IB生物 遗传学 DNA复制 转录翻译 减数分裂
遗传学在国际文凭生物学里落在 Theme D Continuity and Change 这条主线，从 DNA 复制一路延伸到遗传比例的计算。IBO 官网 2023 年 8 月首次教学、2025 年首次评估的这版指南，把过去分散的分子遗传与经典遗传收进同一个主题，SL 与 HL 的内容差别也就集中在后面几节。SL 学生要掌握复制、转录翻译与减数分裂的基本机制，HL 再往上加基因表达与细胞周期调控。很多学生以为背下碱基配对就足够，做到预测遗传比例和卡方检验那一步才发现前面的机制必须真的理解。下面按 D1.1 到 D3.2 的顺序，把十二个最容易失分的环节逐个讲清。
Genetics sits inside Theme D Continuity and Change in the current IB Biology course, running from DNA replication through to the ratios used in inheritance questions. The guide released by IBO for first teaching in 2023 and first assessment in 2025 groups molecular and classical genetics under one theme, so a single question can link them, and the gap between SL and HL is concentrated in the later sections. Many candidates memorise base pairing but cannot explain where a 9:3:3:1 ratio comes from, which is exactly where the marks disappear.
1. DNA 的分子结构与碱基配对
DNA 由四种核苷酸组成，每个核苷酸含一个磷酸、一个脱氧核糖和一个碱基。两条链方向相反，一条是 5' 到 3'，另一条是 3' 到 5'，这种反向平行让碱基对稳定地排在内部。腺嘌呤与胸腺嘧啶之间形成两个氢键，鸟嘌呤与胞嘧啶之间形成三个氢键，所以 G 与 C 含量高的区域更难被打开。糖和磷酸交替构成骨架，真正携带遗传信息的是碱基序列。很多学生以为氢键就是共价键，或者以为出现双螺旋就说明已经形成了两条新链。IBO 官网的 D1.1.1 要求学生把复制写成产生碱基序列完全相同的精确拷贝，把这个定义和结构一起理解才不会写偏。
The double helix only makes sense once the chemistry behind it is clear. Each nucleotide carries one phosphate, one deoxyribose sugar and one base, and the two strands run in opposite directions so the base pairs sit protected on the inside; adenine pairs with thymine through two hydrogen bonds while guanine pairs with cytosine through three, which is why GC-rich regions need more energy to pull apart. A frequent error in exam scripts is calling those hydrogen bonds covalent, or claiming that a double helix automatically means two new strands have been built.
2. 半保留复制与聚合酶的方向限制
复制开始时解旋酶先断开碱基之间的氢键，把双链打开成复制叉。DNA 聚合酶只能把新的核苷酸接到已有链的 3' 端，所以两条模板的工作方式完全不同。沿着 5' 到 3' 方向连续合成的是前导链，另一条模板只能分段合成，形成多个冈崎片段。连接酶随后把片段接成一条完整的链。1958 年 Meselson 与 Stahl 用同位素标记的实验证明复制是半保留的，每个子代分子都保留一条旧链。很多学生以为两条链会同时连续复制，画图时就把后随链画成一条直线。IBO 官网写明这一节只需掌握解旋酶与 DNA 聚合酶的一般作用，不必罗列所有酶的名称。
Replication begins when helicase breaks the hydrogen bonds and opens the double helix into a replication fork. DNA polymerase can only add nucleotides to an existing 3' end, so the two template strands are copied in different ways; one is extended continuously as the leading strand while the other is built in short Okazaki fragments that ligase later joins into one chain. Meselson and Stahl showed in 1958 that each daughter molecule keeps one old strand, and the sketch question is usually lost by candidates who draw both strands as continuous lines.
