上一片认识了基因——说明书里的一个个条目。可这本说明书有 30 亿个字母，它到底收在哪儿？Last leaf we met genes — the entries in the manual. But that manual is 3 billion letters long. Where is it even kept?
第 1 步：打包：DNA 缠成染色体Step 1: Packing: DNA into chromosomes
先看这本超长说明书，怎么从“一条线”变成“一册书”。First, watch the giant manual turn from “one long thread” into “a bound album”.
DNA 要是拉直，一个细胞里的长度约 2 米——根本塞不进细胞核。办法是把它缠到蛋白质线轴上，一圈圈卷紧，最后打包成一根短棒：染色体。一个细胞里的全部 DNA，就靠这种打包才装得下。Stretched out, the DNA in one cell would be about 2 metres long — impossible to fit inside a nucleus. The trick: wrap it around protein spools, coil it tighter and tighter, and pack it into a short rod — a chromosome. Packing is how all that DNA fits.
打包成册以后，一个“人”的说明书一共有多少册？Once packed, how many volumes does one human manual come in?
人的体细胞里有 23 对染色体，一共 46 条；每一对里，一条来自爸爸、一条来自妈妈。精子和卵这类生殖细胞只带 23 条——它们结合成受精卵，两套拼起来，又变回 46 条。A human body cell holds 23 pairs of chromosomes — 46 in total; in each pair, one comes from dad and one from mom. Reproductive cells (sperm and egg) carry only 23. When they join into a fertilized egg, the two sets combine and the count returns to 46.
23 对里，有一对特别有名——它决定你是男孩还是女孩。One of the 23 pairs is especially famous — it decides whether you are a boy or a girl.
第 23 对是性染色体：XX 是女孩，XY 是男孩（Y 由爸爸提供）。数目也会出错：第 21 对多出一条，就是 21 三体（唐氏综合征）——染色体多一条少一条，影响都很大，所以清点染色体是孕期检查的重要内容。Pair 23 is the sex pair: XX for girls, XY for boys (the Y comes from dad). The count can also go wrong: an extra copy of chromosome 21 causes trisomy 21 (Down syndrome) — one chromosome too many or too few matters a lot, which is why counting chromosomes is an important part of prenatal check-ups.
🧬 你来数染色体（30 秒）🧬 Your turn: count the chromosomes (30 seconds)
先点上面的按钮看不同细胞里染色体怎么排，再完成 4 道清点题。Click the buttons to see how chromosomes are arranged in different cells, then answer the 4 counting questions.
一句话记住它：染色体 = DNA 缠上蛋白质打包成的收纳册；人的体细胞 46 条（23 对），生殖细胞 23 条。Remember it in one line: a chromosome is DNA wrapped around proteins into a storage album; human body cells hold 46 (23 pairs), reproductive cells hold 23.
染色体的本质是“打包”：DNA + 蛋白质，为了把约 2 米长的链装进细胞核A chromosome is packing: DNA + proteins, so ~2 metres of thread fits in a nucleus人的体细胞 23 对 46 条；精子、卵各 23 条；受精卵又回到 46 条Body cells: 23 pairs = 46; sperm and egg: 23 each; fertilized egg: back to 46第 23 对 XX/XY 决定性别；数目出错影响很大（如 21 三体），所以孕期要清点染色体Pair 23 (XX/XY) decides sex; a wrong count matters (e.g. trisomy 21) — hence prenatal counting
内容参考 OpenStax 等公开教材，多来源核对 · AI 生成、人工审核 · 发现错误欢迎指正，帮这片叶子长得更好。Based on OpenStax and other open textbooks, cross-checked across sources · AI-generated, human-reviewed · Spotted a mistake? Tell us — help this leaf grow.
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