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
• Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
• Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.

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