3. 转录、模板链与 mRNA 加工
转录把基因的碱基序列抄成 RNA，使用的酶是 RNA 聚合酶。只有一条链作为模板，另一条称为编码链，它的序列与 mRNA 基本相同，只是胸腺嘧啶换成尿嘧啶。RNA 聚合酶在启动子处结合并把双链局部打开，然后沿模板链移动，合成方向同样是 5' 到 3'。真核生物的初级转录本要经过剪接，内含子被切除，外显子连接起来，两端再加上帽子结构和多聚腺苷酸尾巴。原核生物没有这些加工步骤，转录与翻译几乎连在一起进行。常见的错误是把编码链当成模板链，或者忘记 mRNA 上的碱基用 U 代替 T。IBO 官网的 D1.2 把重点放在序列如何被可靠地传递，而不是罗列所有调控因子。
Transcription copies a gene into RNA using RNA polymerase, which binds at the promoter and opens the helix locally before moving along the template. Only one strand acts as the template, and the other is called the coding strand because its sequence matches the mRNA apart from thymine being replaced by uracil; the new RNA is built in the 5' to 3' direction just like DNA. In eukaryotes the primary transcript is spliced to remove introns and a cap plus a poly-A tail are added, whereas bacteria have no such processing and begin translation almost immediately.
4. 翻译、密码子与遗传密码的简并性
核糖体读取 mRNA 上的密码子，每个密码子由三个碱基组成，对应一个氨基酸。转运 RNA 一端的反密码子与密码子互补配对，另一端携带特定的氨基酸。起始密码子 AUG 既编码甲硫氨酸，也标志翻译开始，三个终止密码子不编码任何氨基酸。遗传密码是简并的，大多数氨基酸有多个密码子，这降低了点突变带来的影响。核糖体沿 mRNA 移动时肽键在相邻氨基酸之间形成，多肽从 N 端向 C 端延长。很多学生以为一个密码子可以对应多个氨基酸，或者把 tRNA 的反密码子写成与 mRNA 相同的序列。IBO 官网的 D1.2 要求学生能根据一段模板推出氨基酸序列。
Translation turns the message into a chain of amino acids on the ribosome, which reads the mRNA three bases at a time and matches each codon with a tRNA carrying the corresponding amino acid. AUG both starts translation and codes for methionine, the three stop codons code for nothing, and because the code is degenerate most amino acids have several codons, so many point mutations change nothing at all. A very common slip is writing an anticodon identical to the codon instead of complementary to it.
5. 基因突变类型与镰状细胞贫血的例子
点突变只改变一个碱基，替换可能沉默、可能改变一个氨基酸，也可能产生提前终止的密码子。插入或缺失一个碱基会改变读码框，后面所有密码子都被重新解读，影响通常更大。错义突变不一定有害，有的甚至让酶在偏低温度下更活跃。镰状细胞贫血的成因是血红蛋白β链基因第六位的 GAG 变成 GTG，谷氨酸被缬氨酸替换，异常血红蛋白在低氧条件下聚合并使红细胞变形。这个例子常考，是因为它把一个碱基的改变、蛋白质结构与个体表型连在了一起。IBO 官网的 D1.3 要求讨论突变的后果，包括对多肽结构与功能的影响。常见的错误是只说基因变了，不写出氨基酸和蛋白质层面的变化。
A point mutation changes a single base, and a substitution may be silent, may swap one amino acid, or may create a premature stop codon. Insertions and deletions shift the reading frame so every later codon is read differently, which normally has a larger effect; in sickle cell anaemia the sixth codon GAG becomes GTG, glutamic acid is replaced by valine, and the altered haemoglobin polymerises when oxygen is low so the red cells distort. Examiners want that chain traced through to protein structure and phenotype rather than a bare statement that the gene changed.
6. 基因编辑与 CRISPR-Cas9 的机制
CRISPR-Cas9 来自细菌抵抗噬菌体的免疫机制，如今被改造成精准的编辑工具。向导 RNA 通过碱基配对找到目标序列，Cas9 核酸酶在特定位点切断两条链。细胞修复断裂时容易出错，研究者据此破坏某个基因或者插入新的序列。2023 年 11 月英国药品和健康产品管理局授权了首个基于 CRISPR 的疗法，用于镰状细胞病与输血依赖型β地中海贫血，这是官方监管文件里的时间点。有关的伦理讨论集中在体细胞编辑与可遗传胚胎编辑之间的区别。IBO 官网的 D1.3 把基因编辑列为突变这一节的应用。很多学生以为 CRISPR 只用于医学，其实农业与实验室的基因功能研究同样依赖它。
CRISPR-Cas9 started as a bacterial defence against phages and has been repurposed as an editing tool; a guide RNA pairs with the target sequence and the Cas9 nuclease cuts both strands at that site. Because repair of the break is error-prone, researchers can knock out a gene or insert a new sequence, and in November 2023 the UK medicines regulator authorised the first CRISPR-based therapy for sickle cell disease and transfusion-dependent beta thalassemia. The ethical argument normally turns on the difference between editing body cells and editing an embryo whose changes would be inherited.
7. 细胞周期与有丝分裂的四个阶段
细胞周期分为间期与分裂期，间期又包括 G1、S 与 G2 三个阶段。DNA 在 S 期完成复制，此时每条染色体由两条姐妹染色单体组成，由一个着丝粒连接。有丝分裂的四个阶段是前期、中期、后期与末期，中期染色体排列在赤道板上，是观察与计数的最佳时机。后期着丝粒分裂，姐妹染色单体被拉向两极，末期核膜重建、染色体解螺旋。胞质分裂通常平均分配，卵细胞形成时不均等，目的是把营养物质留给一个细胞。HL 还要理解细胞周期受基因调控，原癌基因突变成癌基因或抑癌基因失活会导致分裂失控。常见的错误是忘记 DNA 复制发生在间期而不是分裂期。
The cell cycle runs through interphase and then division, and interphase itself is split into G1, S and G2; DNA is replicated in S phase, leaving each chromosome as two sister chromatids held at a centromere. During mitosis the chromosomes line up on the equator at metaphase, which is the moment to count them, and in anaphase the centromeres split so the chromatids move to opposite poles before telophase rebuilds the nuclei. At HL the cycle is treated as gene-regulated, so mutations that convert proto-oncogenes into oncogenes or disable tumour suppressor genes lead to uncontrolled division.
8. 减数分裂的两轮分离与变异来源
减数分裂从一个二倍体细胞产生四个单倍体细胞，染色体数目减半。第一轮分离把同源染色体分开，配对时非姐妹染色单体之间可能发生交叉互换。同源染色体对在赤道面上的朝向是随机的，这就是独立分配，人类的二十三对染色体因此能产生极多的组合。第二轮分离把姐妹染色单体分开，过程与有丝分裂相似。IBO 官网的 D2.1.11 明确要求学生理解随机朝向与交叉互换如何产生遗传多样性。同源染色体未分开会造成非整倍体，例如二十一三体导致的唐氏综合征。很多学生以为交叉互换发生在姐妹染色单体之间，画图时就画错了位置。
Meiosis turns one diploid cell into four haploid cells across two rounds of division. In the first round homologous chromosomes separate, and crossing over between non-sister chromatids plus the random orientation of bivalents produce the variety that makes each gamete different; the second round separates sister chromatids in much the same way as mitosis. Non-disjunction leaves a gamete with an extra or missing chromosome, as in trisomy 21, and diagram marks go to candidates who pair the correct chromatids together.
9. 单基因杂交与显隐性判断
基因位于染色体上的固定位置，同一位置的不同版本称为等位基因。二倍体个体每个基因有两个等位基因，纯合时两个相同，杂合时两个不同。显性等位基因在杂合状态下也表现出来，隐性等位基因只有纯合时才影响表型。用旁氏表可以推出配子随机结合后的基因型与表型比例，单基因杂交的杂合子自交在完全显性下得到三比一。测交用隐性纯合个体与待测个体杂交，看后代是否出现隐性表型来判断基因型。ABO 血型是复等位基因与共显性的经典例子，IA 与 IB 同时存在时表现为 AB 型。常见的错误是把表型比例当成基因型比例写进答案。
A gene occupies a fixed locus on a chromosome, and the alternative versions found at that locus are called alleles; a diploid individual carries two of them, identical in a homozygote and different in a heterozygote. In complete dominance the heterozygote shows the dominant trait, so selfing two heterozygous parents gives the familiar 3:1 ratio, while a test cross against a recessive homozygote reveals an unknown genotype. The ABO blood group system shows both multiple alleles and codominance, since IA and IB are expressed together in group AB.
10. 双基因杂交与卡方检验的判断
两对位于不同染色体上的基因遵循独立分配，杂合子互交的理论表型比是九比三比三比一。这个比例只是概率的期望，实际计数会因为样本有限而偏离。卡方检验用来判断偏差是否大到不能只用偶然解释，公式是各个观测值与期望值之差的平方除以期望值再求和。自由度等于表型类别数减一，单基因杂交是三减一即一，卡方临界值在百分之五显著性水平下为三点八四。计算出的卡方小于临界值时接受零假设，认为观测比例与理论比例一致。很多学生以为卡方大就证明假设成立，其实结论正好相反。IBO 官网的 D3.2 要求学生能解释统计检验的结果，而不是只算出一个数字。
Two genes on different chromosomes assort independently, so a dihybrid cross between heterozygotes gives the expected 9:3:3:1 ratio, which real counts only approximate. The chi-squared test compares observed and expected numbers by summing the squared differences divided by the expected values; with one degree of freedom for a monohybrid cross the critical value at the 5% level is 3.84, and a calculated value below it means the deviation can be put down to chance. A large chi-squared value rejects the null hypothesis instead of supporting it.
11. 系谱分析与伴性遗传的判断顺序
系谱图把家族成员的表型用符号连起来，判断遗传方式要先看这个性状有没有隔代出现。常染色体隐性遗传会出现隔代个体，双亲正常也可能生出患病子女，近亲婚配会提高风险。常染色体显性遗传通常每代都有患者，患者的双亲里至少一人患病。X 连锁隐性遗传的典型特征是男性患者远多于女性，患病男性的女儿都是携带者，母亲是携带者时儿子有一半概率患病。人类的性别由性染色体决定，男性是 XY，女性是 XX，男性只有一份 X 染色体上的基因。血友病与红绿色盲常被用作伴性遗传的例子。很多学生以为女性不会患 X 连锁隐性病，其实父亲患病、母亲携带时女儿也可能患病。
A pedigree chart uses symbols to show phenotypes across generations, and reading it starts with the question of whether the trait skips a generation. Recessive autosomal conditions can appear in children of unaffected parents and are more likely after consanguineous marriages, whereas a dominant condition usually shows up in every generation; X-linked recessive traits are far more common in males, who carry only one X and are therefore hemizygous for such genes. Haemophilia and red-green colour blindness are the standard examples used in IB questions.
12. 常见失分点与复习顺序
分子遗传与经典遗传的题目经常混在一起考，能描述复制过程却算不出比例，暴露的往往是理解漏洞。写机制类问题要用准确的术语，解旋酶、DNA 聚合酶、RNA 聚合酶、核糖体各管一件事，答错酶的名字常常整分丢掉。画图题要标出方向与末端，5' 与 3' 的标记是常见扣分点。计算类问题要写出公式与代入过程，只给结果拿不到过程分。IBO 官网的指南把科学探究的内部评估列为 20%，报告上限三千词，评分分成研究设计、数据分析、结论与评价四项，每项六分。复习顺序建议先结构、再机制、最后比例，把序列、蛋白质与表型三层因果串起来。错得最多的是把术语堆在一起却说不清先后关系。
Exam questions mix molecular and classical genetics, so a candidate who can describe replication but cannot finish a ratio calculation is usually revealing a gap in understanding. Use the exact terms, because the marks for naming helicase, DNA polymerase, RNA polymerase and the ribosome are separate, and label the 5' and 3' ends on every diagram; in calculation questions the formula and the substitution carry method marks that a bare answer never earns. The internal scientific investigation is worth 20% of the final grade and is assessed under four criteria of six marks each, with a report limit of 3,000 words.
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